Divergent Expression Regulation of Gonad Development Genes in Medaka Shows Incomplete Conservation of the Downstream Regulatory Network of Vertebrate Sex Determination

Abstract

Genetic control of male or female gonad development displays between different groups of organisms a remarkable diversity of “master sex-determining genes” at the top of the genetic hierarchies, whereas downstream components surprisingly appear to be evolutionarily more conserved. Without much further studies, conservation of sequence has been equalized to conservation of function. We have used the medaka fish to investigate the generality of this paradigm. In medaka, the master male sex-determining gene is dmrt1bY, a highly conserved downstream regulator of sex determination in vertebrates. To understand its function in orchestrating the complex gene regulatory network, we have identified targets genes and regulated pathways of Dmrt1bY. Monitoring gene expression and interactions by transgenic fluorescent reporter fish lines, in vivo tissue-chromatin immunoprecipitation and in vitro gene regulation assays revealed concordance but also major discrepancies between mammals and medaka, notably amongst spatial, temporal expression patterns and regulations of the canonical Hedgehog and R-spondin/Wnt/Follistatin signaling pathways. Examination of Foxl2 protein distribution in the medaka ovary defined a new subpopulation of theca cells, where ovarian-type aromatase transcriptional regulation appears to be independent of Foxl2. In summary, these data show that the regulation of the downstream regulatory network of sex determination is less conserved than previously thought.

Genetic basis and biotechnological manipulation of sexual dimorphism and sex determination in fish

“…These downstream sex-related genes can regulate the expression of sex steroid hormones, and thereby direct the development of functional gonads with the sex phenotype [150152]. A new idea believes that fish sex determination is neither a single genetic cascade reaction nor a result of hierarchical cascades by genetic network, but the reciprocal links between different genetic modules which functionally interact with each other [15,153]. As shown in Figure 1, in fish with XX/XY sex determination system, high expression of the master male sex determination gene on Y chromosome, such as dmy, sox3, amhr2, sdY, or amhy, is able to initiate different modules in genetic network of sex determination and testis differentiation so that activate the essential genes for testis development, such as dmrt1, sox9 and amh, and thereby completes male formation.…”

Foxl2 and Its Relatives Are Evolutionary Conserved Players in Gonadal Sex Differentiation

“…Medaka foxl2 is clearly expressed in a dimorphic fashion with no testicular expression [Nakamoto et al, 2006]. To investigate the possible role of Foxl2 during medaka ovarian differentiation, the distribution of the Foxl2 protein was analyzed by immunohistochemistry [Herpin et al, 2013]. Throughout the transition of germline stem cells to oocytes the Foxl2 protein is first present in the germline stem cells of the cradle and maintained during meiosis until oogenesis [Nakamoto et al, 2006].…”

Genetics of Sexual Development: An Evolutionary Playground for Fish

“…However, recent data from various fish species suggest that, in addition to the flexibility at the top of the cascade, even these key elements are not as conserved as previously assumed. This is evidenced by varying and species-specific expression patterns between sexes and throughout development (see, e.g., Vizziano et al 2007;Ijiri et al 2008;Hale et al 2011;Herpin et al 2013;and Table 1). For example, sox9 and the aromatase cyp19a1 show expression patterns in East African cichlid fishes that are not consistent with conserved testis and ovary functions, respectively (Böhne et al 2013).…”