Cins belirleme sistemi

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Kromozomlra bağlı farklı cins belirleme tipleri.

Cins belirleme sistemi, bir canlıdaki cinsel karakteristiklerin gelişimini belirleyen bir biyolojik sistem. Çoğu cinsel (eşeyli) canlı iki cinse sahiptir. Zaman zaman, bir veya her iki cins yerine hermafroditler bulunabilir. Döllenmenin olmadığı bir dişi üreme davranışı olan partenogenez sayesinde, yalnızca tek bir cinsin olduğu türler de bulunabilir.

Birçok türde, cins belirleme kalıtsaldır: Eriller ve dişiler cinsel morfolojilerini belirleyen farklı alellere ve hatta farklı genlere sahiptirler. Hayvanlarda, buna genellikle XY, ZW, XO, ZO veya haplodiploidy kombinasyonları aracılığıyla, kromozomal farklılıklar eşlik eder. Cinsel farklılaşma, genellikle bir ana gen (bir "cins lokusu") tarafından tetiklenir ve çok sayıda diğer genler de domino etkisinde bunu izler.

Diğer durumlarda, cins çevresel değişkenler (sıcaklık gibi) veya toplumsal değişkenler (örneğin, bir canlının popülasyonunun diğer üyelerine oranla boyutları) tarafından belirlenir. Çevresel cins belirlemesi kuşlar ve memelilerin kalıtsal belirlenen sistemlerinden önce gelir; bir sıcaklığa bağlı amniyotanın cins kromozomlarına sahip amniyotaların ortak atası olduğu düşünülür.[kaynak belirtilmeli]

Some species do not have a fixed sex, and instead change sex based on certain cues. Bazı cins belirleme sistemlerinin ayrıntıları henüz tamamen anlaşılmış değildir.

Kromozomal belirleme[değiştir | kaynağı değiştir]

XX/XY cins kromozomları[değiştir | kaynağı değiştir]

Drosophila cins kromozomları

XX/XY cins belirleme sistemi, insanlarda da bulunduğu gibi, en bilinenidir. Bu sistemde, dişiler aynı çeşit cins kromozomundan (XX) iki tane bulundururken, eriller ise iki ayrı cins kromozomu (XY) bulundurur. XY cins kromozomları, otozomlardan farklı olarak şekil ve boyut açısından birbirlerinden farklıdır ve allozomlar olarak adlandırılırlar. Bazı türler (insanlar dahil) erilliği belirleyen Y kromozomunun üzerinde bir SRY genine sahipken; diğerleri ise ([[Drosofil melanogaster| 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.[1] Because the fruit fly, as well as other species, use the number of Xs to determine sex, they are nonviable with an extra X. SRY-reliant species can have conditions such as XXY and still live.[2] Human sex is determined by containing SRY or not. Once SRY is activated, cells create testosterone and anti-müllerian hormone to turn the genderless sex organs into male.[2] With females, their cells excrete 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.[2] 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.[3] However, when an extra DAX1 gene is placed on the X, the result is a female, despite the existence of SRY.[4] Also, even when there are normal sex chromosomes in XX females, duplication or expression of SOX9 causes testes to develop.[5][6] Gradual sex reversal in developed mice can also occur when the gene FOXL2 is removed from females.[7] 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.[2]

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, Xiphophorus nezahualcoyotl and X. milleri also have a second Y chromosome, known as Y', that creates XY' females and YY' males.[8] 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.[9] 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.[10]

Heredity of sex chromosomes in XO sex determination

XX/X0 sex determination[değiştir | kaynağı değiştir]

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) and Sorex araneus, a shrew species. Transcaucasian mole voles (Ellobius lutescens) also have a form of XO determination, in which both genders lack a second sex chromosome.[4] The mechanism of sex determination is not yet understood.[11]

The nematode C. elegans is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite.[12] 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.[13]

ZW sex chromosomes[değiştir | kaynağı değiştir]

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.[14] In birds, the genes FET1 and ASW are found on the W chromosome for females, similar to how the Y chromosome contains SRY.[2] 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.[12] 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.[12] Because the use of ZW sex determination is varied, it is still unknown how exactly most species determine their sex.[12] However, reportedly, the silkworm Bombyx mori uses a single female-specific piRNA as the primary determiner of sex.[15] 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.[16] The chicken's Z chromosome also seems to be related to the X chromosomes of the platypus.[17] When a ZW species, such as the Komodo Dragon, reproduce 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.[18]

UV sex chromosomes[değiştir | kaynağı değiştir]

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.[19]

Haplodiploid sex chromosomes

Haplodiploidy[değiştir | kaynağı değiştir]

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.[20] 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.[21]

Ayrıca bakınız[değiştir | kaynağı değiştir]

Şablon:Wikipedia books

Kaynakça[değiştir | kaynağı değiştir]

  1. ^ Penalva, Luiz O. F.; Sánchez (Eylül 2003). "RNA Binding Protein Sex-Lethal (Sxl) and Control of Drosophila Sex Determination and Dosage Compensation". Microbiology and Molecular Biology. 67 (3), s. 343–359. doi:10.1128/MMBR.67.3.343-359.2003. PMC 193869 $2. PMID 12966139. 
  2. ^ a b c d e Hake, Laura (2008). "Genetic Mechanisms of Sex Determination". Nature Education. 1 (1). Erişim tarihi: 8 Aralık 2011. 
  3. ^ Goodfellow, P. N.; Camerino, G. (1999). "DAX-1, an 'antitestis' gene". Cellular and Molecular Life Sciences. 55 (6–7), s. 857–863. doi:10.1007/PL00013201. PMID 10412368. 
  4. ^ a b Chandra, H. S. (25 Nisan 1999). "Another way of looking at the enigma of sex determination in Ellobius lutescens". Current Science. 76 (8), s. 1072. 
  5. ^ Cox, James J.; Willatt, L; Homfray, T; Woods, C. G. (6 Ocak 2011). "A SOX9 Duplication and Familial 46,XX Developmental Testicular Disorder". New England Journal of Medicine. 364 (1), s. 91–93. doi:10.1056/NEJMc1010311. PMID 21208124. 
  6. ^ Huang, Bing; Wang, S; Ning, Y; Lamb, A. N.; Bartley, J (7 Aralık 1999). "Autosomal XX sex reversal caused by duplication of SOX9". American Journal of Medical Genetics. 87 (4), s. 349–353. doi:10.1002/(SICI)1096-8628(19991203)87:4<349::AID-AJMG13>3.0.CO;2-N. PMID 10588843. 
  7. ^ Uhlenhaut, Henriette N.; Jakob, S; Anlag, K; Eisenberger, T; Sekido, R; Kress, J; Treier, A. C.; Klugmann, C; Klasen, C; Holter, N. I.; Riethmacher, D; Schütz, G; Cooney, A. J.; Lovell-Badge, R; Treier, M (11 Aralık 2009). "Somatic Sex Reprogramming of Adult Ovaries to Testes by FOXL2 Ablation". Cell. 139 (6), s. 1130–1142. doi:10.1016/j.cell.2009.11.021. PMID 20005806. 
  8. ^ Schartl, Manfred (Temmuz 2004). "A comparative view on sex determination in medaka". Mechanisms of Development. 121 (7–8), s. 639–645. doi:10.1016/j.mod.2004.03.001. PMID 15210173. Erişim tarihi: 6 Aralık 2011. 
  9. ^ Warren, W.C.; Hillier, Ladeana W.; Marshall Graves, Jennifer A.; Birney, Ewan; Ponting, Chris P.; Grützner, Frank; Belov, Katherine; Miller, Webb; ve diğerleri. (2008). "Genome analysis of the platypus reveals unique signatures of evolution". Nature. 453 (7192), s. 175–U1. Bibcode:2008Natur.453..175W. doi:10.1038/nature06936. PMC 2803040 $2. PMID 18464734. 
  10. ^ 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), s. 75–88. doi:10.1007/s00412-005-0034-4. PMID 16344965. 
  11. ^ Kuroiwa A, Handa S, Nishiyama C, Chiba E, Yamada F, Abe S, Matsuda Y; Handa; Nishiyama; Chiba; Yamada; Abe; Matsuda (8 Haziran 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 (5), s. 635–44. doi:10.1007/s10577-011-9223-6. PMID 21656076. 
  12. ^ a b c d Majerus 2003, s. 60
  13. ^ Patricia E. Kuwabara, Peter G. Okkema, Judith Kimble; Okkema; Kimble (Nisan 1992). "tra-2 Encodes a Membrane Protein and May Mediate Cell Communication in the Caenorhabditis elegans Sex Determination Pathway". Molecular Biology of the Cell. 3 (4), s. 461–73. doi:10.1091/mbc.3.4.461. PMC 275596 $2. PMID 1498366.  49. harf sırasında bulunan |başlık= parametresi line feed character içeriyor (yardım)
  14. ^ Smith CA, Roeszler KN, Ohnesorg T; ve diğerleri. (Eylül 2009). "The avian Z-linked gene DMRT1 is required for male sex determination in the chicken". Nature. 461 (7261), s. 267–71. Bibcode:2009Natur.461..267S. doi:10.1038/nature08298. PMID 19710650. 
  15. ^ Kiuchi, Takashi; Koga, Hikaru; Kawamoto, Munetaka; Shoji, Keisuke; Sakai, Hiroki; Arai, Yuji; Ishihara, Genki; Kawaoka, Shinpei; Sugano, Sumio; Shimada, Toru; Suzuki, Yutaka; Suzuki, Masataka; Katsuma, Susumu (14 Mayıs 2014). "A single female-specific piRNA is the primary determiner of sex in the silkworm". Nature. 509 (7502), s. 633–636. Bibcode:2014Natur.509..633K. doi:10.1038/nature13315. 
  16. ^ Stiglec R, Ezaz T, Graves JA; Ezaz; Graves (2007). "A new look at the evolution of avian sex chromosomes". Cytogenet. Genome Res. 117 (1–4), s. 103–9. doi:10.1159/000103170. PMID 17675850. 
  17. ^ Grützner, F.; Rens, W., Tsend-Ayush, E., El-Mogharbel, N., O'Brien, P. C. M., Jones, R. C., Ferguson-Smith, M. A. and Marshall, J. A.; Tsend-Ayush, Enkhjargal; El-Mogharbel, Nisrine; O'Brien, Patricia C. M.; Jones, Russell C.; Ferguson-Smith, Malcolm A.; Marshall Graves, Jennifer A. (2004). "In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes". Nature. 432 (7019), s. 913–917. Bibcode:2004Natur.432..913G. doi:10.1038/nature03021. PMID 15502814. 
  18. ^ "Virgin births for giant lizards". BBC News. 20 Aralık 2006. Erişim tarihi: 13 Mart 2008. 
  19. ^ Bachtrog, D.; Kirkpatrick, M.; Mank, J.E.; McDaniel, S.F.; Pires, J.C.; Rice, W.; Valenzuela, N. (2011). "Are all sex chromosomes created equal?". Trends in genetics : TIG. 27 (9), s. 350–357. doi:10.1016/j.tig.2011.05.005. 
  20. ^ Edward O. Wilson (12 Eylül 2005). "Kin selection as the key to altruism: its rise and fall" (PDF). Social Research. Cilt 72, s. 1–8. Erişim tarihi: 25 Mart 2011. 
  21. ^ Ellen van Wilgenburg; Driessen, Gerard; Beukeboom, Leow (5 Ocak 2006). "Single locus complementary sex determination in Hymenoptera: an "unintelligent" design?". Frontiers in Zoology. 3 (1), s. 1. doi:10.1186/1742-9994-3-1. Erişim tarihi: 22 Kasım 2011. 

Bibliyografi[değiştir | kaynağı değiştir]

Şablon:Sex determination and differentiation Şablon:Chromo