The origin and evolution of tuna sex chromosomes

Vertebrate species exhibit a variety of different systems for sex determination, with the most varied found among teleost fish. The genomic events through which new mechanisms are established are fundamental to our understanding of evolution. Here we describe a genomic rearrangement that gave rise to new sex chromosomes in tuna. We assembled highly complete genome sequences for all eight living true tuna species (genus Thunnus), skipjack tuna (Katsuwonus pelamis), and narrow-barred Spanish mackerel (Scomberomorus commerson) - a close relative of the tunas. We identified an inter-chromosomal translocated gene duplication, producing a single gene candidate controlling sex determination, SULT1-Y. This male-specific gene was shared among all tuna species, including skipjack. However, it was absent in Spanish mackerel, as well as more distantly related members of family Scombridae. We hypothesise that this genomic rearrangement, which appears to coincide with the origin of the Thunnus/Katsuwonus clade, conferred a new mode of genetic sex determination. In doing so, we speculate that it may have played a role in their speciation. Finally, we applied this new knowledge to develop SexXY'Plex, a DNA test for tuna sex assignment, providing an important new tool for tuna genetics research, fisheries management and conservation.