The understanding of the evolution of variable sex determination mechanisms across taxa requires comparative studies among closely related species. Following the fate of a known master sex-determining gene, we traced the evolution of sex determination in an entire teleost order Esociformes.
We discovered that the northern pike Esox lucius master sex-determining gene originated from a 65 to 90 million-year-old gene duplication event and that it remained sex linked on undifferentiated sex chromosomes for at least 56 million years in multiple species.
We identified several independent species- or population-specific sex determination transitions, including a recent loss of a Y chromosome. These findings highlight the diversity of evolutionary fates of master sex-determining genes and the importance of population demographic history in sex determination studies. We hypothesize that occasional sex reversals and genetic bottlenecks provide a non-adaptive explanation for sex determination transitions.
Genetic sex determination GSD evolved independently and repeatedly in diverse taxa, including animals, plants, and fungi Tree of Sex Consortium et al. In mammals and birds, conserved male or female heterogametic sex determination SD systems have been maintained over a long evolutionary time with conserved master sex-determining MSD genes Marshall Graves, In contrast, teleost fishes display both genetic and environmental sex determination ESD , and a remarkable evolutionary lability driven by rapid turnovers of sex chromosomes and MSD genes Kikuchi and Hamaguchi, ; Pan et al.
These characteristics make teleosts an attractive group in which to study the evolution of SD systems. In the past two decades, the identity of a variety of MSD genes has been revealed in teleosts thanks to advances in sequencing technologies.
