Micro-evo-devo analysis of morphological evolution in Drosophila

There is a growing knowledge of the molecular basis of morphological differences between species. However, to fully understand how these differences evolved the relative contribution of variation in the ancestral population and novel mutations to this process must be determined. This requires the synthesis of evo-devo and population genetics or `micro-evo-devo`. Therefore, we propose to characterise a morphological difference that varies within and between species at the molecular and population levels. In Drosophila melanogaster, the trichome pattern on the posterior of the femurs of the second legs differs between strains. Some strains exhibit a large patch of cuticle along the proximo-distal axis that is void of trichomes - the `naked-valley` - while in other strains the naked valley is reduced to a small proximal region. D. simulans strains and other sibling species exhibit only the large naked valley phenotype. A previous study demonstrated that most of the variation seen between the small naked valley of D. melanogaster and the large naked valley of D. simulans is caused by the evolution of Ubx. Intriguingly, mapping experiments indicated that Ubx was not responsible for intraspecific variation in the naked valley in D. melanogaster. However, this mapping did not take into account the inversion In(3R)Payne, which encompasses the mapped region and Ubx. In(3R)Payne is segregating at high frequency in different populations of D. melanogaster and is associated with variation in other traits such as body size and thermotolerence. We propose to investigate the association between In(3R)Payne and the size of the naked valley and identify homosequential lines with large and small naked valleys. Using these lines, we will map the evolved locus to high resolution using an introgression strategy. This will determine if the evolution of Ubx underlies variation both within and between species or whether a different locus is responsible for the intraspecific variation in D. melanogaster. We anticipate that the mapping will also allow the identification of the individual nucleotide changes in the evolved gene(s), thus providing further information on the patterns of nucleotide changes within genes that facilitate morphological evolution, as well as the positions of such genes within networks. In addition, we will use the power of population genetics to test for adaptation at the evolved gene(s) in local populations of D. melanogaster from different geographical locations. Therefore, our proposed study will not only allow a better insight into the molecular basis of morphological evolution within and between species, but will address how natural selection shapes the evolution of such features in populations.