![Bonélie (Bonellia viridis) PC301461](https://upload.wikimedia.org/wikipedia/commons/b/b3/Bon%C3%A9lie_%28Bonellia_viridis%29_PC301461.JPG)
Environmental sex determination is the
establishment of sex by a non-genetic cue, such as nutrient availability, experienced within a discrete period after fertilization. Environmental factors which often influence sex determination during development or sexual maturation include light intensity and photoperiod, temperature, nutrient availability, and pheromones emitted by surrounding plants or animals. This is in contrast to
genotypic sex determination
A sex-determination system is a biology, biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two sexes.
In some species there are ...
, which establishes sex at fertilization by genetic factors such as
sex chromosome
A sex chromosome (also referred to as an allosome, heterotypical chromosome, gonosome, heterochromosome, or idiochromosome) is a chromosome that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical ...
s. Under true environmental sex determination, once sex is determined, it is fixed and cannot be switched again. Environmental sex determination is different from some forms of
sequential hermaphroditism
Sequential hermaphroditism (called dichogamy in botany) is a type of hermaphroditism that occurs in many fish, gastropods, and plants. Sequential hermaphroditism occurs when the individual changes its sex at some point in its life. In particular, ...
in which the sex is determined flexibly after fertilization throughout the organism’s life.
Adaptive Significance
Environmental sex determination is similar to certain forms of sexual selection in that there are oftentimes different and opposing selective pressures on males and females because of the costs of reproduction.
Sexual selection
Sexual selection is a mode of natural selection in which members of one biological sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of t ...
is common throughout the tree of life (most known in birds); often resulting in
sexual dimorphism
Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...
, or size and appearance differences between sexes in the same species. In environmental sex determination,
selective pressure
Any cause that reduces or increases reproductive success in a portion of a population potentially exerts evolutionary pressure, selective pressure or selection pressure, driving natural selection. It is a quantitative description of the amount of ...
s over evolutionary time have selected for flexibility in sex determination to optimize fitness in a heterogenous environment because of the different
costs of sex in males and females. Certain environmental conditions differentially affect each sex such that it would be beneficial to become one sex and not the other. This is especially pertinent for sessile organisms that cannot move to a different environment. In plants, for example, female sexual function is often more energetically expensive because once fertilized they must use significant stored energy to produce
fruit
In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering.
Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in particu ...
s,
seed
A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiospe ...
s, or
sporophyte
A sporophyte () is the diploid multicellular stage in the life cycle of a plant or alga which produces asexual spores. This stage alternates with a multicellular haploid gametophyte phase.
Life cycle
The sporophyte develops from the zygote pr ...
s whereas males must only produce
sperm
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
(and sperm-containing structure;
antheridium
An antheridium is a haploid structure or organ producing and containing male gametes (called ''antherozoids'' or sperm). The plural form is antheridia, and a structure containing one or more antheridia is called an androecium. Androecium is also ...
in seedless plants, and
pollen
Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...
in seed plants).
Mechanisms
Lacking genetic information coding for separate sexes such as
sex chromosome
A sex chromosome (also referred to as an allosome, heterotypical chromosome, gonosome, heterochromosome, or idiochromosome) is a chromosome that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical ...
s, individuals that exhibit environmental sex determination contain genetic information coding for both sexes on
autosomes. In general, once exposed to certain environmental cues,
epigenetic changes cause developing individuals to become either male or female. Environmental cues that often trigger the development of males or females include temperature, nutrient (or food in the case of animals) and water availability, photoperiod, competitive stress, and
pheromone
A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
s from conspecific individuals. Specific mechanisms and cues vary between species.
Taxonomic range
Crustaceans
The
amphipod crustacean
Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...
''
Gammarus duebeni
''Gammarus'' is an Amphipoda, amphipod crustacean genus in the family (biology), family Gammaridae. It contains more than 200 described species, making it one of the most species-rich genera of crustaceans. Different species have different optimu ...
'' produces males early in the mating season, and females later, in response to the length of daylight, the
photoperiod. Because male fitness improves more than female fitness with increased size, environmental sex determination is adaptive in this system by permitting males to experience a longer growing season than females.
The branchiopod crustacean ''
Daphnia magna
''Daphnia magna'' is a small planktonic crustacean (adult length 1.5–5.0 mm) that belongs to the subclass Phyllopoda.
Description
''D. magna'' is a typical water flea of the genus ''Daphnia''. The females reach up to 5 mm in size, ...
''
parthenogenetically produces male progeny in response to a combination of three environmental factors, namely a reduced photoperiod in autumn, shortage of food and raised population density.
Annelids
''
Bonellia viridis
''Bonellia viridis'', the green spoonworm, is a marine worm (class Polychaeta , phylum Annelida) noted for displaying exceptional sexual dimorphism and for the biocidal properties of a pigment in its skin.Murina, G. (2008). Bonellia viridis Rol ...
'', a marine worm, has location-dependent sex determination; sex depends on where the larvae land.
Vertebrates
![Crocnest](https://upload.wikimedia.org/wikipedia/commons/7/78/Crocnest.JPG)
The sex of most
amniote
Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are disti ...
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, ...
s, such as mammals and birds, is determined genetically. However, some reptiles have
temperature-dependent sex determination Temperature-dependent sex determination (TSD) is a type of environmental sex determination in which the temperatures experienced during embryonic/larval development determine the sex of the offspring. It is only observed in reptiles and teleost fish ...
, where sex is permanently determined by thermal conditions experienced during the middle third of embryonic development.
The sex of
crocodilians and
sphenodontian
Rhynchocephalia (; ) is an order of lizard-like reptiles that includes only one living species, the tuatara (''Sphenodon punctatus'') of New Zealand. Despite its current lack of diversity, during the Mesozoic rhynchocephalians were a diverse g ...
s is exclusively determined by temperature. In contrast,
squamate
Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With over 10,900 species, it ...
s (
lizard
Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia alt ...
s and
snake
Snakes are elongated, Limbless vertebrate, limbless, carnivore, carnivorous reptiles of the suborder Serpentes . Like all other Squamata, squamates, snakes are ectothermic, amniote vertebrates covered in overlapping Scale (zoology), scales. Ma ...
s) and
turtle
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...
s exhibit both genotypic sex determination and temperature-dependent sex determination, although temperature dependence is much more common in turtles than in squamates.
Ferns
![Fern_young_gametophyte](https://upload.wikimedia.org/wikipedia/commons/a/a5/Fern_young_gametophyte.jpg)
Most fern species (with a few exceptions, namely the
Salvineales) are
homosporous
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...
and lack
sex chromosome
A sex chromosome (also referred to as an allosome, heterotypical chromosome, gonosome, heterochromosome, or idiochromosome) is a chromosome that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical ...
s. Lacking genetic information coding for separate sexes, every fern spore has the capacity to become a male, female, or hermaphroditic
gametophyte
A gametophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has on ...
depending on the environment. In many fern species, including ''
Ceratopteris richardii'', environmental sex determination is linked to
breeding systems. Fern gametophytes exhibit a wide variety of breed systems which can be divided into
outcrossing
Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing distantly related genetic material into a breeding line, thereby increasing genetic diversity.
Outcrossing can be a usefu ...
and
inbreeding
Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and o ...
. To promote outcrossing, female gametophytes release a chemical pheromone known as Antheridiogen which controls the sex of nearby developing gametophytes. Antheridiogen secreted by females promotes the development of nearby asexual gametophytes into males. This is adaptive because inducing maleness increases the probability of
outcrossing
Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing distantly related genetic material into a breeding line, thereby increasing genetic diversity.
Outcrossing can be a usefu ...
as males provide sperm for the females rather than the females becoming
hermaphroditic
In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes.
Many taxonomic groups of animals (mostly invertebrates) do not have s ...
(or bisexual) and self-fertilizing. However, if no
fertilization
Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...
occurs, the female gametophyte can still become hermaphroditic and self-fertilize if the conditions are conducive to growth, ultimately resulting in
inbreeding depression
Inbreeding depression is the reduced biological fitness which has the potential to result from inbreeding (the breeding of related individuals). Biological fitness refers to an organism's ability to survive and perpetuate its genetic material. In ...
.
Additionally, similar to crocodilians,
homosporous
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...
fern gametophyte sex is determined by the
abiotic
In biology and ecology, abiotic components or abiotic factors are non-living chemical and physical parts of the environment that affect living organisms and the functioning of ecosystems. Abiotic factors and the phenomena associated with them under ...
environment in accordance with the
size-advantage model. In stressful environments (crowding or nutrient stress), gametophytes are smaller and develop into males. While in more favorable growing conditions, gametophytes are larger and develop into females.
Moss
![Moss_007](https://upload.wikimedia.org/wikipedia/commons/3/32/Moss_007.jpg)
Moss gametophytes can be either asexual, female, male, or
hermaphroditic
In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes.
Many taxonomic groups of animals (mostly invertebrates) do not have s ...
like ferns. Unlike homosporous ferns, moss gametophytes can be either monoicous or
dioicous
Dioicy () is a sexual system where archegonia and antheridia are produced on separate gametophytes. It is one of the two main sexual systems in bryophytes. Both dioicous () and monoicous gametophytes produce gametes in gametangia by mitosis ra ...
(similar to monoecious and dioecious in vascular plants), with most studied dioicous species exhibiting genetic sex determination via the UV sex chromosome sex determination system. Some monoicous moss species such as ''Splachnum ampullaceum'' exhibit environmental sex determination during early development, with low light, low pH, and low nutrient availability all promoting male development. In the presence of
auxin, a widespread plant hormone, or
gibberellin
Gibberellins (GAs) are plant hormones that regulate various developmental processes, including stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. GAs are one of the longest-known classes of p ...
s, compounds similar to Antheridiogen in ferns, both female and male individuals invest more in sexual structures (
antheridia
An antheridium is a haploid structure or organ producing and containing male gametes (called ''antherozoids'' or sperm). The plural form is antheridia, and a structure containing one or more antheridia is called an androecium. Androecium is also ...
and
archegonia
An archegonium (pl: archegonia), from the ancient Greek ''ἀρχή'' ("beginning") and ''γόνος'' ("offspring"), is a multicellular structure or organ of the gametophyte phase of certain plants, producing and containing the ovum or female ga ...
). Environmental sex determination in moss is fundamentally different from the spatial segregation of sexes, the occurrence of environmentally mediated sex ratios in moss patches, observed in sexually ''static'' moss species. Spatial segregation of the sexes in mosses is caused by differential ''survival'' rates between sexes as a result of the competitive advantage of female moss. This leads to female dominated populations maintained by asexual reproduction and minimal sexual reproduction. In contrast, environmental sex determination is the ''dynamic development'' of females or males in different environmental conditions.
Angiosperms
![Catasetum_viridiflavum](https://upload.wikimedia.org/wikipedia/commons/2/22/Catasetum_viridiflavum.jpg)
Many angiosperms exhibit
sequential hermaphroditism
Sequential hermaphroditism (called dichogamy in botany) is a type of hermaphroditism that occurs in many fish, gastropods, and plants. Sequential hermaphroditism occurs when the individual changes its sex at some point in its life. In particular, ...
, meaning that they can switch sexes continually throughout their life based on the current conditions and resource availability to optimize
fitness each flowering season. But sequential hermaphroditism and environmental sex determination are not mutually exclusive. For example,
Catasetum viridiflavum, an
epiphyte (plant that grows on another plant) in the
Orchidaceae
Orchids are plants that belong to the family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant.
Along with the Asteraceae, they are one of the two largest families of flowering ...
family exhibits sequential hermaphroditism where the younger, smaller individuals have male
inflorescence
An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Morphologically, it is the modified part of the shoot of seed plants where flowers are formed o ...
s and the older, larger individuals have female inflorescences, but sex expression is also strongly influenced by light intensity. Individuals in high light are more often female and individuals in the low light are more often male, regardless of size.
In higher light, individuals produce more
ethylene, a common plant hormone, which promotes the formation of female flowers.
References
{{reflist, 30em
Sex-determination systems