Hyperproliferation of mitotically active germ cells due to defective anti-Müllerian hormone signaling mediates sex reversal in medaka

Abstract

SUMMARYThe function of AMH (Anti-Müllerian hormone), a phylogenetically ancient member of the TGF family of proteins, in lower vertebrates is largely unknown. Previously, we have shown that the gene encoding the type II anti-Müllerian hormone receptor, amhrII, is responsible for excessive germ cell proliferation and male-to-female sex reversal in the medaka hotei mutant. In this study, functional analyses in cultured cells and of other amhrII mutant alleles indicate that lack of AMH signaling causes the hotei phenotype. BrdU incorporation experiments identified the existence of both quiescent and mitotically active germ cells among the self-renewing, type I population of germ cells in the developing gonad. AMH signaling acts in supporting cells to promote the proliferation of mitotically active germ cells but does not trigger quiescent germ cells to proliferate in the developing gonad. Furthermore, we show that the male-to-female sex reversal phenotype in hotei mutants is not a direct consequence of AMH signaling in supporting cells, but is instead mediated by germ cells. Our data demonstrate that interfollicular AMH signaling regulates proliferation at a specific stage of germ cell development, and that this regulation is crucial for the proper manifestation of gonadal sex directed by sex determination genes.

Gonadal Development in Fish

“…The experiment using the hotei mutant also supports this notion. Maleto-female sex reversal in the XY hotei mutant is not observed in the absence of germ cells, suggesting that the ovarian formation in the sex reversal is mediated by germ cells, and is not a direct defect of the supporting cells Nakamura et al, 2012a]. Interestingly, the supporting cells in the hotei sex-reversed ovary express both dmrt1 and foxl2 .…”

“…However, in teleosts, a tissue equivalent to the Müllerian duct has not been identified. Studies of loss-of-function mutant medaka for AMH type II receptor (amhrII) , named hotei , suggest that AMH signaling is necessary for the proper proliferation of germ cells Nakamura et al, 2012a]. The hotei mutant displays hypertrophic gonads with an excess number of germ cells in both XX and XY fish.…”

“…Although germ cells are largely influenced by gonadal somatic cells, an analysis of germ cell-deficient medaka and hotei mutants indicates that germ cells also positively contribute to the gonadal sex differentiation in medaka [Kurokawa et al, 2007;Morinaga et al, 2007;Nakamura et al, 2012a]. The germ cell-deficient medaka develops as a male irrespective of genetic sex.…”

The Reversible Sex of Gonochoristic Fish: Insights and Consequences

“…Amh and amhr2 together with gsdf appear to play important roles in primordial germ cell proliferation which might explain why they have been recruited to become primary sex determinants [Heule et al, 2014]. In the medaka XY hotei, a mutation in the amhr2 gene causes hyperproliferation of the germ cells and male-to-female sex reversal due to lack of Amh signaling on the mitotic selfrenewing germ cells [Nakamura et al, 2012]. In tilapia amhr2 knockout caused 100% male-to-female sex reversal but in contrast, only 60% were obtained with amhy due to F o generation being mosaic with the mutation varying between individuals [Li et al, 2015].…”

Germline stem cells are critical for sexual fate decision of germ cells

“…AMH/MIS is essential for the regression of female reproductive organs, such as the upper vagina and oviduct. However, an analysis of the mutant of a type II receptor of AMH/MIS in medaka indicated AMH/MIS involvement in the regulation of the germline stem cell numbers, but not in the formation of reproductive organs 53, 54. This led to a hypothesis that the mammalian AMH/MIS acquired its role in the regression of female reproductive organs because the establishment of germline stem cells is dispensable during mammalian ovarian formation.…”