Genetic and ecological divergence of a monophyletic cichlid species pair under fully sympatric conditions in Lake Ejagham, Cameroon

Authors

  • Ulrich Schliewen,

    1. Max-Planck Institut für Verhaltensphysiologie, Abteilung Wickler, 82319 Seewiesen, Germany,
    2. Zoologisches Institut der Universität München, Luisenstr. 14, 80333 München, Germany,
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    • Present addresses: K. Rassmann, SCIL Diagnostics, Fraunhoferstr. 15, 82152 Martinsried, Germany. U. Schliewen, Zoologische Staatssammlung München (ZSM), Münchhausenstr. D-81247 Munich, Germany.

  • Kornelia Rassmann,

    1. Zoologisches Institut der Universität München, Luisenstr. 14, 80333 München, Germany,
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    • Present addresses: K. Rassmann, SCIL Diagnostics, Fraunhoferstr. 15, 82152 Martinsried, Germany. U. Schliewen, Zoologische Staatssammlung München (ZSM), Münchhausenstr. D-81247 Munich, Germany.

  • Melanie Markmann,

    1. Zoologisches Institut der Universität München, Luisenstr. 14, 80333 München, Germany,
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  • Jeff Markert,

    1. Department of Zoology, University of New Hampshire, Biological Science Center, 46 College Road, Durham, New Hampshire 03824, USA
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  • Thomas Kocher,

    1. Department of Zoology, University of New Hampshire, Biological Science Center, 46 College Road, Durham, New Hampshire 03824, USA
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  • Diethard Tautz

    Corresponding author
    1. Zoologisches Institut der Universität München, Luisenstr. 14, 80333 München, Germany,
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Diethard Tautz. ¶Present address: Institut für Genetik, Weyertal 121, 50931 Köln, Germany. Fax: +49 221 470 5975; E-mail:tautz@uni-koeln.de

Abstract

Although there is mounting evidence that speciation can occur under sympatric conditions, unambiguous examples from nature are rare and it is almost always possible to propose alternative allopatric or parapatric scenarios. To identify an unequivocal case of sympatric speciation it is, therefore, necessary to analyse natural settings where recent monophyletic species flocks have evolved within a small and confined spatial range. We have studied such a case with a cichlid species flock that comprises five Tilapia forms endemic to a tiny lake (Lake Ejagham with a surface area of approximately 0.49 km2) in Western Cameroon. Analysis of mitochondrial D-Loop sequences shows that the flock is very young (approximately 104 years) and has originated from an adjacent riverine founder population. We have focused our study on a particular pair of forms within the lake that currently appears to be in the process of speciation. This pair is characterized by an unique breeding colouration and specific morphological aspects, which can serve as synapomorphic characters to prove monophyly. It has differentiated into a large inshore and a small pelagic form, apparently as a response to differential utilization of food resources. Still, breeding and brood care occurs in overlapping areas, both in time and space. Analysis of nuclear gene flow on the basis of microsatellite polymorphisms shows a highly restricted gene flow between the forms, suggesting reproductive isolation between them. This reproductive isolation is apparently achieved by size assortative mating, although occasional mixed pairs can be observed. Our findings are congruent with recent theoretical models for sympatric speciation, which show that differential ecological adaptations in combination with assortative mating could easily lead to speciation in sympatry.

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