Abstract: To provide genetic information for the purpose of selective breeding, comparison of genetic variability among the three cultured stocks of Oreochromis niloticus, O. aureus and<> red tilapia<> O. sp. was made using nine microsatellite loci. In total 60 alleles were detected from 109 individuals. The mean numbers of alleles were 4.11, 1.33 and 3.44 with average polymorphism information contents being 0.580, 0.077 and 0.466, respectively, for the Nile, aurea and red tilapia stocks. The observed heterozygosities averaged 0.528, 0.056 and 0.491, respectively, for the three cultured stocks, while the expected heterozygosities averaged 0.644, 0.091 and 0.526, respectively . D values of heterozygote deviation were - 0.148, - 0.222 and - 0.044, respectively, showing heterozygote deficiency in the three tilapia stocks . Chi-square tests showed that most loci in the three stocks deviated from Hardy-Weinberg equilibrium, suggesting occurrence of genetic drift possibly due to founder effect. S ignificant genetic differences were observed among the three stocks (pairwise genetic differentiation index F _ST : 0.329 to 0.656, P<0.01). The genetic distance between O. niloticus and red tilapia was the lowest ( 0.47 ). These observations indicated that O. niloticus has the highest genetic diversity, whereas O. aureus has the lowest; so there is still some selective breeing potential for O. niloticus and red tilapia , whereas new germplasms of O. aureus should be imported for supplement .

Key words: tilapia, cultured stock, genetic diversity, genetic differentiation, microsatellite DNA