Sex-determination system

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Some chromosomal sex determination systems in animals

A sex-determination system is a biological system that determines the development of sexual characteristics in an organism.[1] Most organisms that create their offspring using sexual reproduction have two common sexes and a few less common intersex variations.

In some species, there are

fertilization
.

In some species, sex determination is genetic: males and females have different

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
.

In other cases, sex of a fetus is determined by environmental variables (such as temperature). The details of some sex-determination systems are not yet fully understood. Hopes [who?] for future fetal biological system analysis include complete-reproduction-system initialized signals that can be measured during pregnancies to more accurately determine whether a determined sex of a fetus is male, or female.[citation needed] Such analysis of biological systems could also signal whether the fetus is hermaphrodite, which includes total or partial of both male and female reproduction organs.

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 sex characteristics of both sexes, a condition called intersex.[3]

While diversity in sex determination systems is common throughout different biological systems, the systems beyond XY/XX/XO in mammals are often left to more advanced courses for those whose studies specialize in genetics of other organisms.

Discovery

Sex determination was discovered in the mealworm by the American geneticist Nettie Stevens in 1903.[4][5][6]

In 1694, J.R. Camerarius, conducted early experiments on pollination and reported the existence of male and female characteristics in plants(Maize).

In 1866,

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.

Chromosomal systems

XX/XY sex chromosomes

Drosophila sex-chromosomes
G-banding
Main article: XY sex-determination system

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

embryogenesis); in others, however, such as fruit flies, sexual differentiation occurs as soon as the egg is fertilized.[7]

Y-centered sex determination

Some species (including humans) have a gene

SRY on the Y chromosome that determines maleness. Members of SRY-reliant species can have uncommon XY chromosomal combinations such as XXY and still live.[7]
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
anti-müllerian hormone which typically ensures the development of a single, male reproductive system.[7] In typical XX embryos, cells secrete estrogen
, 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

adrenal hypoplasia congenita.[8] 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.[9] Even when there are normal sex chromosomes in XX females, duplication or expression of SOX9 causes testes to develop.[10][11] Gradual sex reversal in developed mice can also occur when the gene FOXL2 is removed from females.[12] 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.[7]

X-centered sex determination

Some species, such as fruit flies, use the presence of two X chromosomes to determine femaleness.[13] 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, 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.[14]

At least one

eutherian sex chromosomes.[15] 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.[16]

Heredity of sex chromosomes in XO sex determination

XX/X0 sex chromosomes

Main article:
X0 sex-determination system

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

Ellobius lutescens) also have a form of XO determination, in which both sexes lack a second sex chromosome.[9] The mechanism of sex determination is not yet understood.[17]

The nematode C. elegans is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite.[18] 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.[19]

ZW/ZZ sex chromosomes

Main article: ZW sex-determination system

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.[20] In birds, the genes FET1 and ASW are found on the W chromosome for females, similar to how the Y chromosome contains SRY.[7] 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.[18] Also, while mammals deactivate one of their extra X chromosomes when female, it appears that in the case of Lepidoptera, the males produce double the normal amount of enzymes, due to having two Z's.[18] Because the use of ZW sex determination is varied, it is still unknown how exactly most species determine their sex.[18] However, reportedly, the silkworm Bombyx mori uses a single female-specific piRNA as the primary determiner of sex.[21] 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.[22] The chicken's Z chromosome also seems to be related to the X chromosome of the platypus.[23] 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.[24]

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.

better source needed
]

ZZ/Z0 sex chromosomes

Main article:
Z0 sex-determination system

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.[26][27][28]

UV sex chromosomes

In some bryophyte and some algae 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 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.[29][30]

Haplodiploid sex chromosomes

Haplodiploidy

Main article: Haplodiploidy

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.[31][32][33]
Most females in the Hymenoptera order can decide the sex of their offspring by holding received sperm in their spermatheca and either releasing it into their oviduct or not. This allows them to create more workers, depending on the status of the colony.[34]

Other chromosomal systems

Other uncommon systems include those of the

platyfish, which has W, X, and Y chromosomes. This allows WY, WX, or XX females and YY or XY males.[14]

Mating type in microorganisms 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 seven mating types. Schizophyllum commune, a type of fungus, has 23,328.

Environmental systems

Main article: Environmental sex determination
All alligators determine the sex of their offspring by the temperature of the nest.

Temperature-dependent

Main article: Temperature-dependent sex determination

Many other sex-determination systems exist. In some species of reptiles, including

SRY gene, but have other genes such as DAX1, DMRT1, and SOX9 that are expressed or not expressed depending on the temperature.[36] 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.[14]

It is unknown how exactly temperature-dependent sex determination evolved.[38] 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.[38] In amniotes, environmental sex determination preceded the genetically determined systems of birds and mammals; it is thought that a temperature-dependent

common ancestor of amniotes with sex chromosomes.[39]

Other environmental systems

There are other environmental sex determination 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.[40] Some species, such as some snails, 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. 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 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.[41]

Evolution

The ends of the XY chromosomes, highlighted here in green, are all that is left of the original autosomes that can still cross over with each other.

Sex determination systems may have evolved from mating type, which is a feature of microorganisms.

Chromosomal sex determination may have evolved early in the history of eukaryotes.[42] But in plants it has been suggested to have evolved recently.[43]

The accepted hypothesis of XY and ZW sex chromosome evolution in amniotes is that they evolved at the same time, in two different branches.[44][45]

No genes are shared between the avian ZW and mammal XY chromosomes

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 therian mammals, while the X5 chromosome contains an avian sex-determination gene, further suggesting an evolutionary link.[47]

However, there is some evidence to suggest that there could have been transitions between ZW and XY, such as in

SRY gene, causing that chromosome to designate sex.[45][51][52] After this mutation, the SRY-containing chromosome inverted and was no longer completely homologous with its partner. The regions of the X and Y chromosomes that are still homologous to one another are known as the pseudoautosomal region.[53] Once it inverted, the Y chromosome became unable to remedy deleterious mutations, and thus degenerated.[45] 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.[54][55]

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.[14] Non-inverted Y chromosomes with long histories are found in pythons and emus, each system being more than 120 million years old, suggesting that inversions are not necessarily an eventuality.[56] XO sex determination can evolve from XY sex determination with about 2 million years.[clarification needed][57]

See also

References

  1. ^ Schnebly, Risa Aria (2021). "Sex Determination in Humans". The Embryo Project Encyclopedia. Retrieved 6 July 2022.
  2. ISBN 978-3-319-47829-6
    .
  3. ISBN 978-1108499859. Archived
    from the original on 11 October 2020. Retrieved 11 October 2020.
  4. ^ "Nettie Stevens: A Discoverer of Sex Chromosomes | Learn Science at Scitable". www.nature.com. Archived from the original on 7 April 2019. Retrieved 7 June 2018.
  5. PMID 11620765
    .
  6. ^ "Nettie Maria Stevens (1861–1912) | The Embryo Project Encyclopedia". embryo.asu.edu. Archived from the original on 8 April 2019. Retrieved 7 June 2018.
  7. ^ a b c d e Hake L (2008). "Genetic Mechanisms of Sex Determination". Nature Education. 1 (1). Archived from the original on 19 August 2017. Retrieved 8 December 2011.
  8. S2CID 19764423
    .
  9. ^ a b Chandra, H. S. (25 April 1999). "Another way of looking at the enigma of sex determination in Ellobius lutescens". Current Science. 76 (8): 1072.
  10. PMID 21208124
    .
  11. PMID 10588843
    .
  12. PMID 20005806
    .
  13. PMID 12966139
    .
  14. ^
    S2CID 17401686
    .
  15. PMID 18464734
    .
  16. S2CID 23603889
    .
  17. S2CID 23311263
    .
  18. ^
    ISBN 978-0-691-00981-0
    . Retrieved 4 November 2011.
  19. PMID 1498366
    .
  20. S2CID 4413389
    .
  21. S2CID 205238635
    .
  22. S2CID 12932564
    .
  23. S2CID 4379897
    .
  24. ^ "Virgin births for giant lizards". BBC News. 20 December 2006. Archived from the original on 4 November 2014. Retrieved 13 March 2008.
  25. ^ "Evolution of the Y Chromosome". Annenberg Media. Archived from the original on 4 November 2004. Retrieved 1 April 2008.
  26. S2CID 6885122
    .
  27. ^ "Genetic Mechanisms of Sex Determination - Learn Science at Scitable". www.nature.com. Archived from the original on 19 August 2017. Retrieved 8 December 2011.
  28. ISBN 9783110162103
    – via Google Books.
  29. PMID 21962970
    .
  30. doi:10.1111/jse.12266
    .
  31. PMID 12941271
    .
  32. PMID 23466984
    .
  33. PMID 29408414
    .
  34. PMID 16393347
    .
  35. PMID 17148121
    .
  36. ^
    PMID 12409092
    .
  37. S2CID 85177125
    .
  38. ^ a b Valenzuela N, Janzen FJ (2001). "Nest-site philopatry and the evolution of temperature-dependent sex determination" (PDF). Evolutionary Ecology Research. 3: 779–794. Archived (PDF) from the original on 4 May 2013. Retrieved 7 December 2011.
  39. S2CID 15485510
    .
  40. ISBN 9780878932504
    .
  41. S2CID 4311088
    .
  42. PMID 25323972
    .
  43. ISBN 978-0-520-29248-2
    .
  44. PMID 19461890
    .
  45. ^
    PMID 17116892
    .
  46. S2CID 12932564
    .
  47. PMID 18463302
    .
  48. PMID 10952906
    .
  49. S2CID 18864471
    .
  50. PMID 21212104
    .
  51. ^
    S2CID 8379688
    .
  52. ^ "The evolution of the sex chromosomes: Step by step" (Press release). University of Chicago Medical Center. 28 October 1999. Retrieved 23 October 2011.
  53. PMID 12952526
    .
  54. S2CID 23740483
    .
  55. PMID 22367542
    .
  56. PMID 24983465
    .
  57. ISBN 978-0-19-163781-0
    .

Further reading

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