Molecular Population Genetics of the α-Esterase5 Gene Locus in Original and Colonized Populations of Drosophila buzzatii and Its Sibling Drosophila koepferae

Abstract

Several studies have suggested that esterase-2 (EST-2) may be the target of natural selection inthe cactophilic fly Drosophila buzzatii. In this work,we analyzed nucleotide variation in a fragment ofa-esterase5 (aE5), the gene encoding EST-2, in original(Argentinian) and colonized (Australian) populationsof D. buzzatii and in its sibling D. koepferae.Estimates of nucleotide heterozygosity in D. buzzatiiwere similar in Australia and Argentina, although wedetected a loss of singletons in colonized populations,suggesting a moderate founder effect. Interspecificcomparisons revealed that D. buzzatii was morepolymorphic for nonsynonymous variation, whereasD. koepferae was more variable for synonymous andnoncoding sites. The two major chromosomalarrangements (2st and 2j) in D. buzzatii displayedsimilar levels of nucleotide variation, whereas 2jz3 wasmonomorphic. The sequenced region allowed thediscrimination of a greater number of EST-2 proteinvariants in the Australian sample than in the Argentineansample. In D. koepferae, nucleotide variation inaE5 does not depart from neutral expectations, althoughtests of population structure were significantfor silent variation. In contrast, D. buzzatii hasprobably undergone a recent population expansion inits South American range. In addition, theMcDonaldand Kreitman test revealed an excess of nonsynonymouspolymorphism in both original and colonizedpopulations of this species.