Population structure in two sympatric species of the Lake Tanganyika cichlid tribe Eretmodini: evidence for introgression

Patterns of genetic differentiation were analysed and compared in two sympa tric species of the endemic Lake Tanganyika cichlid tribe Eretmodini by mea ns of mitochondrial DNA (mtDNA) sequences of the control region and six mic rosatellite DNA loci. The sample area covers a total of 138 km of mostly un interrupted rocky shoreline in the Democratic Republic of Congo and include s the entire distribution range of Tanganicodus cf. irsacae that stretches over a distance of 35 km. Both markers detected significant genetic differe ntiation within and between the two species. T. cf. irsacae contained lower overall genetic variation than Eretmodus cyanostictus, possibly due to its more restricted range of distribution and its smaller effective population sizes. Complete fixation of Tanganicodus mtDNA haplotypes was observed in Eretmodus at two localities, while at two other localities some Tanganicodu s individuals possessed Eretmodus mtDNA haplotypes. Taking into account the relatively large average sequence divergence of 6.2% between the two speci es, as well as the geographical distribution of mtDNA haplotypes in the lak e, the observed pattern is more likely to be a consequence of asymmetric in trogression than of shared ancestral polymorphism. As there is significant population differentiation between sympatric Tanganicodus and Eretmodus pop ulations, the events of introgressions may have happened after secondary co ntact, but our data provide no evidence for ongoing gene now and suggest th at both species are reproductively isolated at present time.