fshr, a fish sex-determining locus shows variable incomplete penetrance across flathead grey mullet populations

Whole-genome sequence data were produced from a single flathead grey mullet female and assembled into a draft genome sequence, whereas publicly available sequence data were used to obtain a male draft sequence. Two pools, each consisting of 60 unrelated individuals respectively of male and female fish were analysed using Pool-Sequencing. Mapping and analysis of Pool-Seq data against the draft genome(s) revealed >30 loci potentially associated with sex, the most promising locus of which, encoding the follicle stimulating hormone receptor (fshr) and harbouring two missense variants, was genotyped on 245 fish from four Mediterranean populations. Genotype data showed that fshr represents a previously unknown sex-determining locus, though the incomplete association pattern between fshr genotype and sex-phenotype, the variability of such pattern across different populations, and the presence of other candidate loci, reveal that a greater complexity underlies sex determination in the flathead grey mullet.