Abstract：Sex determination is a plastic biological developmental process, which has always the intriguing aspect in evolutionary biology and developmental biology. In vertebrates, the sex determination including genetic sex determination (GSD) and environmental sex determination (ESD) acts to differentiate an initially bipotential gonad primordium into either testes or ovaries. GSD is governed by a series of sex-related genes involving in genetic pathway that initiate by sex-determining gene during critical periods of gonadal development. Though many downstream genes in sex determination pathways are conserved, even among vertebrates and invertebrates, the upstream sex determination gene can vary even between closely related species. Up to now, five sex-determining genes (SRY, DMRT1, DMY, DMW and AMHY) had been identified in vertebrates. This paper reviews the recent progress of sex-determining genes in vertebrate and analysis of the differences on the conservation of sex-determining genes between the higher vertebrates and lower vertebrates. We found that the sex-determining genes (SRY and DMRT1) are conserved in therian and aves, respectively, comparing to the sex-determining genes DMW, DMY and AMHY, which are unstable in their respective taxonomic systems. Correspondingly, the sex chromosomes in higher vertebrate attained high differentiation during their evolution, whereas no obvious (or even no) differentiation was observed in the sex chromosome of most extant lower vertebrates. Generally, the appearance of sex-determining gene in certain organism was always accompanied with the evolution of their sex chromosomes, which was defined by coevolution. We thus concluded that the difference of conservation on sex-determining genes between higher vertebrates and lower vertebrates may be caused by the sex chromosome differentiation or not. Following these conclusions, we then put forward two models on the relationship between sex-determining gene and sex chromosome evolution. One is the model of differentiation on sex chromosome that developed through diversification of one region of the progenitor chromosomes. In this model, transposons and repetitive elements were accumulated around a sex-determining gene and its homolog evolved into a pseudo-gene because the recombination was ceased in the sex-determining region. We thus expect a dosage dependent sex-determining gene in homogametic sex or a heteromorphic chromosome-linked (usually Y and W) determining gene in heterogametic sex such as DMRT1 in chicken and SRY in human, respectively. The other model is undifferentiation of sex chromosome where sex-determining gene derived from a duplicated region from elsewhere in the genome that was inserted on it. The large region of sex chromosomes in such case can still recombine, though the duplicated region could accumulate few repetitive elements. In this case, the sex-determining genes would reside on the heterogametic chromosome (usually Y and W) such as DMY, DMW and AMHY. In all, sex determination is a complex biological process; these two models cannot represent the types of sex determination in all species, but above models proposed would make for the isolation of new sex-determining gene.
