GENETIC DIFFERENTIATION AMONG NATURAL-POPULATIONS OF THE NILE TILAPIAOREOCHROMIS-NILOTICUS (TELEOSTEI, CICHLIDAE)

We analysed the genetic differentiation among 17 natural populations o f the Nile tilapia Oreochromis niloticus (Linnaeus, 1758) using allozy mes and restriction fragment length polymorphism (RFLP) of mitochondri al DNA (mtDNA). The populations studied, from the River Senegal to Lak e Tana and from Lake Manzalla to Lake Baringo, represent all subspecie s which have been previously described. Sixteen variable nuclear loci showed that these populations can be clustered in three groups: (1) We st African populations (Senegal, Niger, Volta and Chad drainages), (2) Ethiopian Rift Valley populations (Lakes Awasa, Ziway, Koka and the A wash River) and (3) Nile drainage (Manzalla, Cairo, Lake Edward) and K enyan Rift Valley populations (Lakes Turkana, Baringo and River Suguta ). Nine different mtDNA haplotypes were found in the RFLP analysis of a 1 kb portion of the D-loop region. The network obtained showed that there are three geographically distinct groups; all West African popul ations and O. aureus are clustered, the two Ethiopian Rift Valley popu lations are distinct and between these two groups are the Kenyan and U gandan Rift Valley populations. Nile populations show affinities both with West African populations and with specimens from Lakes Tana and T urkana. Taxonomic and biogeographical implications of these results ar e discussed.