HOME

TheInfoList



OR:

A sex-determination system is a biological system that determines the development of
sexual characteristics Sexual characteristics are physical traits of an organism (typically of a sexually dimorphic organism) which are indicative of or resultant from biological sexual factors. These include both primary sex characteristics, such as gonads, and ...
in an
organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
. Most
organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
s that create their
offspring In biology, offspring are the young creation of living organisms, produced either by sexual reproduction, sexual or asexual reproduction. Collective offspring may be known as a brood or progeny. This can refer to a set of simultaneous offspring ...
using
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 ...
have two common sexes, males and females, and in other species, there are
hermaphrodite A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...
s, organisms that can function reproductively as either female or male, or both. There are also some species in which only one sex is present, temporarily or permanently. This can be due to
parthenogenesis Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...
, the act of a female reproducing without
fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...
. In some plants or algae the gametophyte stage may reproduce itself, thus producing more individuals of the same sex as the parent. In some species, sex determination is genetic: males and females have different
allele An allele is a variant of the sequence of nucleotides at a particular location, or Locus (genetics), locus, on a DNA molecule. Alleles can differ at a single position through Single-nucleotide polymorphism, single nucleotide polymorphisms (SNP), ...
s or even different
gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
s that specify their sexual morphology. In animals this is often accompanied by chromosomal differences, generally through combinations of XY, ZW, XO, ZO chromosomes, or haplodiploidy. The sexual differentiation is generally triggered by a main gene (a "sex locus"), with a multitude of other genes following in a
domino effect A domino effect is the cumulative effect produced when one event sets off a series of similar or related events, a form of chain reaction. The term is an analogy to a falling row of dominoes. It typically refers to a linked sequence of events ...
. In other cases, the sex of a fetus is determined by environmental variables (such as
temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
). The details of some sex-determination systems are not yet fully understood. Some species such as various plants and fish do not have a fixed sex and instead go through life cycles and change sex based on genetic cues during corresponding life stages of their type. This could be due to environmental factors such as seasons and temperature. In some gonochoric species, a few individuals may have conditions that cause a mix of different sex characteristics.


Discovery

Sex determination was discovered in the mealworm by the American geneticist Nettie Stevens in 1903. In 1694, J.R. Camerarius, conducted early experiments on pollination and reported the existence of male and female characteristics in plants(Maize). In 1866, Gregor Mendel published on inheritance of genetic traits. This is known as
Mendelian inheritance Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularize ...
and it eventually established the modern understanding of inheritance from two gametes. In 1902, C.E. McClung identified sex chromosomes in bugs. In 1917, C.E. Allen, discovered sex determination mechanisms in plants. In 1922, C.B. Bridges, put forth the Genic Balance Theory of sex determination. In 1928, Fritz Baltzer first described environmental sex determination.


Chromosomal systems

Among animals, the most common chromosomal sex determination systems are XY, XO, ZW, ZO, but with numerous exceptions. According to the Tree of Sex database (as of 2023), the known sex determination systems are: 1. complex sex chromosomes, homomorphic sex chromosomes, or others


XX/XY sex chromosomes

The XX/XY sex-determination system is the most familiar, as it is found in humans. The XX/XY system is found in most other
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s, as well as some insects. In this system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). The X and Y sex chromosomes are different in shape and size from each other, unlike the rest of the chromosomes ( autosomes), and are sometimes called allosomes. In some species, such as humans, organisms remain sex indifferent for a time during development (
embryogenesis An embryo ( ) is the initial stage of development for 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 ...
); in others, however, such as fruit flies, sexual differentiation occurs as soon as the egg is fertilized.


Y-centered sex determination

Some species (including humans) have a gene SRY on the Y chromosome that determines
male Male (Planet symbols, symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or Egg cell, ovum, in the process of fertilisation. A male organism cannot sexual repro ...
ness. Members of SRY-reliant species can have uncommon XY chromosomal combinations such as XXY and still live. Human sex is determined by the presence or absence of a Y chromosome with a functional SRY gene. Once the SRY gene is activated, cells create
testosterone Testosterone is the primary male sex hormone and androgen in Male, males. In humans, testosterone plays a key role in the development of Male reproductive system, male reproductive tissues such as testicles and prostate, as well as promoting se ...
and anti-müllerian hormone which typically ensures the development of a single, male reproductive system. In typical XX embryos, cells secrete
estrogen Estrogen (also spelled oestrogen in British English; see spelling differences) is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three ...
, which drives the body toward the female pathway. In Y-centered sex determination, the SRY gene is the main gene in determining male characteristics, but multiple genes are required to develop testes. In XY mice, lack of the gene DAX1 on the X chromosome results in sterility, but in humans it causes adrenal hypoplasia congenita. However, when an extra DAX1 gene is placed on the X chromosome, the result is a female, despite the existence of SRY, since it overrides the effects of SRY. Even when there are normal sex chromosomes in XX females, duplication or expression of SOX9 causes testes to develop. Gradual sex reversal in developed mice can also occur when the gene FOXL2 is removed from females. Even though the gene DMRT1 is used by birds as their sex locus, species who have XY chromosomes also rely upon DMRT1, contained on chromosome 9, for sexual differentiation at some point in their formation.


X-centered sex determination

Some species, such as fruit flies, use the presence of two X chromosomes to determine
female An organism's sex is female ( symbol: ♀) if it produces the ovum (egg cell), the type of gamete (sex cell) that fuses with the male gamete (sperm cell) during sexual reproduction. A female has larger gametes than a male. Females and ...
ness. Species that use the number of Xs to determine sex are nonviable with an extra X chromosome.


Other variants of XX/XY sex determination

Some fish have variants of the
XY sex-determination system The XY sex-determination system is a sex-determination system present in many mammals (including humans), some insects (''Drosophila''), some snakes, some fish (guppy, guppies), and some plants (''Ginkgo'' tree). In this system, the sex of an ...
, as well as the regular system. For example, while having an XY format, '' Xiphophorus nezahualcoyotl'' and ''X. milleri'' also have a second Y chromosome, known as Y', that creates XY' females and YY' males. At least one monotreme, the platypus, presents a particular sex determination scheme that in some ways resembles that of the ZW sex chromosomes of birds and lacks the SRY gene. The platypus has sex chromosomes X_1, X_2, X_3, X_4, X_5, Y_1, Y_2, Y_3, Y_4, Y_5. The males have X_1Y_1/X_2Y_2/X_3Y_3/X_4Y_4/X_5Y_5, while females have X_1X_1/X_2X_2/X_3X_3/X_4X_4/X_5X_5. During meiosis, 5 of X form one chain, and 5 of Y form another chain. Thus, they behave effectively as a typical XY chromosomal system, except each of X and Y is broken into 5 parts, with the effect that recombinations occur very frequently at 4 particular points. One of the X chromosomes is homologous to the human X chromosome, and another is homologous to the bird Z chromosome. Although it is an XY system, the platypus' sex chromosomes share no homologues with eutherian sex chromosomes. Instead, homologues with eutherian sex chromosomes lie on the platypus chromosome 6, which means that the eutherian sex chromosomes were autosomes at the time that the monotremes diverged from the therian mammals (marsupials and eutherian mammals). However, homologues to the avian DMRT1 gene on platypus sex chromosomes X3 and X5 suggest that it is possible the sex-determining gene for the platypus is the same one that is involved in bird sex-determination. More research must be conducted in order to determine the exact sex determining gene of the platypus.


XX/X0 sex chromosomes

In this variant of the XY system, females have two copies of the sex chromosome (XX) but males have only one (X0). The ''0'' denotes the absence of a second sex chromosome. Generally in this method, the sex is determined by amount of genes expressed across the two chromosomes. This system is observed in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches (order
Blattodea Blattodea is an order (biology), order of insects that contains cockroaches and termites. Formerly, termites were considered a separate order, Isoptera, but genetics, genetic and molecular evidence suggests they evolved from within the cockroach ...
). A small number of mammals also lack a Y chromosome. These include the Amami spiny rat ('' Tokudaia osimensis'') and the Tokunoshima spiny rat ('' Tokudaia tokunoshimensis'') and ''Sorex araneus'', a
shrew Shrews ( family Soricidae) are small mole-like mammals classified in the order Eulipotyphla. True shrews are not to be confused with treeshrews, otter shrews, elephant shrews, West Indies shrews, or marsupial shrews, which belong to dif ...
species. Transcaucasian mole voles ('' Ellobius lutescens'') also have a form of XO determination, in which both sexes lack a second sex chromosome. The mechanism of sex determination is not yet understood. The
nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...
'' C. elegans'' is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite. Its main sex gene is XOL, which encodes XOL-1 and also controls the expression of the genes TRA-2 and HER-1. These genes reduce male gene activation and increase it, respectively.


ZW/ZZ sex chromosomes

The ZW sex-determination system is found in birds, some reptiles, and some insects and other organisms. The ZW sex-determination system is reversed compared to the XY system: females have two different kinds of chromosomes (ZW), and males have two of the same kind of chromosomes (ZZ). In the chicken, this was found to be dependent on the expression of DMRT1. In birds, the genes FET1 and ASW are found on the W chromosome for females, similar to how the Y chromosome contains SRY. However, not all species depend upon the W for their sex. For example, there are moths and butterflies that are ZW, but some have been found female with ZO, as well as female with ZZW. Also, while mammals deactivate one of their extra X chromosomes when female, it appears that in the case of
Lepidoptera Lepidoptera ( ) or lepidopterans is an order (biology), order of winged insects which includes butterflies and moths. About 180,000 species of the Lepidoptera have been described, representing 10% of the total described species of living organ ...
, the males produce double the normal amount of enzymes, due to having two Z's. Because the use of ZW sex determination is varied, it is still unknown how exactly most species determine their sex. However, reportedly, the silkworm ''Bombyx mori'' uses a single female-specific piRNA as the primary determiner of sex. Despite the similarities between the ZW and XY systems, these sex chromosomes evolved separately. In the case of the chicken, their Z chromosome is more similar to humans' autosome 9. The chicken's Z chromosome also seems to be related to the X chromosome of the platypus. When a ZW species, such as the Komodo dragon, reproduces parthenogenetically, usually only males are produced. This is due to the fact that the haploid eggs double their chromosomes, resulting in ZZ or WW. The ZZ become males, but the WW are not viable and are not brought to term. In both XY and ZW sex determination systems, the sex chromosome carrying the critical factors is often significantly smaller, carrying little more than the genes necessary for triggering the development of a given sex.


ZZ/Z0 sex chromosomes

The ZZ/Z0 sex-determination system is found in some moths. In these insects there is one sex chromosome, Z. Males have two Z chromosomes, whereas females have one Z. Males are ZZ, while females are Z0.


UV sex chromosomes

In some
bryophyte Bryophytes () are a group of embryophyte, land plants (embryophytes), sometimes treated as a taxonomic Division (taxonomy), division referred to as Bryophyta ''Sensu#Common qualifiers, sensu lato'', that contains three groups of non-vascular pla ...
and some
algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
species, the gametophyte stage of the life cycle, rather than being hermaphrodite, occurs as separate male or female individuals that produce male and female gametes respectively. When meiosis occurs in the
sporophyte A sporophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the biological life cycle, life cycles of plants and algae. It is a diploid multicellular organism which produces asexual Spo ...
generation of the life cycle, the sex chromosomes known as U and V assort in spores that carry either the U chromosome and give rise to female gametophytes, or the V chromosome and give rise to male gametophytes.


Mating types

The mating type in
microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...
s is analogous to sex in multi-cellular organisms, and is sometimes described using those terms, though they are not necessarily correlated with physical body structures. Some species have more than two mating types. '' Tetrahymena,'' a type of ciliate, has 7 mating types Mating types are extensively studied in fungi. Among fungi, mating type is determined by chromosomal regions called mating-type loci. Furthermore, it is not as simple as "two different mating types can mate", but rather, a matter of combinatorics. As a simple example, most '' basidiomycete'' have a "tetrapolar heterothallism" mating system: there are two loci, and mating between two individuals is possible if the alleles on ''both'' loci are different. For example, if there are 3 alleles per locus, then there would be 9 mating types, each of which can mate with 4 other mating types. By multiplicative combination, it generates a vast number of mating types. For example, '' Schizophyllum commune,'' a type of fungus, has 9 \times 32 \times 9 \times 9 = 23328 mating types.


Haplodiploidy

Haplodiploidy is found in insects belonging to Hymenoptera, such as
ant Ants are Eusociality, eusocial insects of the Family (biology), family Formicidae and, along with the related wasps and bees, belong to the Taxonomy (biology), order Hymenoptera. Ants evolved from Vespoidea, vespoid wasp ancestors in the Cre ...
s and bees. Sex determination is controlled by the
zygosity Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mos ...
of a complementary sex determiner (''csd'') locus. Unfertilized eggs develop into haploid individuals which have a single, hemizygous copy of the ''csd'' locus and are therefore males. Fertilized eggs develop into diploid individuals which, due to high variability in the ''csd'' locus, are generally heterozygous females. In rare instances diploid individuals may be homozygous, these develop into sterile males. The gene acting as a ''csd'' locus has been identified in the honeybee and several candidate genes have been proposed as a ''csd'' locus for other Hymenopterans. Most females in the Hymenoptera order can decide the sex of their offspring by holding received sperm in their
spermatheca The spermatheca (pronounced : spermathecae ), also called ''receptaculum seminis'' (: ''receptacula seminis''), is an organ of the female reproductive tract in insects, e.g. ants, bees, some molluscs, Oligochaeta worms and certain other in ...
and either releasing it into their oviduct or not. This allows them to create more workers, depending on the status of the colony.


Polygenic sex determination

Polygenic sex determination is when the sex is primarily determined by genes that occur on multiple non- homologous chromosomes. The environment may have a limited, minor influence on sex determination. Examples include African cichlid fish ('' Metriaclima spp.''), lemmings ('' Myopus schisticolor''), green swordtail, medaka, etc. In such systems, there is typically a dominance hierarchy, where one system is dominant over another if in conflict. For example, in some species of cichlid fish from Lake Malawi, if an individual has both the XY locus (on one chromosome pair) and the WZ locus (on another chromosome pair), then the W is dominant and the individual has a female phenotype. The sex-determination system of zebrafish is polygenic. Juvenile zebrafishes (0–30 days after hatching) have both ovary-like tissue to testis tissue. They then develop into male or female adults, with the determination based on a complex interaction genes on multiple chromosomes, but not affected by environmental variations.


Other chromosomal systems

In systems with two sex chromosomes, they can be heteromorphic or homomorphic. Homomorphic sex chromosomes are almost identical in size and gene content. The two familiar kinds of sex chromosome pairs (XY and ZW) are heteromorphic. Homomorphic sex chromosomes exist among pufferfish, ratite birds, pythons, and European tree frogs. Some are quite old, meaning that there is some evolutionary force that resists their differentiation. For example, three species of European tree frogs have homologous, homomorphic sex chromosomes, and this homomorphism was maintained for at least 5.4 million years by occasional recombination. The ''
Nematocera The Nematocera (the name meaning "thread-horns") are a suborder of elongated fly, flies with thin, segmented antenna (biology), antennae and mostly aquatic larvae. This group is paraphyletic and contains all flies except for species from suborder ...
'', particularly the '' Simuliids'' and ''Chironomus'', have sex determination regions that are labile, meaning that one species may have the sex determination region in one chromosome, but a closely related species might have the same region moved to a different non- homologous chromosome. Some species even have the sex determination region different among individuals ''within'' ''the same species'' ( intraspecific variation). In some species, some populations have homomorphic sex chromosomes while other populations have heteromorphic sex chromosomes. The New Zealand frog, '' Leiopelma hochstetteri'', uses a supernumerary sex chromosome. With zero of that chromosome, the frog develops into a male. With one or more, the frog develops into a female. One female had as many as 16 of that chromosome. Different populations of the Japanese frog ''Rana rugosa'' uses different systems. Two use homomorphic male heterogamety, one uses XX/XY, one uses ZZ/ZW. Remarkably, the X and Z chromosomes are homologous, and the Y and W as well. ''Dmrt1'' is on autosome 1 and not sex-linked. This means that an XX female individual is genetically similar to a ZZ male individual, and an XY male individual is to a ZW female individual. The mechanism behind this is yet unclear, but it is hypothesized that during its recent evolution, the XY-to-ZW transition occurred twice. '' Clarias gariepinus'' uses both XX/XY and ZW/ZZ system within the species, with some populations using homomorphic XX/XY while others using heteromorphic ZW/ZZ. A population in Thailand appears to use both systems simultaneously, possibly because ''C. gariepinus'' were not native to Thailand, and were introduced from different source populations which resulted in a mixture. Multiple sex chromosomes like those of platypus also occurs in bony fish. Some moths and butterflies have W_1W_2Z \text /ZZ \text or WZ_1Z_2 \text /Z_1Z_1Z_2Z_2 \text. The Southern platyfish has a complex sex determination system involving 3 sex chromosomes and 4 autosomal alleles. '' Gastrotheca pseustes'' has XY_b \text / XY_a \text C-banding heteromorphism, meaning that both males and females have XY chromosomes, but their Y chromosomes are different on one or more C-bands. '' Eleutherodactylus maussi'' has a \mathrm_1 \mathrm_1 \mathrm_2 \mathrm_2 \text / \mathrm_1 \mathrm_2 \mathrm \text system.


Evolution

See for a review.


Origin of sex chromosomes

Sexual chromosome pairs can arise from an autosomal pair that, for various reasons, stopped recombination, allowing for their divergence. The rate at which recombination is suppressed, and therefore the rate of sex chromosome divergence, is very different across
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
s. In analogy with geological strata, historical events in the evolution of sex chromosomes are called evolutionary strata. The human Y-chromosome has had about 5 strata since the origin of the X and Y chromosomes about 300 Mya from a pair of autosomes. Each stratum was formed when a pseudoautosomal region (PAR) of the Y chromosome is inverted, stopping it from recombination with the X chromosome. Over time, each inverted region decays, possibly due to Muller's ratchet. Primate Y-chromosome evolution was rapid, with multiple inversions and shifts of the boundary of PAR. Among many species of the
salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...
s, the two chromosomes are only distinguished by a pericentric inversion, so that the banding pattern of the X chromosome is the same as that of Y, but with a region near the centromere reversed. (fig 7 ) In some species, the X is pericentrically inverted and the Y is ancestral. In other species it is the opposite. (p. 15 ) The gene content of the X chromosome is almost identical among placental mammals. This is hypothesized to be because the X inactivation means any change would cause serious disruption, thus subjecting it to strong purifying selection. Similarly, birds have highly conserved Z chromosomes.


Neo-sex chromosomes

Neo-sex chromosomes are currently existing sex chromosomes that formed when an autosome pair fused to the previously existing sex chromosome pair. Following this fusion, the autosomal portion undergoes recombination suppression, allowing them to differentiate. Such systems have been observed in insects, reptiles, birds, and mammals. They are useful to the study of the evolution of Y chromosome degeneration and dosage compensation.


Sex-chromosome turnover

The sex-chromosome turnover is an evolutionary phenomenon where sex chromosomes disappear or become autosomal, and autosomal chromosomes become sexual, repeatedly over evolutionary time. Some lineages have extensive turnover, but others don't. Generally, in an XY system, if the Y chromosome is degenerate, mostly different from the X chromosome, and has X dosage compensation, then turnover is unlikely. In particular, this applies to humans. The ZW and XY systems can evolve into to each other due to sexual conflict.


Homomorphism and the fountain of youth

It is an evolutionary puzzle why certain sex chromosomes remain homomorphic over millions of years, especially among lineages of fishes, amphibians, and nonavian reptiles. The fountain-of-youth model states that heteromorphy results from recombination suppression, and recombination suppression results from the male phenotype, not the sex chromosomes themselves. Therefore, if some XY sex-reversed females are fertile and adaptive under some circumstances, then the X and Y chromosomes would recombine in these individuals, preventing Y chromosome decay and maintaining long-term homomorphism. Sex reversal denotes a situation where the phenotypic sex is different from the genotypic sex. While in humans, sex reversal (such as the XX male syndrome) are often infertile, sex-reversed individuals of some species are fertile under some conditions. For example, some XY-individuals in population of Chinook salmon in the
Columbia River The Columbia River (Upper Chinook language, Upper Chinook: ' or '; Sahaptin language, Sahaptin: ''Nch’i-Wàna'' or ''Nchi wana''; Sinixt dialect'' '') is the largest river in the Pacific Northwest region of North America. The river headwater ...
became fertile females, producing YY sons. Since Chinook salmons have homomorphic sex chromosomes, such YY sons are healthy. When YY males mate with XX females, all their progeny would be XY male if grown under normal conditions. Support for the hypothesis is found in the common frog, for which XX males and XY males both suppresses sex chromosome recombination, but XX and XY females both recombine at the same rate.


Environmental systems


Temperature-dependent

Many other sex-determination systems exist. In some species of reptiles, including alligators, some
turtle Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...
s, and the
tuatara The tuatara (''Sphenodon punctatus'') is a species of reptile endemic to New Zealand. Despite its close resemblance to lizards, it is actually the only extant member of a distinct lineage, the previously highly diverse order Rhynchocephal ...
, sex is determined by the temperature at which the egg is incubated during a temperature-sensitive period. There are no examples of temperature-dependent sex determination (TSD) in birds. Megapodes had formerly been thought to exhibit this phenomenon, but were found to actually have different temperature-dependent embryo mortality rates for each sex. For some species with TSD, sex determination is achieved by exposure to hotter temperatures resulting in the offspring being one sex and cooler temperatures resulting in the other. This type of TSD is called ''Pattern I''. For others species using TSD, it is exposure to temperatures on both extremes that results in offspring of one sex, and exposure to moderate temperatures that results in offspring of the opposite sex, called ''Pattern II'' TSD. The specific temperatures required to produce each sex are known as the female-promoting temperature and the male-promoting temperature. When the temperature stays near the threshold during the temperature sensitive period, the
sex ratio A sex ratio is the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. However, many species deviate from an even sex ratio, ei ...
is varied between the two sexes. Some species' temperature standards are based on when a particular enzyme is created. These species that rely upon temperature for their sex determination do not have the SRY gene, but have other genes such as DAX1, DMRT1, and SOX9 that are expressed or not expressed depending on the temperature. The sex of some species, such as the Nile tilapia, Australian skink lizard, and Australian dragon lizard, has an initial bias, set by chromosomes, but can later be changed by the temperature of incubation. It is unknown how exactly temperature-dependent sex determination evolved. It could have evolved through certain sexes being more suited to certain areas that fit the temperature requirements. For example, a warmer area could be more suitable for nesting, so more females are produced to increase the amount that nest next season. In amniotes, environmental sex determination preceded the genetically determined systems of birds and mammals; it is thought that a temperature-dependent
amniote Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial animal, terrestrial and semiaquatic vertebrates. Amniotes evolution, evolved from amphibious Stem tet ...
was the common ancestor of amniotes with sex chromosomes.


Other environmental systems

There are other
environmental sex determination Environmental sex determination is the Sex-determination system, 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 de ...
systems including location-dependent determination systems as seen in the marine worm '' Bonellia viridis'' – larvae become males if they make physical contact with a female, and females if they end up on the bare sea floor. This is triggered by the presence of a chemical produced by the females, bonellin. Some species, such as some
snail A snail is a shelled gastropod. The name is most often applied to land snails, terrestrial molluscs, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gas ...
s, practice sex change: adults start out male, then become female. In tropical clownfish, the dominant individual in a group becomes female while the other ones are male, and bluehead wrasses ('' Thalassoma bifasciatum'') are the reverse. Clownfish live in colonies of several small undifferentiated fish and two large fish (male and female). The male and female are the only sexually mature fish to reproduce. Clownfish are protandrous hermaphrodites, which means after they mature into males, they eventually can transform into females. They develop undifferentiated until they are needed to fill a certain role in their environment, i.e., if they receive the social and environmental cues to do so. Some species, however, have no sex-determination system. Hermaphrodite species include the common earthworm and certain species of snails. A few species of fish, reptiles, and insects reproduce by
parthenogenesis Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...
and are female altogether. There are some reptiles, such as the boa constrictor and Komodo dragon that can reproduce both sexually and asexually, depending on whether a mate is available.


Others

There are exceptional sex-determination systems, neither genetic nor environmental.


Cytoplasmic sex determination

The '' Wolbachia'' genus of parasitic bacteria lives inside the cytoplasm of its host, and is vertically transmitted from parents to children. They primarily infect arthropods and nematodes. Different ''Wolbachia'' can alter the reproductive abilities of its host by a variety of means, including cytoplasmic incompatibility, parthenogenesis, feminization and embryonic male killing. '' Mitochondrial male sterility'': In many flowering plants, the mitochondria can cause hermaphrodite individuals to be unable to father offspring, effectively turning them into exclusive females. This is a form of mother’s curse. It is an evolutionarily adaptive strategy for mitochondria as mitochondria are inherited exclusively from mother to offspring. The first published case of mitochondrial male sterility among metazoans was reported in 2022 in the hermaphroditic snail '' Physa acuta''.


Paternal genome elimination

In some species of insects, springtails and mites, male offspring lose their paternal genome (in whole or in part) during development or in the germline. Males can either be diploid, diploid with missing sex chromosome, functionally haploid or truly haploid, depending on the mechanism of elimination.


Monogeny

In some species of Hymenoptera (ants, bees and wasps), flies and crustaceans, all offspring of a particular individual female are either exclusively male or exclusively female. The underlying mechanisms are diverse and include maternally controlled paternal genome elimination and Mendelian inherited maternal sex-determining factors.


Evolution

Sex determination systems may have evolved from mating type, which is a feature of
microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...
s. Chromosomal sex determination may have evolved early in the history of eukaryotes. But in plants it has been suggested to have evolved recently. The accepted hypothesis of XY and ZW sex chromosome evolution in amniotes is that they evolved at the same time, in two different branches. No genes are shared between the avian ZW and mammal XY chromosomes and the chicken Z chromosome is similar to the human autosomal chromosome 9, rather than X or Y. This suggests not that the ZW and XY sex-determination systems share an origin but that the sex chromosomes are derived from autosomal chromosomes of the common ancestor of birds and mammals. In the platypus, a monotreme, the X1 chromosome shares homology with
theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...
n mammals, while the X5 chromosome contains an avian sex-determination gene, further suggesting an evolutionary link. However, there is some evidence to suggest that there could have been transitions between ZW and XY, such as in '' Xiphophorus maculatus'', which have both ZW and XY systems in the same population, despite the fact that ZW and XY have different gene locations. A recent theoretical model raises the possibility of both transitions between the XY/XX and ZZ/ZW system and environmental sex determination The platypus' genes also back up the possible evolutionary link between XY and ZW, because they have the DMRT1 gene possessed by birds on their X chromosomes. Regardless, XY and ZW follow a similar route. All sex chromosomes started out as an original autosome of an original amniote that relied upon temperature to determine the sex of offspring. After the mammals separated, the reptile branch further split into Lepidosauria and Archosauromorpha. These two groups both evolved the ZW system separately, as evidenced by the existence of different sex chromosomal locations. In mammals, one of the autosome pair, now Y, mutated its SOX3 gene into the SRY gene, causing that chromosome to designate sex. After this mutation, the SRY-containing chromosome inverted and was no longer completely homologous with its partner. The regions of the X and
Y chromosome The Y chromosome is one of two sex chromosomes in therian mammals and other organisms. Along with the X chromosome, it is part of the XY sex-determination system, in which the Y is the sex-determining chromosome because the presence of the ...
s that are still homologous to one another are known as the pseudoautosomal region. Once it inverted, the Y chromosome became unable to remedy deleterious mutations, and thus degenerated. There is some concern that the Y chromosome will shrink further and stop functioning in ten million years: but the Y chromosome has been strictly conserved after its initial rapid gene loss. There are some vertebrate species, such as the medaka fish, that evolved sex chromosomes separately; their Y chromosome never inverted and can still swap genes with the X. These species' sex chromosomes are relatively primitive and unspecialized. Because the Y does not have male-specific genes and can interact with the X, XY and YY females can be formed as well as XX males. Non-inverted Y chromosomes with long histories are found in pythons and
emu The emu (; ''Dromaius novaehollandiae'') is a species of flightless bird endemism, endemic to Australia, where it is the Tallest extant birds, tallest native bird. It is the only extant taxon, extant member of the genus ''Dromaius'' and the ...
s, each system being more than 120 million years old, suggesting that inversions are not necessarily an eventuality. XO sex determination can evolve from XY sex determination with about 2 million years.


See also

* Clarence Erwin McClung, who discovered the role of chromosomes in sex determination * Testis-determining factor * Maternal influence on sex determination * Sequential hermaphroditism * Sex determination and differentiation (human) * Cell autonomous sex identity


References


Further reading

* * {{DEFAULTSORT:Sex-Determination System Epigenetics