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Citrus taxonomy refers to the botanical classification of the
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
, varieties,
cultivars A cultivar is a type of cultivated plant that people have selected for desired traits and when propagated retain those traits. Methods used to propagate cultivars include: division, root and stem cuttings, offsets, grafting, tissue culture, ...
, and graft hybrids within the
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
''
Citrus ''Citrus'' is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, lemons, grapefruits, pomelos, and limes. The genus ''Citrus'' is native to ...
'' and related genera, found in cultivation and in the wild. Citrus taxonomy is complex and controversial. Cultivated citrus are derived from various citrus species found in the wild. Some are only selections of the original wild types, many others are hybrids between two or more original species, and some are backcrossed hybrids between a hybrid and one of the hybrid's parent species. Citrus plants hybridize easily between species with completely different morphologies, and similar-looking citrus fruits may have quite different ancestries. Some differ only in disease resistance. Conversely, different-looking varieties may be nearly genetically identical, and differ only by a bud mutation. Detailed genomic analysis of wild and domesticated citrus cultivars has suggested that the progenitor of modern citrus species expanded out of the Himalayan foothills in a rapid radiation that has produced at least 11 wild species in South and East Asia and Australia, with more than a half-dozen additional candidates for which either insufficient characterization prevents definitive species designation, or there is a lack of consensus for their placement within the ''Citrus'' genus rather than sister genera. Most commercial cultivars are the product of hybridization among these wild species, with most coming from crosses involving
citron The citron (''Citrus medica''), historically cedrate, is a large fragrant citrus fruit with a thick rind. It is said to resemble a 'huge, rough lemon'. It is one of the original citrus fruits from which all other citrus types developed throu ...
s, mandarins and
pomelo The pomelo ( ), ''Citrus maxima'', is the largest citrus fruit from the family Rutaceae and the principal ancestor of the grapefruit. It is a natural, non-hybrid, citrus fruit, native to Southeast Asia. Similar in taste to a sweet grapefr ...
s. and Supplement and Supplement Many different phylogenies for the non-hybrid citrus have been proposed, and the phylogeny based on their nuclear genome does not match that derived from their chloroplast DNA, probably a consequence of the rapid initial divergence. Taxonomic terminology is not yet settled. Most hybrids express different ancestral traits when planted from seeds ( F2 hybrids) and can continue a
stable A stable is a building in which livestock, especially horses, are kept. It most commonly means a building that is divided into separate stalls for individual animals and livestock. There are many different types of stables in use today; the ...
lineage only through
vegetative propagation Vegetative reproduction (also known as vegetative propagation, vegetative multiplication or cloning) is any form of asexual reproduction occurring in plants in which a new plant grows from a fragment or cutting of the parent plant or spec ...
. Some hybrids do reproduce
true to type A true-breeding organism, sometimes also called a purebred ''(biology slang: pure line'' or ''true-breeding line)'', is an organism that always passes down certain phenotypic traits (i.e. physically expressed traits) to its offspring of many genera ...
via nucellar seeds in a process called
apomixis In botany, apomixis is asexual reproduction without fertilization. Its etymology is Greek for "away from" + "mixing". This definition notably does not mention meiosis. Thus "normal asexual reproduction" of plants, such as propagation from cuttin ...
. As such, many hybrid species represent the clonal progeny of a single original F1 cross, though others combine fruit with similar characteristics that have arisen from distinct crosses.


Genetic history

All of the wild 'pure' citrus species trace to a common ancestor that lived in the Himalayan foothills, where a late-
Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
citrus fossil, '' Citrus linczangensis'', has been found. At that time, a lessening of the
monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscil ...
s and resultant drier climate in the region allowed the citrus ancestor to expand across south and east Asia in a rapid genetic radiation. After the plant crossed the
Wallace line The Wallace Line or Wallace's Line is a faunal boundary line drawn in 1859 by the British naturalist Alfred Russel Wallace and named by English biologist Thomas Henry Huxley that separates the biogeographical realms of Asia and Wallacea, a trans ...
a second radiation took place in the early
Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58micranthas (''C. micrantha''), the Ichang papeda (''C. cavaleriei''), the mangshanyegan (''C. mangshanensis''), and the oval (Nagami)
kumquat Kumquats (; zh, 金桔), or cumquats in Australian English, are a group of small fruit-bearing trees in the flowering plant family Rutaceae. Their taxonomy is disputed. They were previously classified as forming the now-historical genus ''For ...
(''Fortunella margarita'' or ''C. japonica'' var. ''margarita''). Three from Australia were identified: the desert lime (''C. glauca''), round lime (''C. australis'') and the finger lime (''C. australasica''). Many other cultivars previously identified as species were found to be closely related variants (
subspecies In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics ( morphology), but that can successfully interbreed. Not all specie ...
or varieties) or hybrids of these species, though not all cultivars were evaluated. Subsequent studies have added two additional species to this list of pure species: a mandarin native to the
Ryukyu Islands The , also known as the or the , are a chain of Japanese islands that stretch southwest from Kyushu to Taiwan: the Ōsumi, Tokara, Amami, Okinawa, and Sakishima Islands (further divided into the Miyako and Yaeyama Islands), with Yon ...
designated '' C. ryukyuensis'', and a rare wild species from Southeast Asia, the
mountain citron ''Citrus halimii'', or mountain citron, is a citrus with sour fruit. Historically placed within the polyphyletic grouping of papedas, it has since been determined to be a wild species most closely related to the kumquats, and is not related to t ...
. A number of further species originally placed in other genera have recently been subsumed into ''Citrus'' as a result of phylogenetic analysis, but these have yet to be characterized on a phylogenomic level to confirm their status as unique pure species. Artificial interbreeding seems possible among all citrus plants, though there are certain limitations to natural interbreeding due to plant physiology and differences in natural breeding seasons. This ability to cross-pollinate extends to some related species that some classifications place in distinct genera. The ability of citrus hybrids to self-pollinate and to reproduce sexually also helps create new varieties, as does spontaneous mutation and genome duplication. The three most predominant ancestral citrus taxa are
citron The citron (''Citrus medica''), historically cedrate, is a large fragrant citrus fruit with a thick rind. It is said to resemble a 'huge, rough lemon'. It is one of the original citrus fruits from which all other citrus types developed throu ...
(''C. medica''),
pomelo The pomelo ( ), ''Citrus maxima'', is the largest citrus fruit from the family Rutaceae and the principal ancestor of the grapefruit. It is a natural, non-hybrid, citrus fruit, native to Southeast Asia. Similar in taste to a sweet grapefr ...
(''C. maxima''), and mandarin (''C. reticulata''). These taxa interbreed freely, despite being quite genetically distinct, having arisen through
allopatric speciation Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...
, with citrons evolving in northern
Indochina Mainland Southeast Asia, also known as the Indochinese Peninsula or Indochina, is the continental portion of Southeast Asia. It lies east of the Indian subcontinent and south of Mainland China and is bordered by the Indian Ocean to the west an ...
, pomelos in the
Malay Archipelago The Malay Archipelago ( Indonesian/ Malay: , tgl, Kapuluang Malay) is the archipelago between mainland Indochina and Australia. It has also been called the " Malay world," " Nusantara", "East Indies", Indo-Australian Archipelago, Spices Arc ...
, and mandarins in
Vietnam Vietnam or Viet Nam ( vi, Việt Nam, ), officially the Socialist Republic of Vietnam,., group="n" is a country in Southeast Asia, at the eastern edge of mainland Southeast Asia, with an area of and population of 96 million, making ...
,
southern China South China () is a geographical and cultural region that covers the southernmost part of China. Its precise meaning varies with context. A notable feature of South China in comparison to the rest of China is that most of its citizens are not n ...
, and
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the n ...
. The hybrids of these taxa include familiar citrus fruits like oranges,
grapefruit The grapefruit (''Citrus'' × ''paradisi'') is a subtropical citrus tree known for its relatively large, sour to semi-sweet, somewhat bitter fruit. The interior flesh is segmented and varies in color from pale yellow to dark pink. Grapefruit ...
,
lemon The lemon (''Citrus limon'') is a species of small evergreen trees in the flowering plant family Rutaceae, native to Asia, primarily Northeast India (Assam), Northern Myanmar or China. The tree's ellipsoidal yellow fruit is used for culin ...
s, and some
limes Limes may refer to: * the plural form of lime (disambiguation) Lime commonly refers to: * Lime (fruit), a green citrus fruit * Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide * Lime (color), a ...
and
tangerine The tangerine is a type of citrus fruit that is orange in color. Its scientific name varies. It has been treated as a separate species under the name ''Citrus tangerina'' or ''Citrus'' × ''tangerina'', or treated as a variety of '' Citrus reti ...
s. These three have also been hybridized with other citrus taxa, for example, the Key lime arose from a citron crossing with a micrantha. In many cases, the varieties are propagated asexually, and lose their characteristic traits if bred. Some of the hybrids have in turn interbred with one another hybrid or with the original taxa, making the citrus family tree a complicated network.
Kumquat Kumquats (; zh, 金桔), or cumquats in Australian English, are a group of small fruit-bearing trees in the flowering plant family Rutaceae. Their taxonomy is disputed. They were previously classified as forming the now-historical genus ''For ...
s do not naturally interbreed with core taxa due to different flowering times, but hybrids (such as the
calamansi Calamansi (''Citrus'' × ''microcarpa''), also known as calamondin, Philippine lime, or Philippine lemon, is an economically important citrus hybrid predominantly cultivated in the Philippines. It is native to the Philippines, Borneo, Sumatra, ...
) exist.
Australian lime Australian limes are species of the plant genus ''Citrus'' that are native to Australia and Papua New Guinea. These species were formerly included in the genera ''Microcitrus'' and ''Eremocitrus''. They have been used as a food source by indigeno ...
s are native to
Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
and
Papua New Guinea Papua New Guinea (abbreviated PNG; , ; tpi, Papua Niugini; ho, Papua Niu Gini), officially the Independent State of Papua New Guinea ( tpi, Independen Stet bilong Papua Niugini; ho, Independen Stet bilong Papua Niu Gini), is a country i ...
, so they did not naturally interbreed with the core taxa, but they have been crossbred with mandarins and
calamansi Calamansi (''Citrus'' × ''microcarpa''), also known as calamondin, Philippine lime, or Philippine lemon, is an economically important citrus hybrid predominantly cultivated in the Philippines. It is native to the Philippines, Borneo, Sumatra, ...
s by modern breeders. Humans have deliberately bred new citrus fruits by propagating seedlings of spontaneous crosses (e.g. clementines), creating or selecting mutations of hybrids, (e.g.
Meyer lemon ''Citrus'' × ''meyeri'', the Meyer lemon (), is a hybrid citrus fruit native to China. It is a cross between a citron and a mandarin/pomelo hybrid. Mature trees are around tall with dark green shiny leaves. The flowers are white with a purp ...
), and crossing different varieties (e.g. 'Australian Sunrise', a finger lime and
calamansi Calamansi (''Citrus'' × ''microcarpa''), also known as calamondin, Philippine lime, or Philippine lemon, is an economically important citrus hybrid predominantly cultivated in the Philippines. It is native to the Philippines, Borneo, Sumatra, ...
cross).


Citrus naming systems

Initially, many citrus types were identified and named by individual taxonomists, resulting in a large number of identified species: 870 by a 1969 count. Some order was brought to citrus taxonomy by two unified classification schemes, those of
Chōzaburō Tanaka , often Romanized as Tyôzaburô Tanaka (November 3, 1885 in Osaka – June 28, 1976), was a Japanese botanist and mycologist. He established one of the two major taxonomic classification systems for citrus and related genera currently in use, an ...
and
Walter Tennyson Swingle Walter Tennyson Swingle (January 8, 1871 – January 19, 1952) was an American agricultural botanist who contributed greatly to the classification and taxonomy of citrus. Biography Swingle was born in Canaan, Pennsylvania, and moved with his ...
, that can be viewed as extreme alternative visions of the genus. Swingle's system divided the Citrinae subtribe into three groups, the 'primitive citrus' distant relatives, the closer 'near citrus' including citrus-related genera like ''
Atalantia ''Atalantia'' is a genus of flowering plants in the citrus family, the Rutaceae.Swingle, W. T., rev. P. C. ReeceChapter 3: The Botany of ''Citrus'' and its Wild Relatives.In: ''The Citrus Industry'' vol. 1. Webber, H. J. (ed.). Berkeley: Universit ...
'', and the "true citrus", for the species that had historically been placed in ''Citrus'' but many of which he elevated to separate genera: ''Poncirus'' (trifoliate orange), ''Fortunella'' (kumquat), ''Eremocitrus'' (desert limes), ''Microcitrus'' (finger and round limes), as well as an additional genus, '' Clymenia'', formerly thought to be a citrus hybrid. His ''Citrus'' he likewise subdivided into two subgenera: citrons, pomelos, mandarins, oranges, grapefruits and lemons were placed in subgenus ''Eucitrus'' (later called simply subgenus ''Citrus''), while the hardy but slow-growing trees with relatively unpalatable fruit he placed in subgenus '' Papeda''. His genus ''Citrus'' consisted of just 16
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
, dividing them further into varieties, and lastly cultivars or hybrids. The Swingle system is generally followed globally today with much modification; there are still large differences in nomenclature between countries and individual scientists. The 'Tanaka system' (1954) instead provides a separate species name for each cultivar, regardless of whether it is pure or a hybrid of two or more species or varieties, and resulted in 159 identified species. It thus represents an example of taxonomic " splitting", and in assigning separate species names to horticultural variants does not conform to the standard species concept. Tanaka also divided into subgenera, but different than in Swingle's system, introducing ''Archicitrus'' (which he subdivided into five sections, Papeda, Limonellus, Aruntium, Citrophorum and Cephalocitrus) and ''Metacitrus'' (divided into Osmocitrus, Acrumen and Pseudofortunella). This system is commonly used in Tanaka's native Japan. A 1969 analysis by Hodgson intended to harmonize the two schemes accepted 36 species. These initial attempts at ''Citrus'' systematization all predated the recognition, which began to gain traction in the mid-1970s, that the majority of cultivars represent hybrids of just three species, citron, mandarin and pomelo. Phylogenetic analysis confirms this hybrid origin of most citrus cultivars, indicating a small number of founder species. While the subgenera suggested by Tanaka proved similar to the phylogenetic divisions of pure founder species, Swingle's subgenera were
polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...
, and hence do not represent valid taxonomy. His novel genera also fail to withstand phylogenetic analysis. Swingle had elevated kumquats into a separate genus ''Fortunella'', while two genera were suggested by him for the Australian limes, ''Microcitrus'' and ''Eremocitrus''. However, genomic analysis shows these groups nested within the phylogenetic tree of ''Citrus''. Since their placement in distinct genera would make ''Citrus'' a paraphyletic grouping, it has been suggested that all of these are correctly members of the genus ''Citrus''. Similarly, genomic analysis has suggested that other genera previously split off from ''Citrus'' likewise belong within this expanded phylogenetic concept of the genus ''Citrus'', including '' Clymenia'', '' Oxanthera'' (false oranges, moved out of ''Citrus'' to a different branch of the tribe
Citreae Citreae is one of the two tribes of the flowering plant family Rutaceae, subfamily Aurantioideae, the other being Clauseneae. Subtribes and genera Three sub-tribes are included: * Triphasiinae ** '' Luvunga'' Buch.-Ham. ex Wight & Arn. ** '' ...
and given a new genus by Swingle) and more controversially ''
Poncirus The trifoliate orange, ''Citrus trifoliata'' or ''Poncirus trifoliata'', is a member of the family Rutaceae. Whether the trifoliate oranges should be considered to belong to their own genus, ''Poncirus'', or be included in the genus ''Citrus'' is ...
'', which is the earliest branching of Swingle's genera, and a genus not previously recognized as a close citrus relative, ''
Feroniella ''Feroniella'' is a genus in the family Rutaceae, the only species being ''Feroniella lucida''. The genus is placed within ''Citrus'' by some sources, with the species becoming ''Citrus lucida''. ''Feroniella lucida'' is a fruit-bearing tree nativ ...
''. There remains a lack of consensus as to which wild plants and hybrids merit distinct species status, a phenomenon exacerbated by the prior failure to correctly identify the genetically pure citrus strains and distinguish them from hybrids. In 2020, a new taxonomic system was proposed by Ollitrault, Curk and Krueger, with the goal of harmonizing traditional naming systems with the new genomic data that have both allowed the pure ancestral species to be distinguished from hybrids, and enabled the ancestry of those hybrids to be identified among the ancestral species. In their system, each ancestral species has a binomial name, while a unique species name is reserved for each combination of ancestral species, independent of the specific order of crossing or proportional representation of the ancestral species in a given hybrid. Individual hybrids of each type are then distinguished by a variety name. Thus hybrids that are crosses between mandarin (''C. reticulata'') and pomelo (''C. maxima'') would all be ''C.'' × ''aurantium'', with specific crosses including: ''C.'' × ''aurantium'' var. ''sinensis'' for the sweet orange, ''C.'' × ''aurantium'' var. ''paradisi'' for grapefruit, and ''C.'' × ''aurantium'' var. ''clementina'' for the clementine. Likewise, hybrids combining mandarins and citrons would all be varieties of ''C.'' × ''limonia'', those of pomelo and citron, ''C.'' x ''lumia'', while tri-species hybrids of citrons, pomelos and mandarins would be ''C.'' × ''limon'', and a tetra-species cross involving these three species along with ''C. micrantha'' would be ''C.'' × ''latifolia''. This naming system focused on the four species ancestral to most commercial hybrids, and did not include similar species designations for more exotic hybrids involving other citrus species, such as the Ichang papeda, kumquat, or trifoliate orange. Likewise, Ollitrault, Curk and Krueger accepted that the whole-genome characterization necessary to unambiguously assign a hybrid species name under their system is not available for many varieties.


Core species and hybrids

Most commercial varieties are descended from one or more of the 'core species',
citrons The citron (''Citrus medica''), historically cedrate, is a large fragrant citrus fruit with a thick rind. It is said to resemble a 'huge, rough lemon'. It is one of the original citrus fruits from which all other citrus types developed throu ...
, mandarins, and pomelos, which share in common a complex floral anatomy that gives rise to more complex fruit. These core species, and to a lesser extent other citrus, have given rise to a wide variety of hybrids for which the naming is inconsistent. The same
common name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often contrast ...
s may be given to different species, citrus hybrids or mutations. For example, citrus with green fruit tend to be called 'limes' independent of their origin: Australian limes, musk limes, Key limes,
kaffir lime ''Citrus hystrix'', called the kaffir lime or makrut lime, (, ) is a citrus fruit native to tropical Southeast Asia. Its fruit and leaves are used in Southeast Asian cuisine, and its essential oil is used in perfumery. Its rind and crushed leav ...
s, Rangpur limes, sweet limes and
wild lime ''Zanthoxylum fagara'' or wild lime, is a species of flowering plant that—despite its name—is not part of the genus ''Citrus'' with real limes and other fruit, but is a close cousin in the larger citrus family, Rutaceae. It is native to sout ...
s are all genetically distinct. Fruit with similar ancestry may be quite different in name and traits (e.g. grapefruit, common oranges, and ponkans, all pomelo-mandarin hybrids). Many traditional citrus groups, such as true sweet oranges and lemons, seem to be bud sports, clonal families of cultivars that have arisen from distinct spontaneous mutations of a single hybrid ancestor. Novel varieties, and in particular seedless or reduced-seed varieties, have also been generated from these unique hybrid ancestral lines using gamma irradiation of budwood to induce mutations.


Ancestral species


Mandarins

Mandarin orange The mandarin orange (''Citrus reticulata''), also known as the mandarin or mandarine, is a small citrus tree fruit. Treated as a distinct species of orange, it is usually eaten plain or in fruit salads. Tangerines are a group of orange-colou ...
s (tangerines, satsumas – ''Citrus reticulata'') are one of the basic species, but the name mandarin is also used more generally for all small, easily peeled citrus, including a large range of hybrids. Swingle saw three species of mandarin, while Tanaka identified five groups with a total of 36 species. Webber (1948) divided them into four groups, king, satsuma, mandarin, and tangerine, and Hodgson (1967) saw in them four species. Genomic analysis suggests just one mainland-Asian species, ''Citrus reticulata''. In an observation originally made in a study of their hybrid progeny, a subspecies-level division has been characterized in this mainland-Asian species. Wang, ''et al.'', found that domesticated mandarins fell into two genetic clusters that linked to different branches of the tree of wild mandarins, had different deduced population histories and had distinct patterns of pomelo introgression, suggesting that they derive from separate domestication events. Wu, ''et al.'', would later extend this observation, similarly detecting two divergent subspecies within the wild populations that gave rise to Wang's northern and southern domesticate classes, which they described as 'common mandarins' and ''mangshanyeju'' (Mangshan wild mandarins). It was specifically in the latter that a genetic mutation caused by the insertion of a
transposable element A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Transp ...
adjacent to the ''CitRKD1'' gene led to the ability of these mandarins to reproduce asexually through
apomixis In botany, apomixis is asexual reproduction without fertilization. Its etymology is Greek for "away from" + "mixing". This definition notably does not mention meiosis. Thus "normal asexual reproduction" of plants, such as propagation from cuttin ...
, a characteristic passed down to the subspecies' hybrid descendants such as hybrid mandarins, oranges, lemons and grapefruit. A distinct class of mandarins are native to the Japanese and neighboring islands. Initial characterization of one of these, the
Tachibana orange The tachibana orange (''Citrus tachibana,'' or ''Citrus reticulata tachibana'') is a variety of mandarin orange, a citrus fruit. They grow wild in the forests of Japan and are referred to in the poetry of the early Japanese and Ryukyu Islands kin ...
(Tanaka's ''Citrus tachibana''), native to Taiwan, the Ryukyu Islands and southern Japan, classified it as a
subspecies In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics ( morphology), but that can successfully interbreed. Not all specie ...
nesting within the wild mandarins of the East-Asian mainland. However, a directed study of these island cultivars revealed the existence of a second mandarin true-species that diverged from the mainland species between 2.2 and 2.8 million years ago, following the geographical isolation of the islands through rising sea levels. Unlike the mainland species, this Ryukyu mandarin, named ''C. ryukyuensis'', reproduces sexually. The previously-characterized island cultivars, including the Tachibana, proved to be either natural
F1 hybrid An F1 hybrid (also known as filial 1 hybrid) is the first filial generation of offspring of distinctly different parental types. F1 hybrids are used in genetics, and in selective breeding, where the term F1 crossbreed may be used. The term is some ...
s between this native Ryukyu mandarin and mainland mandarin species that had recolonized the islands after a period of isolation, or else later agricultural hybrids with introduced Asian cultivars. All characterized commercial varieties called mandarins are actually inter-species hybrids. Wu, ''et al.'', divided mandarins into three types, based on their degree of hybridization. In addition to genetically pure mandarins, a second type are the result of hybridization with pomelos followed by subsequent
backcrossing Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, to achieve offspring with a genetic identity closer to that of the parent. It is used in horticulture, animal breeding, and produ ...
with mandarins to retain only a few pomelo traits. The third type arose more recently from the crossing of these hybrids again with pomelos or sweet oranges (which are themselves crosses of hybrid mandarins and pomelos). This produces mandarins with more, longer stretches of pomelo DNA. Some commercial mandarins are hybrids with lemons, while several were found to have a significant contribution (35–65%) from papedas. 'Mangshan wild mandarin' is a name used for all of the similar-looking wild mandarin-like fruit of the Mangshan area, but has been found to include two genetically-distinct groups, one representing pure, wild "true" mandarins (the ''mangshanyeju'' subspecies of ''C. reticulata''), and the other the genetically-distinct and only distantly-related species, the '' mangshanyegan'' (''C. mangshanensis''), akin to another local fruit known as the ''yuanju'', and found to be the most distant branch of all the citrus. In a limited genomic analysis, ''
Feroniella ''Feroniella'' is a genus in the family Rutaceae, the only species being ''Feroniella lucida''. The genus is placed within ''Citrus'' by some sources, with the species becoming ''Citrus lucida''. ''Feroniella lucida'' is a fruit-bearing tree nativ ...
'' was surprisingly found to cluster with ''C. reticulata'' deep within ''Citrus'', leading botanist David Mabberley to propose that the sole member of this genus, ''F. lucida'', be moved to the genus ''Citrus'' and rechristened ''C. lucida''. Though this has received a degree of acceptence, two modern phylogenetic studies obtained results in conflict with Mabberley's, and retained ''Feronioella'' as a distinct genus closely related to '' Luminia'', with which Swingle had placed ''Feroniella'' in a grouping referred to as 'wood apples'.


Pomelos

The
pomelo The pomelo ( ), ''Citrus maxima'', is the largest citrus fruit from the family Rutaceae and the principal ancestor of the grapefruit. It is a natural, non-hybrid, citrus fruit, native to Southeast Asia. Similar in taste to a sweet grapefr ...
(''Citrus maxima''), a second of the core species from which most citrus hybrids have derived, is native to southeast Asia. Among the hybrids deriving from mandarin/pomelo crosses, there is a direct correlation between the proportion of pomelo DNA in the hybrid and fruit size, while the more palatable mandarins are those that have received specific genes from pomelos that alter their acidity. Some of the more common pomelos are genetically pure, while a number have a single small region of introgressed mandarin DNA on one chromosome, the result of a cross followed by extensive backcrossing with pomelo.


Citrons

Varieties of true (non-hybrid)
citron The citron (''Citrus medica''), historically cedrate, is a large fragrant citrus fruit with a thick rind. It is said to resemble a 'huge, rough lemon'. It is one of the original citrus fruits from which all other citrus types developed throu ...
(''Citrus medica'') have distinctly different forms. The citron usually propagates by cleistogamy, a
self-pollination Self-pollination is a form of pollination in which pollen from the same plant arrives at the stigma of a flower (in flowering plants) or at the ovule (in gymnosperms). There are two types of self-pollination: in autogamy, pollen is transferr ...
within an unopened flower, and this results in the lowest levels of heterozygosity among the citrus species. Because of this, it will generally serve as the male parent of any hybrid progeny. Many citron varieties were proven to be non-hybrids despite their rather dramatic morphological differences; however, the florentine citron is probably of hybrid origin. Genetic analysis of citrons has shown that they divide into three groups. One cluster consists of wild citrons that originated in China and produce non-fingered fruit with pulp and seeds. A second cluster, also native to China, consist of the fingered citrons, most of which are seedless and must be propagated artificially. The third cluster represents the Mediterranean citrons, thought to have originally been introduced there from India. Some
fingered citron ''Citrus medica'' var. ''sarcodactylis'', or the fingered citron, is an unusually-shaped citron variety whose fruit is segmented into finger-like sections, resembling those seen on representations of the Buddha. It is called Buddha's hand in m ...
varieties are used in
buddhist offering In Buddhism, symbolic offerings are made to the Triple Gem, giving rise to contemplative gratitude and inspiration. Typical material offerings involve simple objects such as a lit candle or oil lamp, burning incense, flowers, food, fruit, water ...
s, and some more common varieties are used as the
etrog Etrog ( he, אֶתְרוֹג, plural: '; Ashkenazi Hebrew: ', plural: ') is the yellow citron or ''Citrus medica'' used by Jews during the week-long holiday of Sukkot as one of the four species. Together with the ''lulav'', ''hadass'', and '' a ...
in the Jewish harvest festival of
Sukkot or ("Booths, Tabernacles") , observedby = Jews, Samaritans, a few Protestant denominations, Messianic Jews, Semitic Neopagans , type = Jewish, Samaritan , begins = 15th day of Tishrei , ends = 21st day of Tis ...
. There is also a specific variety of citron called
etrog Etrog ( he, אֶתְרוֹג, plural: '; Ashkenazi Hebrew: ', plural: ') is the yellow citron or ''Citrus medica'' used by Jews during the week-long holiday of Sukkot as one of the four species. Together with the ''lulav'', ''hadass'', and '' a ...
. The
Mountain citron ''Citrus halimii'', or mountain citron, is a citrus with sour fruit. Historically placed within the polyphyletic grouping of papedas, it has since been determined to be a wild species most closely related to the kumquats, and is not related to t ...
is a complex citrus hybrid that only includes trace amounts of true citron.


Papedas

Swingle coined the ''Citrus'' subgenus ''Papeda'' to separate its members from the more edible citrus that also differ from other citrus in having stamens that grow separately, not united at their base. He included in this group the
kaffir lime ''Citrus hystrix'', called the kaffir lime or makrut lime, (, ) is a citrus fruit native to tropical Southeast Asia. Its fruit and leaves are used in Southeast Asian cuisine, and its essential oil is used in perfumery. Its rind and crushed leav ...
(''Citrus hystrix''), as well as its likely taxonomic synonym the micrantha (''Citrus micrantha''), Ichang papeda (''Citrus cavaleriei''). Since the latter two species locate to different branches of the citrus phylogenetic tree, the group would be
polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...
and not a valid division. Both the micrantha and the Ichang papeda have also given rise to hybrids with other citrus. Sometimes included among the papedas was the mountain citron, not affiliated with the true citrons, and subsequently found to be a pure species most closely related to kumquats.


Kumquats

Kumquats Kumquats (; zh, 金桔), or cumquats in Australian English, are a group of small fruit-bearing trees in the flowering plant family Rutaceae. Their taxonomy is disputed. They were previously classified as forming the now-historical genus ''Fortun ...
were originally classified by
Carl Peter Thunberg Carl Peter Thunberg, also known as Karl Peter von Thunberg, Carl Pehr Thunberg, or Carl Per Thunberg (11 November 1743 – 8 August 1828), was a Swedish naturalist and an "apostle" of Carl Linnaeus. After studying under Linnaeus at Uppsala U ...
as ''Citrus japonica'' in his 1784 book '' Flora Japonica''. In 1915, Swingle reclassified them in a separate genus, '' Fortunella'', named in honor of Robert Fortune. He divided the kumquats into two subgenera, the ''Protocitrus'', containing the primitive Hong Kong kumquat (''F. hindsii''), and ''Eufortunella'', comprising the round (''F. japonica''), oval kumquat (''F. margarita'') and Meiwa kumquats (''F. crassifolia''), to which Tanaka added two others, the Malayan kumquat (which he christened ''F. swinglei'' but more commonly called ''F. polyandra'') and the Jiangsu kumquat (''F. obovata''), and Huang added another, ''F. bawangica''. Since the kumquat is a cold-hardy species, there are many hybrids between common citrus members and the kumquat. Swingle coined a separate hybrid genus for these, which he called '' × Citrofortunella''. Subsequent study of the many commercial citrus lineages revealed such complexity that the genera could not be separated, and genomic analysis rooted ''Fortunella'' within the polyphyletic tree of ''Citrus''. As a result there is growing acceptance for the restoration of kumquats to ''Citrus'', though the assignment of individual species among the kumquats remains controversial due in part to insufficient genomic data on the variants. ''The Flora of China'' unites all kumquats as the single species, ''Citrus japonica''. Based on chromosomal analysis, Yasuda, et al., identified Jiangsu and Malayan kumquats as hybrids and see the remainder of the ''Eufortunella'' subgenus as a single species, while retaining a distinct species designation for the Hong Kong kumquat.


Australian and New Guinean species

Australian and New Guinean citrus species had been viewed as belonging to separate genera by Swingle, who placed in ''Microcitrus'' all but the desert lime, which he assigned to ''
Eremocitrus ''Citrus glauca'', commonly known as the desert lime, is a thorny shrub or small tree native to Queensland, New South Wales, and South Australia.Burkill, Isaac Henry. 1932. Gardens' Bulletin, Straits Settlements 5 (Index): 3. ''Citrus glauca''. ...
''. However, genomic analysis shows that though they form a distinct clade from other citrus, this is nested within the citrus phylogenetic tree, most closely related to kumquats, suggesting that all these species should be included in the genus ''Citrus''. Wu, ''et al.'', found that several of the finger lime cultivars were actually hybrids with round lime, and concluded there were just three species among those tested, desert lime (''C. glauca''), round lime (''C. australis'') and the finger lime (''C. australasica''), though their analysis did not include other types previously identified as distinct species. In more limited genomic analysis, the
New Guinea wild lime ''Citrus warburgiana''. the kakamadu or New Guinea wild lime, grows on the south coast of the Papuan Peninsula near Alotau in Papua-New Guinea. It is a poorly known tree species. It has dark green, spherical fruits about in diameter. It is t ...
, '' Clymenia'' and '' Oxanthera'' (false orange) all cluster with the Australian limes as members of ''Citrus''. The outback lime is a desert lime agriculturally selected for more commercial traits, while some commercial varieties of the Australian lime are hybrids with mandarins, lemons, and/or sweet oranges. ''Clymenia'', will hybridize with
kumquat Kumquats (; zh, 金桔), or cumquats in Australian English, are a group of small fruit-bearing trees in the flowering plant family Rutaceae. Their taxonomy is disputed. They were previously classified as forming the now-historical genus ''For ...
s and some
limes Limes may refer to: * the plural form of lime (disambiguation) Lime commonly refers to: * Lime (fruit), a green citrus fruit * Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide * Lime (color), a ...
.


Trifoliate orange

The trifoliate orange is a cold-hardy plant distinguishable by its compound leaves with three leaflets and its deciduous nature, but is close enough to the genus ''Citrus'' to be used as a rootstock. Swingle moved the trifoliate orange from ''Citrus'' to its own
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
, ''
Poncirus The trifoliate orange, ''Citrus trifoliata'' or ''Poncirus trifoliata'', is a member of the family Rutaceae. Whether the trifoliate oranges should be considered to belong to their own genus, ''Poncirus'', or be included in the genus ''Citrus'' is ...
'', but Mabberley and Zhang reunited the genera Swingle had separated back into ''Citrus''. Early phylogenetic analysis nested ''Poncirus'' within the citrus, consistent with a single genus, but the genomic sequencing of Wu, ''et al.'', placed it outside the cluster representing ''Citrus'', and the authors retained a separate genus ''Poncirus''. Ollitrault, Curk and Krueger indicate that the majority of data are consistent with the enlarged ''Citrus'' that includes the trifoliate orange, though they recognize that many botanists still follow Swingle. A further complication to the placement of ''Poncirus'' is the conflicting phylogenetic data: its nuclear genome places ''Poncirus'' as an outgroup to other citrus, while its
chloroplast DNA Chloroplast DNA (cpDNA) is the DNA located in chloroplasts, which are photosynthetic organelles located within the cells of some eukaryotic organisms. Chloroplasts, like other types of plastid, contain a genome separate from that in the cell n ...
(cpDNA) nests within a ''Citrus'' subclade. This led Talon, ''et al.'' to conclude that the trifoliate orange likely is either the progeny of an ancient hybridization between a core citrus and an unidentified more distant relative, or at some time in its history acquired an introgressed cpDNA genome from another species. A second trifoliate orange, ''Poncirus polyandra'', was discovered in
Yunnan Yunnan , () is a landlocked province in the southwest of the People's Republic of China. The province spans approximately and has a population of 48.3 million (as of 2018). The capital of the province is Kunming. The province borders the ...
(
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
) in the 1980s. Zhang and Mabberley later concluded this was likely a hybrid between the trifoliate orange and some other ''Citrus''. However, recent genomic analysis of ''P. polyandra'' showed low heterozygosity, the opposite of what one would expect for a hybrid. Were ''Poncirus'' subsumed into ''Citrus'', ''C. polyandra'' would be unavailable, so ''C. polytrifolia'' has been suggested as a replacement species name for this Yunnan trifoliate orange. File:Mandarin Oranges (Citrus Reticulata).jpg,
Mandarin orange The mandarin orange (''Citrus reticulata''), also known as the mandarin or mandarine, is a small citrus tree fruit. Treated as a distinct species of orange, it is usually eaten plain or in fruit salads. Tangerines are a group of orange-colou ...
is a true species (''Citrus reticulata''); it is one of the progenitors of most cultivated
citrus ''Citrus'' is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, lemons, grapefruits, pomelos, and limes. The genus ''Citrus'' is native to ...
File:Citrus grandis - Honey White.jpg, The
pomelo The pomelo ( ), ''Citrus maxima'', is the largest citrus fruit from the family Rutaceae and the principal ancestor of the grapefruit. It is a natural, non-hybrid, citrus fruit, native to Southeast Asia. Similar in taste to a sweet grapefr ...
(''Citrus maxima'') File:Citron (8448513463).jpg, These varieties of
citron The citron (''Citrus medica''), historically cedrate, is a large fragrant citrus fruit with a thick rind. It is said to resemble a 'huge, rough lemon'. It is one of the original citrus fruits from which all other citrus types developed throu ...
(''Citrus medica''),
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
and fingered, have distinctly different appearances File:3 etrog.JPG, Three varieties of ''
etrog Etrog ( he, אֶתְרוֹג, plural: '; Ashkenazi Hebrew: ', plural: ') is the yellow citron or ''Citrus medica'' used by Jews during the week-long holiday of Sukkot as one of the four species. Together with the ''lulav'', ''hadass'', and '' a ...
im'' (''Citrus medica'' acceptable for Jewish ritual use) that are all true non-hybrid citrons File:Fort-hindsii.jpg, Hong Kong kumquat File:Citrus glauca fruit.jpg, The Australian desert lime, '' Citrus glauca'', hangs from a branch File:Poncirusfruits2001HD.jpg, Trifoliate orange


Hybrids

Citrus hybrids include many varieties and
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
that have been selected by
plant breeders Plant breeding is the science of changing the traits of plants in order to produce desired characteristics. It has been used to improve the quality of nutrition in products for humans and animals. The goals of plant breeding are to produce cro ...
. This is done not only for the useful characteristics of the fruit, but also for plant size and growth characteristics such as cold-tolerance. Some citrus hybrids occurred naturally, and others have been deliberately created, either by cross pollination and selection among the progeny, or (rarely, and only recently) as somatic hybrids. The aim of plant breeding of hybrids is to use two or more different citrus varieties or species, in order to get traits intermediate between those of the parents, or to transfer individual desirable traits of one parent into the other. In some cases, particularly with the natural hybrids, it has been viewed as
hybrid speciation Hybrid speciation is a form of speciation where hybridization between two different species leads to a new species, reproductively isolated from the parent species. Previously, reproductive isolation between two species and their parents was thou ...
and the new plants have been viewed as different species from any of their parents. In older taxonomic systems, citrus hybrids have often been given unique hybrid names, marked with a multiplication sign after the word ''Citrus'' (or abbreviation ''C.''); for example, the Key lime is ''Citrus'' × ''aurantifolia'', and also are referred to by joining the names of the crossed species or hybrids that produced them, as with
sunquat A sunquat, also known as lemonquat or lemondrop, is a variety of citrus fruit, having an edible rind. It was initially created by Leslie Cude in Beeville, Texas, as a chance hybrid between a lemon The lemon (''Citrus limon'') is a spec ...
– ''C. limon'' × ''japonica''. Styling a hybrid as such a cross between two species can present challenges. In some cases the parental species that gave rise to a hybrid have yet to be determined, while genotyping reveals some hybrids to descend from three or more ancestral species. In the Ollitrault system, a hybrid will be given a species name corresponding to the ancestral species contributions to it, as well as a distinctive variety name.


Labelling of hybrids

Hybrid taxonomy is inconsistent. There is disagreement over whether to assign species names to hybrids, and even modern hybrids of known parentage are sold under general common names that give little information about their ancestry, or even imply technically incorrect identity. This can be a problem for those who cannot eat some citrus varieties. Drug interactions with chemicals found in some citrus, including
grapefruit The grapefruit (''Citrus'' × ''paradisi'') is a subtropical citrus tree known for its relatively large, sour to semi-sweet, somewhat bitter fruit. The interior flesh is segmented and varies in color from pale yellow to dark pink. Grapefruit ...
and Seville oranges, make the ancestry of citrus fruit of interest: many commonly sold citrus varieties are grapefruit hybrids or
pomelo The pomelo ( ), ''Citrus maxima'', is the largest citrus fruit from the family Rutaceae and the principal ancestor of the grapefruit. It is a natural, non-hybrid, citrus fruit, native to Southeast Asia. Similar in taste to a sweet grapefr ...
-descended grapefruit relatives. One medical review has advised patients on medication to avoid all citrus juice, although some citrus fruits contain none of the problematic
furanocoumarin The furanocoumarins, or furocoumarins, are a class of organic chemical compounds produced by a variety of plants. Most of the plant species found to contain furanocoumarins belong to a handful of plant families. The families Apiaceae and Rutace ...
s. Citrus allergies can also be specific to only some fruit or some parts of some fruit.


Major citrus hybrids

The most common citrus hybrids that are sometimes treated as a
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
by themselves, especially in folk taxonomy, are: * Orange: a name used for several distinct crosses between
pomelo The pomelo ( ), ''Citrus maxima'', is the largest citrus fruit from the family Rutaceae and the principal ancestor of the grapefruit. It is a natural, non-hybrid, citrus fruit, native to Southeast Asia. Similar in taste to a sweet grapefr ...
and a
mandarin orange The mandarin orange (''Citrus reticulata''), also known as the mandarin or mandarine, is a small citrus tree fruit. Treated as a distinct species of orange, it is usually eaten plain or in fruit salads. Tangerines are a group of orange-colou ...
. They have the
orange color Orange is the colour between yellow and red on the spectrum of visible light. Human eyes perceive orange when observing light with a dominant wavelength between roughly 585 and 620 nanometres. In traditional colour theory, it is a secondary ...
of the mandarin in their outer peels and segments, and are easier to peel than the pomelos. Oranges are all intermediate between the two ancestors in size, flavor and shape. The
bitter orange Bitter orange, Seville orange, bigarade orange, or marmalade orange is the citrus tree ''Citrus'' × ''aurantium'' and its fruit. It is native to Southeast Asia and has been spread by humans to many parts of the world. It is probably a cross be ...
and sweet orange both arose from mandarin-pomelo crosses, the former involving a pure mandarin, the latter with a mandarin already containing small amounts of pomelo. *
Grapefruit The grapefruit (''Citrus'' × ''paradisi'') is a subtropical citrus tree known for its relatively large, sour to semi-sweet, somewhat bitter fruit. The interior flesh is segmented and varies in color from pale yellow to dark pink. Grapefruit ...
: grapefruits, like oranges, include genetic contributions from both mandarin and pomelo, but more of the latter, arising from a natural backcross of a sweet orange with a pomelo. The 'cocktail grapefruit', or mandelo, is distinct, instead the product of a low-acid pomelo variety hybridized with a mandarin that itself was a cross between two distinct mandarin stocks. *
Lemon The lemon (''Citrus limon'') is a species of small evergreen trees in the flowering plant family Rutaceae, native to Asia, primarily Northeast India (Assam), Northern Myanmar or China. The tree's ellipsoidal yellow fruit is used for culin ...
: "true" lemons derive from one common hybrid ancestor, having diverged by mutation. The original lemon was a hybrid between a male citron and a female sour orange, itself a pomelo/pure-mandarin hybrid; citrons contribute half of the genome, while the other half is divided between pomelo and mandarin. There are other hybrids also known as 'lemons'.
Rough lemon Rough lemon (''Citrus'' × ''jambhiri'' Lush.) is the fruit and the tree of a citrus hybrid. Like the rangpur, it is a cross between mandarin orange and citron. Rough lemon is a cold-hardy citrus and can grow into a large tree. The rough lemon ...
s arose from a cross between citron and mandarin, without the pomelo contribution found in true lemons, while the
Meyer lemon ''Citrus'' × ''meyeri'', the Meyer lemon (), is a hybrid citrus fruit native to China. It is a cross between a citron and a mandarin/pomelo hybrid. Mature trees are around tall with dark green shiny leaves. The flowers are white with a purp ...
comes from a citron crossed with a sweet (as opposed to sour) orange. *
Limes Limes may refer to: * the plural form of lime (disambiguation) Lime commonly refers to: * Lime (fruit), a green citrus fruit * Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide * Lime (color), a ...
: A highly diverse group of hybrids go by this name. Rangpur limes, like rough lemons, arose from crosses between citron and mandarin. The sweet limes, so-called due to their low acid pulp and juice, come from crosses of citron with either sweet or sour oranges, while the Key lime arose from a cross between a citron and a micrantha. All of these hybrids have in turn been bred back with their parent stocks or with other pure or hybrid citrus to form a broad array of fruits. Naming of these is inconsistent, with some bearing a variant of the name of one of the parents or simply another citrus with superficially-similar fruit, a distinct name, or a
portmanteau A portmanteau word, or portmanteau (, ) is a blend of wordsPonderosa lemon (''Citrus limon'' × ''medica'') and Florentine citron (''Citrus'' × ''limonimedica'') are both true lemon/citron hybrids, the
Bergamot orange ''Citrus bergamia'', the bergamot orange (pronounced ), is a fragrant citrus fruit the size of an orange, with a yellow or green color similar to a lime, depending on ripeness. Genetic research into the ancestral origins of extant citrus cult ...
is a sweet orange/lemon hybrid and the
Oroblanco An oroblanco, oro blanco (white gold), Pomelit (Israel) or sweetie (''Citrus grandis'' Osbeck × '' C. Paradisi'' Macf.) is a sweet seedless citrus hybrid fruit similar to grapefruit. It is often referred to as oroblanco grapefruit. Development ...
is a grapefruit/pomelo mix, while
tangelo The tangelo ( , ; ''C. reticulata × C. maxima'' or ''× C. paradisi''), ''Citrus × tangelo, is'' a citrus fruit hybrid of a ''Citrus reticulata'' variety, such as mandarin orange or tangerine, and a ''Citrus maxima'' variety, such as a pome ...
s are tangerine (mandarin)/pomelo or mandarin/grapefruit hybrids,
orangelo An orangelo (Spanish ''chironja'' – ''C. paradisi'' × ''C. sinensis'') is a hybrid citrus fruit believed to have originated in Puerto Rico. The fruit, a cross between a grapefruit and an orange, had spontaneously appeared in the shade-providi ...
s result from grapefruit backcrossed with sweet orange, and a sweet orange backcrossed with a tangerine gives the tangor. One lumia, a member of the sweet lemons, is the product of crossing a lemon with a pomelo/citron hybrid, though another lumia variety, the Pomme d'Adam, is a micrantha/citron cross, like the Key lime. The most common and commercially popular 'limes', the
Persian lime Persian lime (''Citrus'' × ''latifolia''), also known by other common names such as seedless lime, Bearss lime and Tahiti lime, is a citrus fruit species of hybrid origin, known only in cultivation. The Persian lime is a triploid cross between ...
s, are Key lime/lemon hybrids that combine the genetic lineages of four ancestral citrus species: mandarin, pomelo, citron and micrantha. While most other citrus are
diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectiv ...
, many of the Key lime hybrid progeny have unusual chromosome numbers. For example, the Persian lime is triploid, deriving from a diploid Key lime
gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...
and a haploid lemon
ovule In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the '' integument'', forming its outer layer, the ''nucellus'' (or remnant of the megasporangium), and the ...
. A second group of Key lime hybrids, including the Tanepao lime and Madagascar lemon, are also triploid but instead seem to have arisen from a backcross of a diploid Key lime ovule with a citron haploid gamete. The "Giant Key lime" owes its increased size to a spontaneous duplication of the entire diploid Key lime genome to produce a
tetraploid Polyploidy is a condition in which the cells of an organism have more than one pair of ( homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contains ...
. Historically, hybrids with similar characteristics have been placed together in a number of hybrid species, yet relatively recent genomic analysis has revealed some hybrids assigned to the same species to be of quite distinct ancestry. No alternative system of grouping fruit in hybrid species has been adopted.


Hybrids from other citrus species

While most citrus hybrids derive from the three core species, hybrids have also been derived from the micrantha, Ichang papeda,
kumquat Kumquats (; zh, 金桔), or cumquats in Australian English, are a group of small fruit-bearing trees in the flowering plant family Rutaceae. Their taxonomy is disputed. They were previously classified as forming the now-historical genus ''For ...
, Australian limes, and trifoliate orange. The best known hybrid from micrantha is the Key lime (or Mexican lime), derived from the breeding of a male citron and a female micrantha. Several citrus varieties are Ichang papeda/mandarin crosses (for which Swingle coined the term ichandarin), including Sudachi and
Yuzu Yuzu (''Citrus junos'', from Japanese or ) is a citrus fruit and plant in the family Rutaceae of East Asian origin. Yuzu has been cultivated mainly in East Asia, though recently also in New Zealand, Australia, Spain, Italy, and France. ...
(which also includes smaller contributions from pomelo and kumquat). Other more exotic citrus have likewise proved hybrids that include papeda. For example, the Indian wild orange, once suggested as a possible ancestor of today's cultivated citrus fruits, yielded conflicting phylogenetic placements in more limited genetic analysis, but study of nuclear markers and chloroplast DNA showed it to be of maternal citron lineage, with further genetic contributions from mandarin and papeda.


''Citrofortunella''

A large group of commercial hybrids involve the
kumquat Kumquats (; zh, 金桔), or cumquats in Australian English, are a group of small fruit-bearing trees in the flowering plant family Rutaceae. Their taxonomy is disputed. They were previously classified as forming the now-historical genus ''For ...
, ''Fortunella'' in the Swingle system. ''
Citrofortunella Citrofortunella are a large group of commercial hybrids that cross the kumquat with other citrus. In the system of citrus taxonomy established by Swingle, kumquats were placed in a different genus, '' Fortunella'', from ''Citrus'', which included ...
'' was coined as a genus containing intergeneric hybrids between members of the ''Citrus'' and the ''Fortunella'', and is named after its parent genera. Such hybrids often combine the cold hardiness of the kumquat with some edibility properties of the other ''Citrus'' species. As members of a hybrid genus, these crosses were marked with the multiplication sign before the genus name, for example × ''Citrofortunella microcarpa''. With the return of kumquats to within ''Citrus'', ''Citrofortunella'' are no longer viewed as being intergeneric hybrids and thus likewise belong in ''Citrus'', while ''Citrofortunella'' as a distinct genus name would no longer be valid. Examples of the ''Citrofortunella'' include the
calamansi Calamansi (''Citrus'' × ''microcarpa''), also known as calamondin, Philippine lime, or Philippine lemon, is an economically important citrus hybrid predominantly cultivated in the Philippines. It is native to the Philippines, Borneo, Sumatra, ...
,
limequat The limequat is a citrofortunella hybrid that is the result of a cross between the Key lime and the kumquat, hybridized by Walter Tennyson Swingle in 1909. Description It is a small tree that grows into a contained bushy form. The leaves are ...
, and
yuzuquat A ''yuzuquat'' is a trigeneric hybrid between a Yuzu lemon (''Citrus ichangensis'' x ''C. reticulata'') and 'Nagami' kumquat (''C. japonica'' 'Nagami'). It was developed by John Brown in Texas. The fruit often is used as a lemon The lemon ...
, crossing kumquat with
tangerine The tangerine is a type of citrus fruit that is orange in color. Its scientific name varies. It has been treated as a separate species under the name ''Citrus tangerina'' or ''Citrus'' × ''tangerina'', or treated as a variety of '' Citrus reti ...
, Key lime, and
yuzu Yuzu (''Citrus junos'', from Japanese or ) is a citrus fruit and plant in the family Rutaceae of East Asian origin. Yuzu has been cultivated mainly in East Asia, though recently also in New Zealand, Australia, Spain, Italy, and France. ...
respectively.


''Citroncirus''

As with kumquats, the trifoliate orange does not naturally interbreed with core taxa due to different flowering times, but hybrids have been produced artificially between ''Poncirus'' and members of the genus ''Citrus''. Genomic analysis of several such hybrids showed all to have involved ''P. trifoliata'' and not ''P. polyandra''. In the Swingle system the name coined for these intra-generic crosses, represented as a hybrid genus, is "× ''Citroncirus''". The group includes the citrange, a hybrid between the trifoliate and sweet oranges, and the citrumelo, a hybrid of trifoliate orange and 'Duncan'
grapefruit The grapefruit (''Citrus'' × ''paradisi'') is a subtropical citrus tree known for its relatively large, sour to semi-sweet, somewhat bitter fruit. The interior flesh is segmented and varies in color from pale yellow to dark pink. Grapefruit ...
. As with Citrofortunella, were ''Poncirus'' subsumed into ''Citrus'', these hybrids would no longer be intergeneric, and would likewise fall within ''Citrus'', rendering ''Citroncirus'' invalid.


Graft hybrids

Due to the sterility of many of the genetic hybrids as well as disease- or temperature-sensitivity of some ''Citrus'' trees, domesticated citrus cultivars are usually propagated via grafting to the rootstock of other, often hardier though less palatable citrus or close relatives. As a result, graft hybrids, also called
graft-chimaera In horticulture, a graft-chimaera may arise in grafting at the point of contact between rootstock and scion and will have properties intermediate between those of its "parents". A graft-chimaera is not a true hybrid but a mixture of cells, each wi ...
s, can occur in ''Citrus''. After
grafting Grafting or graftage is a horticultural technique whereby tissues of plants are joined so as to continue their growth together. The upper part of the combined plant is called the scion () while the lower part is called the rootstock. The succ ...
, the
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
from the scion and
rootstock A rootstock is part of a plant, often an underground part, from which new above-ground growth can be produced. It could also be described as a stem with a well developed root system, to which a bud from another plant is grafted. It can refer to a ...
are not somatically fused, but rather the cells of the two intermix at the graft site, and can produce shoots from the same tree that bear different fruit. For example, the 'Faris' lemon, has some branches with purple immature leaves and flowers with a purple blush that give rise to sour fruit, while other branches produce genetically distinct sweet lemons coming from white flowers, with leaves that are never purple. Graft hybrids can also give rise to an intermixed shoot that bears fruit with a combination of the characteristics of the two contributing species due to the presence of cells from both in that fruit. In an extreme example, on separate branches Bizzaria produces fruit identical to each of the two contributing species, but also fruit that appears to be half one species and half the other, unmixed. In taxonomy, graft hybrids are distinguished from genetic hybrids by designating the two contributing species with a plus sign between the individual names (''Citrus medica'' + ''C. aurantium'').


See also

* ''
Zanthoxylum fagara ''Zanthoxylum fagara'' or wild lime, is a species of flowering plant that—despite its name—is not part of the genus ''Citrus'' with real limes and other fruit, but is a close cousin in the larger citrus family, Rutaceae. It is native to sout ...
'' — the so-called "wild lime", which is of the
Rutaceae The Rutaceae is a family, commonly known as the rueRUTACEAE
in BoDD – Botanical Der ...
family but not a citrus or near relative *
Japanese citrus A number of citrus fruits are grown in or strongly associated with Japan. Many of these fruits are of Chinese origin, but have been modified or specially bred for cultivation in Japan. Japanese taxonomy Japan usually follows the botanical name ...
*
Plant taxonomy Plant taxonomy is the science that finds, identifies, describes, classifies, and names plants. It is one of the main branches of taxonomy (the science that finds, describes, classifies, and names living things). Plant taxonomy is closely alli ...


References


External links


USDA Citrus Genome Database

USDA Classification


a comprehensive article on citrus taxonomy




GRIN database for Species of Citrus
{{Taxonomy of... Citrus
Citrus ''Citrus'' is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, lemons, grapefruits, pomelos, and limes. The genus ''Citrus'' is native to ...
* Botanical nomenclature
Citrus ''Citrus'' is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, lemons, grapefruits, pomelos, and limes. The genus ''Citrus'' is native to ...