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Mastacembelid eels support Lake Tanganyika as an evolutionary hotspot of diversification.

ABSTRACT: BACKGROUND:Lake Tanganyika (LT) is the oldest of the African Rift Lakes and is one of the richest freshwater ecosystems on Earth, with high levels of faunal diversity and endemism. The endemic species flocks that occur in this lake, such as cichlid fishes, gastropods, catfish and crabs, provide unique comparative systems for the study of patterns and processes of speciation. Mastacembelid eels (Teleostei: Mastacembelidae) are a predominately riverine family of freshwater fish, occurring across Africa and Asia, but which also form a small species flock in LT. METHODS:Including 25 species across Africa, plus Asian representatives as outgroups, we present the first molecular phylogenetic analysis for the group, focusing particularly on the evolutionary history and biodiversity of LT mastacembelid eels. A combined matrix of nuclear and mitochondrial genes based on 3118 bp are analysed implementing different phylogenetic methods, including Bayesian inference and maximum likelihood. RESULTS:LT Mastacembelus are recovered as monophyletic, and analyses reveal the rapid diversification of five main LT lineages. Relaxed molecular clock dates provide age estimates for the LT flock at approximately 7-8 Myr, indicating intralacustrine diversification, with further speciation events coinciding with periods of lower lake level. Our analyses also reveal as yet undescribed diversity of lacustrine and riverine species. A Southern-Eastern African clade, that is younger than the LT flock, is also recovered, while West African taxa are basal members of the African mastacembelid clade. CONCLUSIONS:That the LT species flock of mastacembelid eels appears to have colonised and immediately diversified soon after the formation of the lake, supports the view of LT as an evolutionary hotspot of diversification. We find evidence for biogeographic clades mirroring a similar pattern to other ichthyological faunas. In addition, our analyses also highlight a split of African and Asian mastacembelid eels at approximately 19 Myr that is considerably younger than the split between their associated continents, suggesting a dispersal scenario for their current distribution.

SUBMITTER: Brown KJ

PROVIDER: S-EPMC2903574 | biostudies-literature | 2010 Jun

REPOSITORIES: biostudies-literature

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Mastacembelid eels support Lake Tanganyika as an evolutionary hotspot of diversification.

Brown Katherine J KJ Rüber Lukas L Bills Roger R Day Julia J JJ

BMC evolutionary biology 20100619

<h4>Background</h4>Lake Tanganyika (LT) is the oldest of the African Rift Lakes and is one of the richest freshwater ecosystems on Earth, with high levels of faunal diversity and endemism. The endemic species flocks that occur in this lake, such as cichlid fishes, gastropods, catfish and crabs, provide unique comparative systems for the study of patterns and processes of speciation. Mastacembelid eels (Teleostei: Mastacembelidae) are a predominately riverine family of freshwater fish, occurring ...[more]

PMID: 20565906

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Project description:BackgroundUnderstanding the causes of disparities in species diversity across taxonomic groups and regions is a fundamental aim in evolutionary biology. Addressing these questions is difficult because of the need for densely sampled phylogenies and suitable empirical systems.Methodology/principal findingsHere we investigate the cichlid fish radiation of Lake Tanganyika and show that per lineage diversification rates have been more than six times slower than in the species flocks of Lakes Victoria and Malawi. The result holds even at peak periods of diversification in Lake Tanganyika, ruling out the age of the lake as an explanation for slow average rates, and is robust to uncertainties over the calibration of cichlid radiations in geological time. Moreover, Lake Tanganyika lineages, irrespective of different biological characteristics (e.g. sexually dichromatic versus sexually monochromatic clades), have diversified at similar rates, falling within typical estimates across a range of plant and animal clades. For example, the mostly sexually dichromatic haplochromines, which have speciated explosively in Lakes Victoria and Malawi, have displayed modest rates in Lake Tanganyika (where they are called Tropheini).Conclusion/significanceOur results show that either the Lake Tanganyika environment is less conducive for cichlid speciation or the remarkable diversifying abilities of the haplochromines were inhibited by the prior occupancy of older radiations. Although the results indicate a dominant role for the environment in shaping cichlid diversification, differences in the timing of diversification among the Tanganyikan tribes indicate that biological differences were still important for the dynamics of species build-up in the lake. While we cannot resolve the timing of the radiation relative to the origin of the lake, because of the lack of robust geological date calibrations for cichlids, our results are consistent with a scenario that the different clades reflect independent adaptive radiations into different broad niches in the lake.

Project description:BACKGROUND: Cichlid fishes are notorious for their wealth of intra- and interspecific colour pattern diversity. In Lake Tanganyika, the endemic genus Tropheus represents the most impressive example for geographic variation in the pattern and hue of integument colouration, but the taxonomy of the over 100 mostly allopatric colour morphs remains to a large degree unresolved. Previous studies of mitochondrial DNA sequence data revealed polyphyly of the six nominally described species and complex phylogeographic patterns influenced by lake level fluctuations and population admixture, and suggested the parallel evolution of similar colour patterns in divergent evolutionary lineages. A gene tree of a rapidly radiating group may be subject to incomplete and stochastic lineage sorting, and to overcome this problem we used multi-locus, nuclear AFLP data in comparison with mtDNA sequences to study diversification, migration and introgression in Tropheus colour morphs in Lake Tanganyika. RESULTS: Significant incongruence between phylogenetic reconstructions from mitochondrial and AFLP data suggested incomplete sorting of mitochondrial haplotypes as well as frequent introgression between differentiated lineages. In contrast to the mitochondrial phylogeny, the AFLP phenogram was largely congruent with species classifications, colour pattern similarities, and in many cases also with the current geographic distribution of populations, and did not produce evidence of convergent colour pattern evolution. Homoplasy in the AFLP data was used to identify populations that were strongly affected by introgression. CONCLUSION: Different evolutionary processes were distinguished by the combination of mitochondrial and AFLP data. Mitochondrial phylogeographic patterns retained signals of large-scale migration events triggered by historical, major lake level fluctuations, whereas AFLP data indicated genetic cohesion among local groups of populations resulting from secondary contact of adjacent populations in the course of the more frequently occurring, minor lake level fluctuations. There was no support for the parallel evolution of similar colour patterns in the AFLP data. Genetic signatures of introgression and hybridisation detected in several populations suggest that lake level fluctuations drove the stunning diversification of Tropheus morphs not only through population fragmentation, but also by promoting hybridisation between differentiated morphs in secondary contact.

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Mastacembelid eels support Lake Tanganyika as an evolutionary hotspot of diversification.

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Mastacembelid eels support Lake Tanganyika as an evolutionary hotspot of diversification.

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