Evidence of two XX/XY sex-determining loci in the Stirling stock of Nile tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) has been the subject of much research on sex determination due to the problems caused by maturation and reproduction in culture before harvest, and because of the emerging complexity of sex determination in this species. Analysis of the stock held at the University of Stirling, derived directly from Lake Manzala in Egypt about 41 years ago, detected an XX/XY locus in linkage group (LG) 1, and then a locus in LG20 that affected sex-ratio in some families, and was associated with the effects of elevated temperature on sexratio. Another XX/XY locus in LG23 with the candidate male-specific Amh gene was found recently to control sex determination in a Nile tilapia stock held at the Agricultural Research Organization (ARO) in Israel which was derived from a stock held at the University of Swansea, which was in turn derived from the Stirling stock. Extensive screening of the Stirling Nile tilapia stock for a marker in this male-specific Amh variant (Y-linked deletion in Amh exon VII) showed that this was present at a low frequency (7 out of 51 male broodstock, 13.73%) and associated with male phenotypic sex. It seems most likely that the dominance of the LG23 locus in sex determination in the ARO stock came about through founder effects associated with serial transfer. The presence of the previously undetected LG23 sex-determining locus may have led to some confusion in earlier attempts to develop YY males in the Stirling stock. Increased knowledge of sex determination in this species should aid marker-assisted selection (MAS) to produce all male fish to prevent unwanted reproduction in tilapia aquaculture, although the presence of multiple sex-determining loci in single stocks is likely to complicate such efforts.

Automated summary

Nile tilapia has been the focus of research on sex determination due to problems caused by maturation and reproduction before harvest, with multiple sex-determining loci identified in different populations. Understanding sex determination in this species is crucial for preventing unwanted reproduction in tilapia aquaculture.