Abstract
A central question of evolutionary biology concerns the study of evolvability. A growing body of theory implicates the pattern of genetic effect interactions underlying a trait, referred to as the genetic architecture of a trait, as an important component of evolutionary dynamics. The model organism, Drosophila melanogaster, belongs to a group of fly lineages, the Acalyptrate Diptera, that exhibit evolutionary stasis in their wing shape. The small variation in wing shape between the member species of such a disparate group, indicates either a uniform stabilizing selection of wing shape for all species or a lack of evolvability in the trait. This thesis investigates the genetic architecture of wing shape in D. melanogaster as a possible constraint on its evolvability. I conduct a line-cross analysis for estimating the genetic effects of various percentages of alleles from a parental population in the genetic background of another parental population; and, a chromosome substitution analysis for estimating the genetic effects of various X-chromosomes substituted into different genetic backgrounds. For both experiments, I use fly populations established by Houle et al. that had undergone thirty generations of artificial selection on an index of wing shape. The line-cross analysis suggests that the two lines selected to increase or decrease the wing shape index score had decanalized genetic architectures with respect to their second generation hybrids. This is consistent with the idea that wing shape evolutionary stasis is due, in part, to low evolvability of the trait. The chromosome substitution experiment was hampered by the extinction of over fifty percent of the substitution lines in the experimental design, and so the results had little power to estimate the genetic architecture of the trait. However, the best estimate of the analysis suggests a negative directional epistasis, which is also consistent with the hypothesis that the wings are canalized.
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