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MBE Advance Access published online on October 14, 2008
Molecular Biology and Evolution, doi:10.1093/molbev/msn227
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© The Author 2008. 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

Nucleotide and copy-number polymorphism at the odorant receptor genes Or22a and Or22b in Drosophila melanogaster

Montserrat Aguadé

Departament de Genètica, Facultat de Biologia, Universitat de Barcelona

Corresponding author: Montserrat Aguadé, Departament de Genètica, Facultat de Biologia, Universitat de Barcelona, Av. Diagonal 645 Edifici Annex 2a planta, 08028 Barcelona, Spain, Tel 34 93 402 1493 Fax 34 93 403 44 20, e-mail : maguade{at}ub.edu

Received for publication May 24, 2008. Revision received September 26, 2008. Accepted for publication September 27, 2008.

In Drosophila, odorant receptors are encoded by an old and moderately sized multigene family. Or22a and Or22b are two tandemly-arranged genes of this family that have proved to be the result of a rather young duplication. Nucleotide variation in the region spanning both duplicates was surveyed in 4 natural populations (2 African and 2 non-African) of Drosophila melanogaster and also analyzed in species of the melanogaster subgroup. The intraspecific survey revealed a particular copy-number polymorphism in some of the studied populations, with the two genes (Or22a and Or22b) present in the long variant and a single chimeric gene (Or22ab) present in the short variant. Estimated nucleotide diversity was higher in the short than in the long variant, despite the ancestral character of the latter variant in D. melanogaster. The general skew towards low-frequency variants detected in the non-African long variant and its reduced level of silent polymorphism relative to divergence is consistent with the recent fixation of an advantageous mutation at, or nearby, the Or22 long variant region. The non-negligible frequency of the short variant and the presence of a highly divergent haplotype in the East African sample would point to direct or indirect selection for its maintenance in the species. There was evidence for a generally more rapid evolution of the Or22b copy at both synonymous and nonsynonymous sites. However, an excess of nonsynonymous substitutions was only detected in the early history of this copy.

Key Words: Drosophila melanogaster • nucleotide polymorphism • odorant receptors


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