Research article

Reticulate phylogeny of gastropod-shell-breeding cichlids from Lake Tanganyika – the result of repeated introgressive hybridization

Stephan Koblmüller¹ , Nina Duftner² , Kristina M Sefc¹ , Mitsuto Aibara³ , Martina Stipacek¹ , Michel Blanc¹ , Bernd Egger¹ and Christian Sturmbauer¹

¹  Department of Zoology, University of Graz, Universitätsplatz 2, 8010 Graz, Austria

²  Section of Integrative Biology, University of Texas at Austin,1 University Station, #C0930, Austin, TX 78712, USA

³  Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, B21-4259, Nagatsuta-cho, Midori-ku, Yokohama 226-8501, Japan

author email corresponding author email

BMC Evolutionary Biology 2007, 7:7doi:10.1186/1471-2148-7-7

Published:	25 January 2007

Abstract

Background

The tribe Lamprologini is the major substrate breeding lineage of Lake Tanganyika's cichlid species flock. Among several different life history strategies found in lamprologines, the adaptation to live and breed in empty gastropod shells is probably the most peculiar. Although shell-breeding arose several times in the evolutionary history of the lamprologines, all obligatory and most facultative shell-breeders belong to the so called "ossified group", a monophyletic lineage within the lamprologine cichlids. Since their distinctive life style enables these species to live and breed in closest vicinity, we hypothesized that these cichlids might be particularly prone to accidental hybridization, and that introgression might have affected the evolutionary history of this cichlid lineage.

Results

Our analyses revealed discrepancies between phylogenetic hypotheses based on mitochondrial and nuclear (AFLP) data. While the nuclear phylogeny was congruent with morphological, behavioral and ecological characteristics, several species – usually highly specialized shell-breeders – were placed at contradicting positions in the mitochondrial phylogeny. The discordant phylogenies strongly suggest repeated incidents of introgressive hybridization between several distantly related shell-breeding species, which reticulated the phylogeny of this group of cichlids. Long interior branches and high bootstrap support for many interior nodes in the mitochondrial phylogeny argue against a major effect of ancient incomplete lineage sorting on the phylogenetic reconstruction. Moreover, we provide morphological and genetic (mtDNA and microsatellites) evidence for ongoing hybridization among distantly related shell-breeders. In these cases, the territorial males of the inferred paternal species are too large to enter the shells of their mate, such that they have to release their sperm over the entrance of the shell to fertilize the eggs. With sperm dispersal by water currents and wave action, trans-specific fertilization of clutches in neighboring shells seem inevitable, when post-zygotic isolation is incomplete.

Conclusion

From the direct observation of hybrids we conclude that hybridization between distantly related gastropod-shell-breeding cichlids of Lake Tanganyika follows inevitably from their ecological specialization. Moreover, the observed incongruence between mtDNA and nuclear multilocus phylogeny suggests that repeated hybridization events among quite distantly related taxa affected the diversification of this group, and introduced reticulation into their phylogeny.