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Population structure on breeding grounds of Lake Malawi’s ‘twilight zone’ cichlid fishes

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Abstract Aim 

Free-ranging benthopelagic fishes often have large population sizes and high rates of dispersal. These traits can act to homogenize population structure across the distributional range of a species and to reduce the likelihood of allopatric speciation. The apparent absence of any barriers to gene flow among populations, together with prior molecular evidence for panmixia across the ranges of three species, has resulted in Diplotaxodon, a genus of benthopelagic cichlid fishes of Lake Malawi, being proposed as a candidate case of sympatric speciation. Our aim was to further investigate this possibility by testing for intraspecific genetic subdivision among breeding populations, and intraspecific differences in breeding habitat. Location 

Lake Malawi, central-east Africa. Methods 

We analysed eight microsatellite DNA loci to test for spatial genetic differences among populations on breeding grounds of eight Diplotaxodon species. We also tested for temporal population genetic differences within breeding grounds of three species. Records of ripe Diplotaxodon encountered during sampling were analysed to test if spatial variation in assemblage structure was linked to nearshore water depth and geographic proximity of sampling sites. Results 

Consistent with previous molecular evidence, within four of the eight species tested we found no evidence of spatial genetic structuring among breeding populations. However, within the other four species we found slight yet significant spatial genetic differences, indicating restricted gene flow among breeding grounds. There was no evidence of temporal genetic differences within sites. Analyses of the distributions of ripe Diplotaxodon revealed differences in assemblage structure linked to nearshore water depth. Main conclusions 

Together, these results demonstrate both the evolution of fidelity to deep-water breeding locations in some Diplotaxodon species, and differences in breeding habitat among species. These findings are consistent with a role for divergence of breeding habitat in speciation of these cichlids, possibly promoted by dispersal limitation among geographically segregated spawning aggregations.
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Keywords: Allopatric speciation; Diplotaxodon; Lake Malawi; dispersal; genetic diversity; natal homing; pelagic fishes; sympatric speciation

Document Type: Research Article

Affiliations: 1: Department of Biological Sciences, University of Hull, Hull HU6 7RX, UK 2: School of Biological Sciences, Royal Holloway University of London, Egham TW20 0EX, UK 3: School of Biological Sciences, Bangor University, Bangor, Gwynedd LL57 2UW, UK

Publication date: 2010-02-01

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