Expression pattern of anti-Müllerian hormone (amh) in the hybrid fish complex of Squalius alburnoides

In fish of the Squalius alburnoides complex, hybridisation and polyploidy have affected sex ratios, resulting in strong correlations between sex and genotype. The preponderance of females among triploids and the occurrence of an all male lineage among diploids seem to imply that sex ratio deviations should have a strong genetic basis. Until now, no information has been gathered regarding the molecular basis of sex determination in this intricate hybrid system. Thus, putative regulatory elements of the cascade that potentially are involved in sex determination in S. alburnoides have to be investigated. Being reported to have an important role in teleost sex determination, and more particularly in male gonad development, the anti-Müllerian hormone, amh was a good initial candidate. Here we report the isolation, cloning and characterization of the amh ortholog in S. alburnoides and the ancestral species S. pyrenaicus. In adult S. alburnoides and S. pyrenaicus of both sexes, amh shows a gonad specific expression pattern, restricted to the Sertoli cell lineage in testis and to granulosa cells in ovaries. During development, it plays an early role in male gonad differentiation in S. alburnoides. Overall the observed patterns are similar to what has been reported in other teleost species. This suggests a conserved role of amh and implies that its expression dynamics cannot be directly responsible for the sex ratio deviations reported in S. alburnoides. It is possible that a conjunction of other factors could be contributing for sex ratio imbalance. The present results constitute the starting point in the characterization of the S. alburnoides sex determination cascade, a process that we expect to shed some light on the molecular basis of sex distribution, within the context of hybrid system evolution.