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 in the cactophilic fly 'Drosophila buzzatii'. In this work, we analyzed nucleotide variation in a fragment of α-esterase5 (αE5), the gene encoding EST-2, in original (Argentinian) and colonized (Australian) populations of 'D. buzzatii' and in its sibling 'D. koepferae'. Estimates of nucleotide heterozygosity in 'D. buzzatii' were similar in Australia and Argentina, although we detected a loss of singletons in colonized populations, suggesting a moderate founder effect. Interspecific comparisons revealed that 'D. buzzatii' was more polymorphic for nonsynonymous variation, whereas 'D. koepferae' was more variable for synonymous and noncoding sites. The two major chromosomal arrangements (2st and 2j) in 'D. buzzatii' displayed similar levels of nucleotide variation, whereas 2jz3 was monomorphic. The sequenced region allowed the discrimination of a greater number of EST-2 protein variants in the Australian sample than in the Argentinean sample. In 'D. koepferae', nucleotide variation in αE5 does not depart from neutral expectations, although tests of population structure were significant for silent variation. In contrast, 'D. buzzatii' has probably undergone a recent population expansion in its South American range. In addition, the McDonald and Kreitman test revealed an excess of nonsynonymous polymorphism in both original and colonized populations of this species.