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MBE Advance Access published online on August 3, 2007
Molecular Biology and Evolution, doi:10.1093/molbev/msm159
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© The Author 2007. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Research Article

Positive Selection and Gene Conversion in SPP120, a Fertilization-related Gene, During the East African Cichlid Fish Radiation

Dave T. Gerrard and Axel Meyer*

Lehrstuhl für Zoologie und Evolutionsbiologie, Department of Biology, University of Konstanz, 78467 Konstanz, Germany

* Corresponding author: Email: axel.meyer{at}uni-konstanz.de ; Phone: +49-7531-88 4163; Fax: +49-7531-88 3018.

Received for publication April 26, 2007. Revision received July 17, 2007. Accepted for publication July 26, 2007.

The ability to infer historical natural selection from sequence data aides in finding genes that might be important in adaptation and the formation of new species. As the fastest evolving and largest known vertebrate radiation, the cichlid fish of the African Great Lakes exhibit a wide range of recent morphological diversification. We used DNA databases, mostly of expressed sequence tags, to find candidate orthologous coding sequences from two tribes of cichlids and, using an automated procedure, scanned these sequence pairs for high dN/dS, the signal of positive selection and protein adaptation. The results included vertebrate genes commonly found to be under selection (e.g. MHC loci) as well as genes known to be important specifically in the cichlid radiation (e.g. LWS opsins). Further investigation focused on a gene encoding a fertilization related protein, SPP120, which was previously known only from cichlids. Using maximum likelihood analysis on novel SPP120 cDNA sequences from a range of African cichlids, we demonstrate the influence of positive selection in a specific sub-region of the protein. We also show that SPP120 is a tandemly arranged, multi-copy gene evolving with occasional inter-locus gene conversion. A search of the Medaka genome database also revealed a tandem arrangement of multiple SPP120 copies and evolutionary rate differences between Medaka gene sub-regions mirroring those found for cichlids. Combined, these results suggest that SPP120 has been under repeated diversifying selection for over 100 Million years.

Key Words: reproductive genes • natural selection • gene conversion • cichlid • SPP120


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