Persistence of neutral polymorphisms in Lake Victoria cichlid fish - PubMed (original) (raw)

Persistence of neutral polymorphisms in Lake Victoria cichlid fish

S Nagl et al. Proc Natl Acad Sci U S A. 1998.

Abstract

Phylogenetic trees for groups of closely related species often have different topologies, depending on the genes used. One explanation for the discordant topologies is the persistence of polymorphisms through the speciation phase, followed by differential fixation of alleles in the 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 of neutral polymorphisms was tested in the endemic haplochromine species flock of Lake Victoria cichlid fish. Putative noncoding region polymorphisms were identified at four randomly selected nuclear loci and tested on a collection of 12 Lake Victoria species and their putative riverine ancestors. At all loci, the same polymorphism was found to be present 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 polymorphisms must have therefore arisen after the flocks now inhabiting the three great lakes diverged from one another, but before the riverine ancestors of the Lake Victoria flock colonized the Lake. Calculations based on the mtDNA clock suggest that the polymorphisms have persisted for about 1.4 million years. To maintain neutral polymorphisms for such a long time, the population size must have remained large throughout the entire period.

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Figures

Figure 1

Figure 1

Geography of East Africa. Drainage systems are indicated by different shadings. A, Lake Victoria; B, Lake Malawi; C, Lake Tanganyika; D, Lake Manyara; E, Lake Chala; F, Lake Nabugabo; G, Lake Kayania; H, Lake Kayugi. Cichlids were caught at localities indicated by numbers.

Figure 2

Figure 2

Informative nucleotide sites of the loci HN49 and SN-Y. The top line shows the position of the nucleotide in the consensus sequence. A dash indicates identity to the consensus; an asterisk indicates a deletion.

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