Mammalian sexual fate is determined by the presence or absence of
sex determining region of the Y chromosome (
Sry) in the “bipotential” gonads. Recent studies have demonstrated that both male and female sexual development are induced by distinct and active genetic pathways. Breeding the Y chromosome from
Mus m. domesticus poschiavinus (POS) strains into C57BL/6J (B6J) mice (B6J-XY
POS) has been shown to induce sex reversal (75%: bilateral ovary, 25%: true hermaphrodites). However, our B6N-XY
POS mice, which were generated by backcrossing of B6J-XY
POS on an inbred B6N-XX, develop as males (36%: bilateral testis with fertility as well as bilateral ovary (34%), and the remainder develop as true hermaphrodites. Here, we investigated in detail the expressions of essential sex-related genes and histological features in B6N-XY
POS mice from the fetal period to adulthood. The onsets of both Sry and SRY-box 9 (Sox9) expressions as determined spatiotemporally by whole-mount immunohistochemistry in the B6N-XY
POS gonads occurred 2–3 tail somites later than those in B6N-XY
B6 gonads, but earlier than those in B6J-XY
POS, respectively. It is possible that such a small difference in timing of the Sry expression underlies testicular development in our B6N-XY
POS. Our study is the first to histologically show the expression and ectopic localization of a female-related gene in the XY
POS testes and a male-related gene in the XY
POS ovaries. The results from these and previous experiments indicate that the interplay between genome variants, epigenetics and developmental gene regulation is crucial for testis development.
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