Evolutionary transition to XY sex chromosomes associated with Y-linked duplication of a male hormone gene in a terrestrial isopod

Sex chromosomes are highly variable in some taxonomic groups, but the evolutionary mechanisms underlying this diversity are not well understood. In terrestrial isopod crustaceans, interactions with Wolbachia, a vertically transmitted endosymbiont causing male-to-female sex reversal, are thought to drive rapid evolutionary turnovers in sex chromosomes.

Here, we use surgical manipulations and genetic crosses, plus genome sequencing, to examine sex chromosomes in the terrestrial isopod Trachelipus rathkei. Although an earlier cytogenetics study suggested a ZZ/ZW sex chromosome system in this species, we surprisingly find that in our study population, sex is determined by an XX/XY system. Consistent with a recent evolutionary origin for this XX/XY system, the putative male-specific region of the genome is small.

The genome shows evidence of sequences horizontally acquired from past Wolbachia infections, as well as evidence of Y-linked duplications of the androgenic gland hormone gene, thought to be a possible target for sex reversal by Wolbachia. Overall, these results are consistent with the hypothesis that reproductive endosymbionts such as Wolbachia can promote quick turnover of sex determination mechanisms in their hosts.