Analysis of Medaka <em>sox9</em> Orthologue Reveals a Conserved Role in Germ Cell Maintenance

<div><p>The sex determining gene is divergent among different animal species. However, <em>sox9</em> is up-regulated in the male gonads in a number of species in which it is the essential regulator of testis determination. It is therefore often discussed that the sex determining gene-<em>sox9</em> axis functions in several vertebrates. In our current study, we show that <em>sox9b</em> in the medaka (<em>Oryzias latipes</em>) is one of the orthologues of mammalian <em>Sox9</em> at syntenic and expression levels. Medaka <em>sox9b</em> affects the organization of extracellular matrices, which represents a conserved role of <em>sox9</em>, but does not directly regulate testis determination. We made this determination via gene expression and phenotype analyses of medaka with different copy numbers of <em>sox9b</em>. <em>Sox9b</em> is involved in promoting cellular associations and is indispensible for the proper proliferation and survival of germ cells in both female and male medaka gonads. Medaka mutants that lack <em>sox9b</em> function exhibit a seemingly paradoxical phenotype of sex reversal to male. This is explained by a reduction in the germ cell number associated with aberrant extracellular matrices. Together with its identified roles in other vertebrate gonads, a testis-determining role for <em>Sox9</em> in mammals is likely to have been neofunctionalized and appended to its conserved role in germ cell maintenance.</p>