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Sex-determination system

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This article is about the biological cause of sexual differentiation. For other uses, see Sex determination (disambiguation).

A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most sexual organisms have two sexes. Occasionally there are hermaphrodites in place of one or both sexes. There are also some species that are only one sex due to parthenogenesis. In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology. In animals, this is often accompanied by chromosomal differences. Determination genetically is generally through chromosome combinations of XY, ZW, XO, or haplodiploid. In other cases, sex is determined by environmental variables (such as temperature) or social variables (the size of an organism relative to other members of its population). Environmental sex determination occurred before genetic; it is thought that a temperature-dependent reptile was the common ancestor to sex chromosomes. Some species do not have a set sex, and instead change it based on certain cues. Sexual differentiation is generally started by a main gene, a sex locus, then a multitude of other genes follow in a domino effect. The details of some sex-determination systems are not yet fully understood.

Origin of sex chromosomes

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

The accepted hypothesis of XY and ZW sex chromosome evolution is that they evolved at the same time, in two different branches.[1][2] 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.[3][4] 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.[5] Regardless, XY and ZW follow a similar route. All sex chromosomes started out as an original autosome of an original reptile that relied upon temperature to determine the sex of offspring. After the mammals separated, the 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.[2] In mammals, one of the autosome pair, now Y, mutated its SOX3 gene into the SRY gene, causing that chromosome to designate sex.[2][5][6] 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.[7] Once it inverted, Y chromosome continued to receive both positive and negative mutations throughout the years, and without a partner to combine with, it slowly decayed.[2] However, because both the chromosomes were necessary for reproduction, the Y chromosome remained, despite its inability to pair with the X. This is becoming a problem, due to the fact that the Y chromosome is steadily shrinking and losing its genes. Without the ability to swap genes with its pair, it is racking up with mutations and may stop functioning in 10 million years.[8]

There are some species, such as the medaka fish that evolved sex chromosomes separately, whose Y chromosome never inverted and can still swap genes with the X. These species are still in an early phase of evolution of their sex chromosomes. 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.[9]


Chromosomal determination

XX/XY sex chromosomes

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

The XX/XY sex-determination system is the most familiar, as it is found in humans. In the system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). The XY sex chromosomes are different in shape and size from each other, unlike the autosomes, and are termed allosomes. Some species (including humans) have a gene SRY on the Y chromosome that determines maleness; others (such as the fruit fly) use the presence of two X chromosomes to determine femaleness.[10] Because the fruit fly, as well as other species, use the number of Xs to determine sex, they are nonviable with an extra X.[11] Human sex is determined by containing SRY or not. Once SRY is activated, cells create testosterone and AMH to turn the genderless sex organs into male.[11] With females, their ovaries create estrogen, driving the body down the female pathway. Not all organisms remain gender indifferent for a time after they're created; for example, fruit flies differentiate into specific sexes as soon as the egg is fertilized.[11] In Y-centered sex determination, the SRY gene, is not the only gene to have an influence on sex. Despite the fact that SRY seems to be the main gene in determining male characteristics, it requires the action of multiple genes 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.[12] However, when an extra DAX1 gene is placed on the X, the result is a female, despite the existence of SRY.[13] Also, even when there are normal sex chromosomes in XX females, duplication or expression of SOX9 causes testes to develop.[14][15] Gradual sex reversal in developed mice can also occur when the gene FOXL2 is removed from females.[16] 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.[11]

The XX/XY system is also found in most other mammals, as well as some insects. Some fish also have variants of this, as well as the regular system. For example, while it has an XY format, the Xiphophorus variatus also has a second Y chromosome, known as Y', that creates XY' females and YY' males.[9] 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 also lacks the SRY gene, whereas some rodents, such as several Arvicolinae (voles and lemmings), are also noted for their unusual sex determination systems. The platypus has ten sex chromosomes; males have an XYXYXYXYXY pattern while females have ten X chromosomes. Although it is an XY system, the platypus' sex chromosomes share no homologues with eutherian sex chromosomes.[17] 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.[18]


Heredity of sex chromosomes in XO sex determination

XX/X0 sex determination

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 Orthoptera and in cockroaches (order Blattodea). 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). Voles also have a form of XO determination in which both genders lack a second sex chromosome.[13] The mechanism of sex determination is not yet understood.[19]

The nematode C. elegans is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite.[20] Its main sex gene is xol, which controls the expression of the genes tra-2 and her-1. These genes reduce male gene activation and increase it, respectively.[21]

ZW sex chromosomes

Main article: ZW sex-determination system

The ZW sex-determination system is found in birds, reptiles, 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.[22]In birds, the genes FET1 and ASW are found on the W chromosome for females, similar to how the Y chromosome contains SRY.[11] However, not all species depend upon the W for their gender. For example, there are moths and butterflies that are ZW, but some have been found female with ZO, as well as female with ZZW.[20] Also, while mammals inactivate 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.[20] Because the use of ZW sex determination is varied, it is still unknown how exactly most species determine their sex.[20] Despite the similarities between ZW and XY, the sex chromosomes do not line up correctly and evolved separately. In the case of the chicken, their Z chromosome is more similar to humans' autosome 9.[23] The chicken's Z chromosome also seems to be related to the X chromosomes of the platypus.[24]

Haplodiploid sex chromosomes

Haplodiploidy

Main articles: Ploidy and Haplodiploid sex-determination system

Haplodiploidy is found in insects belonging to Hymenoptera, such as ants and bees. Unfertilized eggs develop into haploid individuals, which are the males. Diploid individuals are generally female but may be sterile males. Males cannot have sons or fathers. If a queen bee mates with one drone, her daughters share ¾ of their genes with each other, not ½ as in the XY and ZW systems. This is believed to be significant for the development of eusociality, as it increases the significance of kin selection, but it is debated.[25] 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.[26]


Non-genetic sex-determination systems

All alligators determine sex of their offspring by the temperature of the nest.

Temperature dependent sex determination

See also: Temperature-dependent sex determination

Many other sex-determination systems exist. In some species of reptiles, including alligators, some turtles, and the tuatara, sex is determined by the temperature at which the egg is incubated during a temperature sensitive period. In some species, this is achieved by hotter temperatures being one and cooler temperatures being the other, while in others the extreme temperatures are one sex and the middle temperature is the other. These temperature thresholds are known as Pattern I and Pattern II, respectively. The temperatures required for the specific sexes are known as the female promoting temperature and the male promoting temperature.[27] When the temperature stays right near the threshold during the temperature sensitive period, the sex ratio is varied between the two sexes.[28] Some species set their temperature standards based on when an 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.[27] Some species such as the Nile Tilapia, Austrailian skink lizard, and Australian dragon lizard have sex determined by chromosomes, but this can later be switched by the temperature of incubation.[9] These species seem to be in a transitional state of evolution.

It is unknown how exactly temperature sex determination evolved.[29] 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. However, at this stage it's mostly hypotheses.[29]

Other sex determination systems

While temperature dependent sex determination is relatively common, there are many other environmental processes. Some species, such as some snails, practice sex change: adults start out male, then become female. In tropical clown fish, the dominant individual in a group becomes female while the other ones are male, and bluehead wrasses (Thalassoma bifasciatum) are the reverse. 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. Some species, however, have no sex-determination system. Hermaphrodites 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 sexually and asexually, depending if a mate is available.[30] In the case of the boa constrictor, it can create both male and female offspring parthenogenetically.

In some arthropods, sex is determined by infection, as when bacteria of the genus Wolbachia alter their sexuality; some species consist entirely of ZZ individuals, with sex determined by the presence of Wolbachia.[31]

Other unusual systems (this section is still being researched):

  • Swordtail fish[9]
  • The Chironomus midge species
  • The Platypus has 10 sex chromosomes[17] but lacks the mammalian sex-determining gene SRY, meaning that the process of sex determination in the Platypus remains unknown.[18]
  • Zebrafish go through juvenile hermaphroditism, but what triggers this is unknown.[9]
  • The Platyfish has W, X, and Y chromosomes. This allows WY, WX, or XX females or YY and XY males.[9]

See also

Template:Wikipedia books

References

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  3. ^ Graves, Jennifer (1 September 2000). "Human Y Chromosome, Sex Determination, and Spermatogenesis—A Feminist View". Biology of Reproduction. 63 (3): 667–676. doi:10.1095/​biolreprod63.3.667b. {{cite journal}}: |access-date= requires |url= (help); zero width space character in |doi= at position 9 (help)
  4. ^ Ezaz, Tariq (5 September 2006). "Relationships between Vertebrate ZW and XY Sex Chromosome System". Current Biology (16). doi:10.1016/j.cub.2006.08.021. {{cite journal}}: |access-date= requires |url= (help)
  5. ^ a b Graves, Jennifer (10 March 2006). "Sex Chromosome Specialization and Degeneration in Mammals". Cell. 124 (5): 901–914. doi:10.1016/j.cell.2006.02.024. {{cite journal}}: |access-date= requires |url= (help)
  6. ^ "The evolution of the sex chromosomes: Step by step" (Press release). University of Chicago Medical Center. 28 October 1999. Retrieved 23 October 2011.
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  8. ^ Graves, Jennifer (July 22 2004). "The degenerate Y chromosome – can conversion save it?". Reproduction, Fertility, and Development. 16 (5): 527–34. doi:10.1071/RD03096. {{cite journal}}: Check date values in: |date= (help)
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  10. ^ Penalva, Luiz O. F. (September 2003). "RNA Binding Protein Sex-Lethal (Sxl) and Control of Drosophila Sex Determination and Dosage Compensation". Microbiology and Molecular Biology. 67 (3): 343–359. doi:10.1128/​MMBR.67.3.343-359.2003. {{cite journal}}: zero width space character in |doi= at position 9 (help)
  11. ^ a b c d e Hake, Laura (2008). "Genetic Mechanisms of Sex Determination". Nature Education. 1 (1). Retrieved 8 December 2011.
  12. ^ Goodfellow, P.N. (1999). "DAX-1, an 'antitestis'gene". Cellular and Molecular Life Sciences. 55: 857–863. doi:10.1007/PL00013201.
  13. ^ 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). {{cite journal}}: Text "pages1072-1072" ignored (help)
  14. ^ Cox, James J. (6 January 2011). "A SOX9 Duplication and Familial 46,XX Developmental Testicular Disorder". New England Journal of Medicine. 364: 91–93.
  15. ^ Huang, Bing (7 December 1999). "Autosomal XX sex reversal caused by duplication of SOX9". American Journal of Medical Genetics. 87 (4): 349–353.
  16. ^ Uhlenhaut, Henriette N. (11 December 2009). "Somatic Sex Reprogramming of Adult Ovaries to Testes by FOXL2 Ablation". Cell. 139: 1130–1142. doi:10.1016/j.cell.2009.11.02.
  17. ^ a b Warren, W.C.; Hillier, Ladeana W.; Marshall Graves, Jennifer A.; Birney, Ewan; Ponting, Chris P.; Grützner, Frank; Belov, Katherine; Miller, Webb; Clarke, Laura (2008). "Genome analysis of the platypus reveals unique signatures of evolution". Nature. 453 (7192): 175–U1. doi:10.1038/nature06936. PMC 2803040. PMID 18464734.
  18. ^ a b Gruetzner, F., T. Ashley, D. M. Rowell, and J. A. M. Graves. (2006). "Analysis of the platypus reveals unique signatures of evolution". Chromosoma. 115 (2): 75–88. doi:10.1007/s00412-005-0034-4. PMID 16344965.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  19. ^ Kuroiwa A, Handa S, Nishiyama C, Chiba E, Yamada F, Abe S, Matsuda Y (8 June 2011). "Additional copies of CBX2 in the genomes of males of mammals lacking SRY, the Amami spiny rat (Tokudaia osimensis) and the Tokunoshima spiny rat (Tokudaia tokunoshimensis)". Chromosome Res. 19: 635–44. PMID 21656076.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  20. ^ a b c d (Majerus 2003, p. 60)
  21. ^ Patricia E. Kuwabara, Peter G. Okkema, Judith Kimble (1992). "tra-2 Encodes a Membrane Protein and May Mediate Cell Communication in the Caenorhabditis elegans Sex Determination Pathway" (PDF). Molecular Biology of the Cell. 3 (4): 461–73. {{cite journal}}: Unknown parameter |month= ignored (help); line feed character in |title= at position 49 (help)CS1 maint: multiple names: authors list (link)
  22. ^ Smith CA, Roeszler KN, Ohnesorg T; et al. (2009). "The avian Z-linked gene DMRT1 is required for male sex determination in the chicken". Nature. 461 (7261): 267–71. doi:10.1038/nature08298. PMID 19710650. {{cite journal}}: Explicit use of et al. in: |author= (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
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  24. ^ Grützner, F. (2004). "In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes". Nature. 432 (7019): 913–917. doi:10.1038/nature03021. PMID 15502814. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  25. ^ Edward O. Wilson (12 September 2005). "Kin selection as the key to altruism: its rise and fall" (PDF). Social Research. 72: 1–8. Retrieved 25 March 2011.
  26. ^ Ellen van Wilgenburg (5 January 2006). "Single locus complementary sex determination in Hymenoptera: an "unintelligent" design?". Frontiers in Zoology. 3 (1). doi:10.1186/1742-9994-3-1. Retrieved 22 November 2011.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  27. ^ a b Maldonado, L. C. Torres (20 August 2002). "Expression profiles of Dax1, Dmrt1, and Sox9 during temperature sex determination in gonads of the sea turtle Lepidochelys olivacea" (PDF). General and Comparative Endocrinology. 129: 20–26. Retrieved 6 December 2011. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  28. ^ Bull, J. J. (1980). "Sex Determination in Reptiles". The Quarterly Review of Biology. 55 (1): 3–21. {{cite journal}}: Unknown parameter |month= ignored (help)
  29. ^ a b Valenzuela, Nicole (2001). "Nest-site philopatry and the evolution of temperature-dependent sex determination" (PDF). Evolutionary Ecology Research. 3: 779–794. Retrieved 7 December 2011. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  30. ^ Watts, Phillip C. (21 December 2006). "Parthenogenesis in Komodo dragons". Nature. 444: 1021–1022. doi:10.1038/4441021a. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  31. ^ Jiggens, Francis M. (2000). "Sex-ratio-distorting Wolbachia causes sex-role reversal in its butterfly host". Proceedings of the Royal Society. 267 (1438): 69–73. doi:10.1098/rspb.2000.0968. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)

Bibliography

Further reading

Sex chromosomes and sex determination in amniotes

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Sex determination in insects

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Sex determination in plants

  • Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 17038793, please use {{cite journal}} with |pmid=17038793 instead.
  • Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1038/hdy.2009.36, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1038/hdy.2009.36 instead.

Book

  • Bainbridge, David A. (2003). The X in sex: how the X chromosome controls our lives. Cambridge: Harvard University Press. ISBN 0-674-01028-0.

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