Co-option of Sox3 as the male-determining factor on the Y chromosome in the fish Oryzias dancena

Abstract

Sex chromosomes harbour a primary sex-determining signal that triggers sexual development of the organism. However, diverse sex chromosome systems have been evolved in vertebrates. Here we use positional cloning to identify the sex-determining locus of a medaka-related fish, Oryzias dancena, and find that the locus on the Y chromosome contains a cis-regulatory element that upregulates neighbouring Sox3 expression in developing gonad. Sex-reversed phenotypes in Sox3 Y transgenic fish, and Sox3 Y loss-of-function mutants all point to its critical role in sex determination. Furthermore, we demonstrate that Sox3 initiates testicular differentiation by upregulating expression of downstream Gsdf, which is highly conserved in fish sex differentiation pathways. Our results not only provide strong evidence for the independent recruitment of Sox3 to male determination in distantly related vertebrates, but also provide direct evidence that a novel sex determination pathway has evolved through co-option of a transcriptional regulator potentially interacted with a conserved downstream component.

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

“…In the present study, the 460 tilapia gsdf mRNA was dominantly detected in the male gonads in advance of dmrt1 detection, and both gsdf and dmrt1 were localized specifically in germ cell-surrounding cells in morphologically undifferentiated gonads. In medaka, the expression profiles of sex differentiation-related genes suggest that gsdf is downstream of the sex-determining gene dmy/dmrt1Yb during sex determination, but it is expressed earlier than dmrt1 in the Sertoli cell 465 lineage (Kikuchi and Hamaguchi, 2013;Myosho et al, 2012;Shibata et al, 2010;Takehana et al, In the tilapia, upregulation of other genes involving testis differentiation, such as anti-Mullerian hormone (after 15 dph) and sox9 (after 35 dph) were delayed far behind gsdf (5 dph) and dmrt1 (6 dph) upregulation in XY gonads (Ijiri et al, 2008), suggesting important roles of gsdf and 470 dmrt1 as initial switches to promote testis differentiation.…”

Mammalian X homolog acts as sex chromosome in lacertid lizards

“…Known sex-determining genes in vertebrates are usually homologs of genes involved in gonad differentiation (see, for example, Kikuchi and Hamaguchi, 2013;Bachtrog et al, 2014), and thus a better knowledge of the function of many genes in the ancestral mammalian X could allow the future identification of the sexdetermining gene in lacertids. Promising candidates might be genes homologous to Sox3 as in therian mammals and the fish Oryzias dancena (Takehana et al, 2014), or an androgen receptor, the alleles of which play a role in the sex determination of the Japanese frog Rana rugosa (Fujii et al, 2014). Candidates for the sex-determining gene in lacertids might also be found in the part of the lacertid Z chromosome homologous to GGA17.…”

Wild Sex in Zebrafish: Loss of the Natural Sex Determinant in Domesticated Strains

“…A variant of dmrt1 is also the major Y-linked sex determinant of Japanese medaka O. latipes (Matsuda et al 2002;Nanda et al 2002;Kondo et al 2006Kondo et al , 2009. In some species of Oryzias, variants of different genes in the sex-determination pathway, including gsdf and sox3,are at the top of the sex-determination hierarchy (Takehana et al 2007(Takehana et al , 2014Kondo et al 2009;Myosho et al 2012;Kikuchi and Hamaguchi 2013), while in other species of fish, other genes are at the top of the hierarchy, including a variant of irf9 in trout and amhr in fugu (Kamiya et al 2012;Yano et al 2012). The molecular genetic nature of the zebrafish sex determinant is as yet unknown.…”