ALLOZYME VARIATION IN NATURAL AND CULTURED POPULATIONS IN 2 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 zill ii and Oreochromis niloticus. Single and joint segregations were analy sed at 12 polymorphic loci in full-sib families. All the loci showed s ingle segregation in agreement with the Mendelian expectations, and tw o nonrandom joint segregations were found. In total, 26 loci were poly morphic and 12 were diagnostic between the two species. A pronounced d ifferentiation 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 fre shwater species analysed at similar macrogeographical scales. The same pattern of geographical differentiation was found between the west an d east African populations in both species suggesting that this struct uring mainly reflected the same palaeogeographical events. Most of the geographical variation observed in the Nile tilapia populations analy sed here was present in the cultured stocks. In contrast to fish-farme d stocks analysed in other countries, those from the Ivory Coast and N iger displayed neither evidence of loss of genetic diversity nor any t race of introgression with other cultured tilapia species, indicating that these stocks have been properly managed.