Local selective sweeps in Drosophila melanogaster

The identification of local selective sweeps by analyzing variability pattern at neutral markers has recently become a very popular approach in population genetics. This proposal builds on previous work by the applicant applying this approach, which has also been called hitchhiking mapping, to D. melanogaster. The general idea is that D. melanogaster colonized the rest of the world only recently and several mutations are expected to have occurred to facilitate adaptation to the new environment. Such mutations should lead to a reduction in variability around the selected site. Hence, the analysis of multiple microsatellite loci distributed over the genome will identify some loci, which are linked to such a mutation resulting in local adaptation. In the previousl project more than 30 microsatellite loci were identified, which are potentially linked to a mutation associated with an adaptation during the habitat expansion of D. melanogaster from Africa to the rest of the world. In the proposed project, these candidate loci should be further characterized to a) verify their non-neutral pattern b) identify associated genes c) determine sequence polymorphism and expression pattern of the candidate genes e) identify mutations which may have caused the selective sweep. Furthermore, it is proposed to analyze the pattern of variability around the P450 gene, which has been shown to be involved in DTT resistance in D. melanogaster. This pattern of a well-documented sweep will be compared to the pattern observed for inferred out of Africa sweeps.

The project was aiming at a better understanding of the adaptation processes associated with the habitat expansion of Drosophila. New methods for the detection of selective sweeps using microsatellites were developed. One method is an improved single locus method, which has a higher power than the previously developed lnRV test. The second test is a multilocus test statistic, which was developed to take advantage of the information from loci flanking a putative target of selection. The power of the statistical test was evaluated using the recently identified mutation conferring DDT resistance to D. melanogaster. Our analyses indicated that it is possible to detect a selective sweep, but if the beneficial mutation is not novel, the detection is more difficult. The remainder of the project focussed mainly on the characterization of the demographic history of Drosophila and its consequences on the pattern of variation, and thus the power to detect selective sweeps.