The most thoroughly studied group of fish for the genetics of sex determination are the platyfishes and swordtails of the genus Xiphophorus (for detailed reviews see Kallman, 1984, Kazianis et al., 2004 and Volff and Schartl, 2001). These fish nicely demonstrate the variability of sex determination systems in fish.
In the platyfish, X. maculatus, three different sex chromosomes: W, X, and Y occur. Females may have WY, WX or XX genotypes. The males are either YY or XY. Interestingly YY males are fully viable and fertile, indicating that no essential genes have been deleted during the evolution of the Y. WW females have not been found in natural habitats, but can be produced in the laboratory and are viable as well. Sex determination in this three chromosome situation is explained at present by two different hypotheses: The first ( Kallman, 1984) proposes a network of male determining genes and regulatory genes distributed over the different types of sex chromosomes and autosomes, while the second ( Volff and Schartl, 2001) favors a dosage dependent mechanism based on one gene being present in different copy numbers on the three types of sex chromosomes. The stable sex chromosomal determination mechanism in the platyfish can be overridden by rare alleles of autosomal modifiers found in some natural populations (Kallman, 1984) and by exogenous influences, e.g. X-ray irradiation (Anders et al., 1969) and steroid hormone treatment (Dzwillo and Zander, 1967). For the green swordtail, X. hellerii, some strains have been shown to be WZ–ZZ ( Kazianis et al., 2004) while others exhibit a so-called polyfactorial system ( Peters, 1964), meaning that the sex-determining genes are spread over several chromosomes, which additively or cooperatively influence gonad development. Other species like X. variatus and X. xiphidium appear to exhibit a simple XX–XY sex determination mechanism. A modification of the XX–XY system is described for two species, X. nezahualcoyotl and X. milleri. Most males are XY and most females are XX. However, a second Y chromosome, Y′, exists, which together with an autosomal modifier is responsible for XY′ females. Consequently, YY′ males can be produced ( Kallman, 1984). Another modification of the XY system was found in X. nigrensis. In this species all XY fish are males. There is an autosomal locus with two alleles, A and a, which affects sex determination in XX fish. If the genotype is AA, the fish will be female, and if aa, the XX individual will be male. Aa genotypes are predominantly female, but sometimes (approx. 5%) such fish develop into males ( Kallman, 1984). Attempts towards isolation of the sex determining gene(s) in Xiphophorus are underway. BAC contigs starting from marker loci that flank the sex determining region on the X. maculatus X and Y chromosomes have been established covering at present approx. one megabase on each side ( Froschauer et al., 2002). Source: A comparative view on sex determination in medaka
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