The Role of chromosomal rearrangements in adaptation in Drosophila americana

Abstract

Natural environments expose organisms to multifarious selective pressures involving numerous aspects of the overall phenotype, therefore eliciting a response from multiple correlated loci. It has been hypothesized that chromosomal rearrangements play a role facilitating local adaptation by establishing new linkage relationships and modifying the recombination patterns between the different chromosomal forms, allowing coordinated adaptation of several loci. The central aim of my work is to test this hypothesis using Drosophila americana as a model system. This species segregates several inversions and an X-4 centromeric fusion which makes it an excellent model to study the role of chromosomal rearrangements on local adaptation. I tested the hypothesis using several approaches. I determined the geographic distribution of the chromosomal rearrangements through sampling of wild populations from a broad geographic range. I found that several of the chromosomal rearrangements exhibit clinal variation. Furthermore, many of these are in linkage disequilibrium (LD) with each other. The X-4 fusion is highly associated with inversions on the X and 4 chromosome. Also, two inversions on chromosome 5 are in strong negative LD. I studied sequence variation associated with rearrangements of the X using inbred lines. The results show the inversion and the fusion strongly influence variation on this chromosome. Regions of significant population differentiation and linkage with the rearrangements are found interspersed with loci showing neutral variation indicating that in spite of recombination, allelic associations are maintained on this chromosome. I also extended the analysis to flies directly collected from the wild assessing variation with RFLP throughout the X and 4 chromosome. I found long distance LD among loci on both chromosomes, interspersed with regions not showing this pattern.