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BIOLOGICAL SCIENCES / EVOLUTION
Evidence that strong positive selection drives neofunctionalization in the tandemly duplicated polyhomeotic genes in Drosophila
Section of Evolutionary Biology, BioCenter, University of Munich, Grosshaderner Strasse 2, 82152 Planegg-Martinsried, Germany
Edited by Tomoko Ohta, National Institute of Genetics, Mishima, Japan, and approved February 6, 2008 (received for review November 16, 2007)
The polyhomeotic (ph) locus in Drosophila melanogaster consists of the two tandemly duplicated genes ph-d (distal) and ph-p (proximal). They code for transcriptional repressors belonging to the Polycomb group proteins, which regulate homeotic genes and hundreds of other loci. Although the duplication of ph occurred at least 25 million to 30 million years ago, both copies are very similar to each other at both the DNA and the protein levels, probably because of the action of frequent gene conversion. Despite this homogenizing force, differential regulation of both transcriptional units suggests that the functions of the duplicates have begun to diverge. Here, we provide evidence that this functional divergence is driven by positive selection. Based on resequencing of an 
30-kb region around the ph locus in an African sample of D. melanogaster X chromosomes, we identified a selective sweep, estimated its age and the strength of selection, and mapped the target of selection to a narrow interval of the ph-p gene. This noncoding region contains a large intron with several regulatory elements that are absent in the ph-d duplicate. Our results suggest that neofunctionalization has been achieved in the Drosophila ph genes through the action of strong positive selection and the inactivation of gene conversion in part of the gene.
gene duplication | selective sweep
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: The sequences reported in this paper have been deposited in the GenBank database (accession nos. AM943662–AM943805).
*To whom correspondence should be addressed. E-mail: beisswanger{at}lmu.de
© 2008 by The National Academy of Sciences of the USA
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