Polyembryony is the phenomenon of two or more

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

s developing from a single fertilized egg. Due to the embryos resulting from the same egg, the embryos are identical to one another, but are genetically diverse from the parents. The genetic difference between the offspring and the parents, but the similarity among siblings, are significant distinctions between polyembryony and the process of

budding Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is know ...

and typical

sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...

. Polyembryony can occur in humans, resulting in

identical twins Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...

, though the process is random and at a low frequency. Polyembryony occurs regularly in many species of

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

s, and

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...

Evolution of polyembryony

The evolution of polyembryony and the potential evolutionary advantages that may entail have been studied. In parasitoid wasps, there are several hypotheses surrounding the evolutionary advantages of polyembryony, one of them being that it allows female wasps that are small in size to increase the number of potential offspring in comparison to wasps that are mono embryonic. There are limitations to monoembryony, but with this method of development, multiple embryos can be derived from each of the individual eggs that are laid. The potential advantages of polyembryony in competing invasive plant species has been studied as well.

Vertebrates

Armadillo Armadillos (meaning "little armored ones" in Spanish) are New World placental mammals in the order Cingulata. The Chlamyphoridae and Dasypodidae are the only surviving families in the order, which is part of the superorder Xenarthra, along wi ...

s are the most well studied vertebrate that undergoes polyembryony, with six species of armadillo in the genus '' Dasypus'' that are always polyembryonic. The

nine banded armadillo The nine-banded armadillo (''Dasypus novemcinctus''), also known as the nine-banded long-nosed armadillo or common long-nosed armadillo, is a mammal found in North America, North, Central America, Central, and South America, making it the most ...

, for instance, always gives birth to four identical young. There are two conditions that are expected to promote the evolution of polyembryony: the mother does not know the environmental conditions of her offspring as in the case of parasitoids, or a constraint on reproduction. It is thought that nine banded armadillos evolved to be polyembryonic because of the latter. Wasp colony

Invertebrates

A more striking example of the use of polyembryony as a competitive reproductive tool is found in the parasitoid

Hymenoptera Hymenoptera is a large order (biology), order of insects, comprising the sawfly, sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are Par ...

, family

Encyrtidae Encyrtidae is a large family of parasitic wasps, with some 3710 described species in about 455 genera. The larvae of the majority are primary parasitoids on Hemiptera, though other hosts are attacked, and details of the life history can be variab ...

. The progeny of the splitting embryo develop into at least two forms, those that will develop into adults and those that become a type of soldier, called precocious larvae. These latter larvae patrol the host and kill any other parasitoids they find with the exception of their siblings, usually sisters. Obligately polyembryonic insects fall in two classes: Hymenoptera (certain wasps), and

Strepsiptera The Strepsiptera are an order of insects with eleven extant families that include about 600 described species. They are endoparasites in other insects, such as bees, wasps, leafhoppers, silverfish, and cockroaches. Females of most species never ...

. From one egg, these insects can produce over thousands of offspring. Polyembryonic wasps from the Hymenoptera group can be further subdivided into four families including

Braconidae The Braconidae are a family of parasitoid wasps. After the closely related Ichneumonidae, braconids make up the second-largest family in the order Hymenoptera, with about 17,000 recognized species and many thousands more undescribed. One analysis ...

(''

Macrocentrus ''Macrocentrus'' is a genus of braconid wasps in the family Braconidae The Braconidae are a family of parasitoid wasps. After the closely related Ichneumonidae, braconids make up the second-largest family in the order Hymenoptera, with about 1 ...

''),

Platygastridae The hymenopteran family Platygastridae (sometimes incorrectly spelled Platygasteridae) is a large group (over 4000 species) of exclusively parasitoid wasps, mostly very small (1–2 mm), black, and shining, with geniculate (elbowed) antennae ...

(''

Platygaster ''Platygaster'' is a genus of parasitoid wasps in the family Platygastridae The hymenopteran family Platygastridae (sometimes incorrectly spelled Platygasteridae) is a large group (over 4000 species) of exclusively parasitoid wasps, mostly ...

''),

(''

Copidosoma ''Copidosoma'' is a genus of chalcid wasps, which are parasitoids of Lepidoptera. The wasps are polyembryonic, depositing one or two eggs into their host which then develop into multiple offspring and have a soldier caste and a reproductive cas ...

''), and

Dryinidae Dryinidae is a cosmopolitan family of solitary wasps. Its name comes from the Greek ''drys'' for oak: Latreille named the type genus ''Dryinus'' because the first species was collected in an oak plant in Spain. The larvae are parasitoids of the ...

. Polyembryony also occurs in

Bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a l ...

. Through genotype analysis and molecular data, it has been suggested that polyembryony happens in the entire bryozoan order

Cyclostomatida Cyclostomatida, or cyclostomata (also known as cyclostomes), are an ancient order of stenolaemate bryozoans which first appeared in the Lower Ordovician. It consists of 7+ suborders, 59+ families, 373+ genera, and 666+ species. The cyclostome b ...

Plants

The term is also used in

botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek w ...

to describe the phenomenon of

seedling A seedling is a young sporophyte developing out of a plant embryo from a seed. Seedling development starts with germination of the seed. A typical young seedling consists of three main parts: the radicle (embryonic root), the hypocotyl (embry ...

s emerging from one embryo. Around 20 genera of gymnospores undergo polyembryony, termed " cleavage polyembryony," where the original

zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicellula ...

splits into many identical embryos. In some plant taxa, the many embryos of polyembryony eventually gives rise to only a single offspring. The mechanism underlying the phenomenon of a resulting single (or in some cases a few) offspring is described in ''

Pinus sylvestris ''Pinus sylvestris'', the Scots pine (UK), Scotch pine (US) or Baltic pine, is a species of tree in the pine family Pinaceae that is native to Eurasia. It can readily be identified by its combination of fairly short, blue-green leaves and orang ...

'' to be

programmed cell death Programmed cell death (PCD; sometimes referred to as cellular suicide) is the death of a cell (biology), cell as a result of events inside of a cell, such as apoptosis or autophagy. PCD is carried out in a biological process, which usually confers ...

(PCD), which removes all but one embryo. Originally, all embryos have equal opportunity to develop into full seeds, but during the early stages of development, one embryo becomes dominant through

competition Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indivi ...

, and therefore the now dormant seed, while the other embryos are destroyed through PCD. The genus ''

Citrus ''Citrus'' is a genus of flowering plant, flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as Orange (fruit), oranges, Lemon, lemons, grapefruits, pomelos, and lim ...

'' has a number of species that undergo polyembryony, where multiple nucellar-cell-derived embryos exist alongside sexually-derived embryos.

Antonie van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch microbiologist and microscopist in the Golden Age of Dutch science and technology. A largely self-taught man in science, he is commonly known as " the ...

first described polyembryony in 1719 when the seed in ''Citrus'' was observed to have two germinating embryos. In ''Citrus'', polyembryony is genetically controlled by a shared polyembryony locus among the species, determined by

single-nucleotide polymorphism In genetics, a single-nucleotide polymorphism (SNP ; plural SNPs ) is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in a sufficiently lar ...

in the genotypes sequenced. The variation within the species of citrus is based on the amount of embryos that develop, the impact of the environment, and gene expression. As with other species, due to the many embryos developing in close proximity, competition occurs, which can cause variation in seed success or vigor.

References

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