Studying adaptation to larval crowding in D. simulans

The negative impact of high population density is of great interest to evolutionary biologists due to its effect on population growth and extinction. It is apparent that Drosophila species can adapt to high larval density, but we are currently only aware of a single gene that contributes to its genetic basis. We will tackle this question by combining experimental evolution, where we will compare the response of populations evolving under high and low larval density. Using a combination of genomic and transcriptomic analyses we will determine the genetic basis of adaptation to high larval crowding and the phenotypic implications. The results will provide unprecedented insights into the selective forces operating on flies exposed to high density.

Many insect populations are experiencing different larval densities caused by different availability of food and substrate for egg laying. Different larval densities cause a plastic phenotypic response in adults, which is shared among different Drosophila species. This project had two aims. First, we characterized the plastic response of different D. simulans populations to larval crowding. Second, we studied the adaptive response to larval crowding. We show that under high larval densities, adult flies eclose over a much broader time range than under low densities. Using gene expression and phenotypic analyses, we show that flies eclosed at different days are distinct from each other. Interestingly, the transcriptomic response to larval crowding differed among natural D. simulans populations. While the similar GO categories were enriched the reaction norm of individual genes differed among natural D. simulans populations. We attribute this to genetic redundancy, which implies that the expression of different gene sets is responsible to maintain phenotypic stasis at high larval density in different D. simulans populations. Flies evolved under high and low density conditions increase their fitness relative to the ancestor, which most likely reflects adaptation to laboratory culture conditions. Under low density conditions, low density evolved flies were fitter than high density evolved flies, suggesting adaptation to the density conditions. Under high density conditions the laboratory adaptation was also seen, but in addition the relative advantage of the evolved flies was dependent on the timing of eclosion. Flies from the first day of eclosion showed a higher fitness for low density evolved flies. On the second day of eclosion, no significant difference was detected between both cohorts. A higher fitness of high density evolved flies was seen among the flies eclosing on the third day. Further evidence for laboratory adaptation comes from the reduced egg size of evolved populations, with no difference between the two evolved cohorts, independent of assaying conditions. On the genomic level, we found very little sharing of selected SNPs between the high and low density evolved cohorts. Interestingly, the selection response in the high density-evolved flies was stronger than for the low density evolved flies, which resulted in a larger effective population size estimate in the latter. Based on the phenotypic and genetic response, we conclude that laboratory adaptation was one important factor in the experiment, but we noted nevertheless differences in fitness components between the two evolved cohorts. In particular in the high-density environment the differences between the two evolved cohorts became apparent, which is in line with the hypothesis that high density environments impose higher stress than low density environments.