Complex Sex Determination in the Grey Mullet Mugil cephalus Suggested by Individual Whole Genome Sequence Data

Mugil cephalus is a cosmopolitan marine fish highly relevant from ecological and economic perspectives. Previous studies identified sex-associated variants in the follicle-stimulating hormone receptor (fshr) gene following an XX/XY sex determination (SD) system. However, these variants could not be fully associated with sex in all samples. This suggests other genes and/or environmental factors may be involved in the SD of this species, denoting intraspecific variation. In this study, we constructed a new high-quality genome assembly of M. cephalus. We then re-sequenced the whole genome in males and females from two divergent Mediterranean populations to ascertain whether other genetic variants could also be involved in SD. fshr gene variants showed to only partially explain SD, while a new intronic variant in the sestd1 gene appeared to be associated with SD following a ZZ/ZW system. The presence of other putative candidate SD variants showing significant differences between the two populations suggested a regional pattern of variation in SD in the Mediterranean Sea. The incomplete association of all the identified variants also pointed to a potential role for environmental factors.