The highly overlapping actions of Lh signaling and Fsh signaling on zebrafish spermatogenesis

Gonadotropin signaling plays a pivotal role in the spermatogenesis of vertebrates, but how gonadotropins regulate the process in non-mammalian species remains elusive. Using a gene knockout approach in zebrafish, we have previously demonstrated the non-canonical action of gonadotropin signaling on spermatogenesis by analyzing four single mutant lines (lhb, lhr, fshb and fshr) and three double mutant lines (lhb;fshb, lhr;fshr and fshb;lhr). In this study, we further investigated the actions of gonadotropins on the testis by establishing three other double mutant zebrafish lines (lhb;lhr, fshb;fshr and lhb;fshr). All lhb;lhr and fshb;fshr mutant males were fertile. Analysis on gonadosomatic index and testicular histology in lhb;lhr and fshb;fshr mutants demonstrated that Lh signaling and Fsh signaling could functionally compensate each other in the testis. Intriguingly, the lhb;fshr mutant males were also found to be morphologically and histologically normal and functionally fertile, a phenomenon which could be explained by the cross-activation of Lhr by Fsh. We then demonstrated this cross-reactivity for the first time in zebrafish. Fsh was shown to activate Lhr using three different assay systems. Taken together, we conclude that the action of Lh and Fsh signaling is redundant in that either alone can support zebrafish spermatogenesis based on two observations. First, either Lh or Fsh signaling alone is sufficient to support male fertility. Second, the two gonadotropin ligands could promiscuously activate both receptors. Apart from revealing the complexity of gonadotropin signaling in controlling male reproduction in zebrafish, this study also shed light towards a better understanding of the evolution of gonadotropin signaling from fish to mammals.

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