Governing Sex Determination in Fish: Regulatory Putsches and Ephemeral Dictators

Abstract

In contrast to the rather stable regulatory regimes established over more that 100 million years in birds and mammals, sex determination in fish might frequently undergo evolutionary changes bringing the sex-determining cascade under new master sex regulators. This phenomenon, possibly associated with the emergence of new sex chromosomes from autosomes, would explain the frequent switching between sex determination systems observed in fish. In the medaka Oryzias latipes, the Y-specific master sex-determining gene dmrt1bY has been formed through duplication of the autosomal gene dmrt1 onto another autosome, thus generating a new Y chromosome. Dmrt1bY emerged about 10 million years ago and is restricted to several Oryzias species, indicating that the Y chromosome of the medaka is evolutionarily much younger than mammalian and bird sex chromosomes. Fertile males without dmrt1bY have been detected in some medaka populations, and this gene might even have been inactivated in one Oryzias species, indicating the existence of sexual regulators already able to supplant dmrt1bY. Studies on other models have confirmed that fish sex chromosomes are generally young and occurred independently in different fish lineages. The identification of new sex-determining genes in these species will shed new light on the exceptional evolutionary instability governing sex determination in fish.

A Microsatellite-Based Genetic Linkage Map of the Cichlid Fish,Astatotilapia burtoni(Teleostei): A Comparison of Genomic Architectures Among Rapidly Speciating Cichlids

“…On the other hand, Lee et al (2004) found microsatellite markers consistent with a WZ (female heterogametic) system on LG3 in O. aureus. Thus, the mechanism of sex determination appears highly variable among species of African cichlids-as appears to be the case in fishes more generally (Volff et al 2007). The occurrence of Cyp19a1 on LG9 in A. burtoni raises the possibility that a sex-determining factor is located on this chromosome.…”

Linkage Analysis Reveals the Independent Origin of Poeciliid Sex Chromosomes and a Case of Atypical Sex Inheritance in the Guppy (Poecilia reticulata)

“…This is not surprising, considering the general lack of conservation of the master sex-determining gene among teleost species (Mank et al 2006;Volff et al 2007). At least four different chromosomes gave rise to the SDL in different species of Oryzias, suggesting that several master sexdetermining genes may have evolved even in closely related species (Takehana et al 2007a,b;Tanaka et al 2007).…”

“…Hence, in typical fish sex chromosomes, there is only a small region with reduced crossing over, and homologous recombination is still possible throughout the larger pseudoautosomal portion. On the basis of studies of common ancestors and the viability of homozygous YY males, the age of teleost sex chromosomes has been estimated in a range of 2-30 million years, dependent on the species (Volff et al 2007), compared to $180 million years for the human Y chromosome.…”

Gonadal soma-derived factor (gsdf), a TGF-beta superfamily gene, induces testis differentiation in the teleost fish Oreochromis niloticus

“…A major locus on the Y chromosome has been shown to control sex determination, but no sex-determining gene has yet been identified in tilapia (Cnaani et al, 2008;Volff et al, 2007). However, the expression patterns of most of the downstream genes involved 55 in gonadal sex differentiation, especially those of foxl2, cyp19a1a (P450arom), fshr, hsd17b1, dmrt1, and sox9a, have been investigated (Ijiri et al, 2008;Kobayashi et al, 2008Kobayashi et al, , 2003Wang et al 2007;Yan et al, 2012).…”