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MBE Advance Access published online on October 31, 2003
Molecular Biology and Evolution, doi:10.1093/molbev/msh006
Molecular Biology and Evolution © Society for Molecular Biology and Evolution 2003; all rights reserved
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Accepted August 19, 2003
© 2003 Society for Molecular Biology and Evolution

Original Articles

Comparative Molecular Population Genetics of the Xdh Locus in the Cactophilic Sibling Species Drosophila buzzatii and D. koepferae

Romina Piccinali 1*, Montserrat Aguadé 2, and Esteban Hasson 1

1 Departamento de Ecología, Genética y Evolución, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellón 2, Ciudad Universitaria, C1428EHA Buenos Aires, Argentina
2 Departament de Genètica, Facultat de Biologia, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain

* To whom correspondence should be addressed. E-mail: rpicci{at}bg.fcen.uba.ar.


  Abstract

The Xdh (rosy) gene is one of the best studied in Drosophila from an evolutionary viewpoint. Here we analyze nucleotide variation in an ~ 1875-bp fragment of the second exon of Xdh in Argentinian populations of the cactophilic D. buzzatii and its sibling D. koepferae. The major electrophoretic alleles of D. buzzatii not only lack diagnostic amino acids in the region studied but on average differ from each other by four to thirteen amino acid changes. Our data also suggest that D. buzzatii populations belonging to different phytogeographic regions are not genetically differentiated, while D. koepferae exhibits a significant pattern of population structure. The Xdh region studied is twice as polymorphic in D. buzzatii as in D. koepferae. Differences in historical population size or in recombinational environment between species could account for the differences in the level of nucleotide variation. In both species, the Xdh region exhibits a great number of singletons, which significantly departs from the frequency spectrum expected under neutrality for nonsynonymous sites, and also for synonymous sites in D. buzzatii. These excesses of singletons could be the signature of a recent population expansion in D. buzzatii, whereas they may be simply explained as the result of negative selection in D. koepferae.

Key Words: Xdh, Cactophilic Drosophila, Population structure, Population expansion, Negative selection


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