ABSTRACT
Sex-determining regions have been identified in the Nile tilapia on linkage groups (LG) 1, 20 and 23, depending on the domesticated strains used. Sex determining studies on wild populations of this species are scarce. Previous work on two wild populations, from Lake Volta (Ghana) and from Lake Koka (Ethiopia), found the sex-determining region on LG23. These populations have a Y-specific tandem duplication containing two copies of the Anti-Müllerian Hormone amh gene (named amhY and amhΔY). Here, we performed a whole-genome short-reads analysis using male and female pools on a third wild population from Lake Hora (Ethiopia). We found no association of sex with LG23, and no duplication of the amh gene. Furthermore, we found no evidence of sex linkage on LG1 or on any other LGs. Long read whole genome sequencing of a male from each population confirmed the absence of a duplicated region on LG23 in the Lake Hora male. In contrast, long reads established the structure of the Y haplotype in Koka and Kpandu males and the order of the genes in the duplicated region. Phylogenies constructed on the nuclear and mitochondrial genomes, showed a closer relationship between the two Ethiopian populations compared to the Ghanaian population, implying an absence of the LG23Y sex-determination region in Lake Hora males. Our study supports the hypothesis that the amh region is not the sex-determining region in Hora males. The absence of the Y amh duplication in the Lake Hora population reflects a rapid change in sex determination within Nile tilapia populations. The genetic basis of sex determination in the Lake Hora population remains unknown.
ABSTRACT
In domesticated strains of the Nile tilapia, phenotypic sex has been linked to genetic variants on linkage groups 1, 20 and 23. This diversity of sex-loci might reflect a naturally polymorphic sex determination system in Nile tilapia, or it might be an artefact arising from the process of domestication. Here, we searched for sex-determiners in wild populations from Kpandu, Lake Volta (Ghana-West Africa), and from Lake Koka (Ethiopia-East Africa) that have not been subjected to any genetic manipulation. We analysed lab-reared families using double-digest Restriction Associated DNA sequencing (ddRAD) and analysed wild-caught males and females with pooled whole-genome sequencing (WGS). Strong sex-linked signals were found on LG23 in both populations, and sex-linked signals with LG3 were observed in Kpandu samples. WGS uncovered blocks of high sequence coverage, suggesting the presence of B chromosomes. We confirmed the existence of a tandem amh duplication in LG23 in both populations and determined its breakpoints between the oaz1 and dot1l genes. We found two common deletions of ~5 kb in males and confirmed the presence of both amhY and amh∆Y genes. Males from Lake Koka lack both the previously reported 234 bp deletion and the 5 bp frameshift-insertion that creates a premature stop codon in amh∆Y.
