Allozyme variation in natural and cultured populations in two tilapia species: Oreochromis niloticus and Tilapia zillii

We investigated the electrophoretic polymorphism at 30 protein loci in 27 wild and cultured populations of two tilapia species, Tilapia zillii and Oreochromis niloticus. Single and joint segregations were analysed at 12 polymorphic loci in full-sib families. All the loci showed single segregation in agreement with the Mendelian expectations, and two nonrandom joint segregations were found. In total, 26 loci were polymorphic and 12 were diagnostic between the two species. A pronounced differentiation was observed between the Ivory Coast and Nilo-Sudanian T. zillii populations (average Nei's standard genetic distance = 0.13). Within the Nilo-Sudanian region, the level of variation observed in O. niloticus and T. zillii was rather low when compared with other freshwater species analysed at similar macrogeographical scales. The same pattern of geographical differentiation was found between the west and east African populations in both species suggesting that this structuring mainly reflected the same palaeo-geographical events. Most of the geographical variation observed in the Nile tilapia populations analysed here was present in the cultured stocks. In contrast to fish-farmed stocks analysed in other countries, those from the Ivory Coast and Niger displayed neither evidence of loss of genetic diversity nor any trace of introgression with other cultured tilapia species, indicating that these stocks have been properly managed