Repeated Unidirectional Introgression of Nuclear and Mitochondrial DNA Between Four Congeneric Tanganyikan Cichlids

Nevado, B., Fazalova, V., Backeljau, T., Hanssens, M., & Verheyen, E. (2011). Repeated Unidirectional Introgression of Nuclear and Mitochondrial DNA Between Four Congeneric Tanganyikan Cichlids. IIASA Interim Report. IIASA, Laxenburg, Austria: IR-11-026

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Abstract

With an increasing number of reported cases of hybridization and introgression, interspecific gene flow between animals has recently become a widely accepted and broadly studied phenomenon. In this study we examine patterns of hybridization and introgression in Ophthalmotilapia spp, a genus of cichlid fish from Lake Tanganyika, using mitochondrial and nuclear DNA from all four species in the genus and including specimens from over 800 kilometers of shoreline. These four species have very different, partially overlapping distribution ranges, thus allowing us to study in detail patterns of gene flow between sympatric and allopatric populations of the different species. We show that a significant proportion of individuals of the lakewide distributed O. nasuta carry mitochondrial and/or nuclear DNA typical of other Ophthalmotilapia species. Strikingly, all such individuals were found in populations living in sympatry with each of the other Ophthalmotilapia species, strongly suggesting that this pattern originated by repeated and independent episodes of genetic exchange in different parts of the lake, with unidirectional introgression occurring into O. nasuta. Our analysis reject the hypotheses that unidirectional introgression is caused by natural selection favoring heterospecific DNA, by skewed abundances of Ophthalmotilapia species, or by hybridization events occurring during a putative spatial expansion in O. nasuta. Instead, cytonuclear incompatibilities or asymmetric behavioral reproductive isolation seem to have driven repeated, unidirectional introgression of nuclear and mitochondrial DNA into O. nasuta in different parts of the lake.

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