PERSISTENCE OF NEUTRAL POLYMORPHISMS IN LAKE VICTORIA CICHLID FISH

Phylogenetic trees for groups of closely related species often have di fferent topologies, depending on the genes used. One explanation for t he discordant topologies is the persistence of polymorphisms through t he speciation phase, followed by differential fixation of alleles in t he resulting species. The existence of transspecies polymorphisms has been documented for alleles maintained by balancing selection but not for neutral alleles, In the present study, transspecific persistence o f neutral polymorphisms was tested in the endemic haplochromine specie s flock of Lake Victoria cichlid fish. Putative noncoding region polym orphisms were identified at four randomly selected nuclear loci and te sted on a collection of 12 Lake Victoria species and their putative ri verine ancestors. At all loci, the same polymorphism was found to be p resent in nearly all the tested species, both lacustrine and riverine. Different polymorphisms at these loci were found in cichlids of other East African lakes (Malawi and Tanganyika). The Lake Victoria polymor phisms must have therefore arisen after the flocks now inhabiting the three great lakes diverged from one another, but before the riverine a ncestors of the Lake Victoria flock colonized the Lake. Calculations b ased on the mtDNA clock suggest that the polymorphisms have persisted for about 1.4 million years. To maintain neutral polymorphisms for suc h a long time, the population size must have remained large throughout the entire period.