The Genetic Architecture of Life History in Drosophila
Kimber, Christopher M.
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An organism’s entire life history - the timing of development, reproduction, senescence and death - can depend on the nature of selection for reproductive age. The outcome of selection will depend both on the particular features of the selective environment and on the genetic architecture of life history in the population. The genetic architecture is highly complex and itself variable in response to selection. Various features of the architecture, including available genetic variation, pleiotropy, epistasis, intersexual genetic correlation, mitochondrial genomes and mito-nuclear interactions all play a role in determining the trajectory of life history evolution, and to interact with one another in doing so. Using eleven laboratory-adapted populations of Drosophila melanogaster that have been subject to controlled long-term selection for differing ages of reproduction, I undertook several experiments that help characterize how these aspects of genetic architecture have contributed to the life history of each population. These experiments combined assays of life history trait values in the standing genetic variation with manipulations of mutation accumulation rate and hybridization, to better understand the nature of selection shaping each life history. I found surprisingly strong positive pleiotropy between early reproduction and longevity, sufficient to maintain a stable ‘surplus’ life span lasting several weeks post-selection. Selection for extremely early reproduction, on the other hand, induced pleiotropic costs to reproductive traits in each sex in a trade-off with development time. Characterization of the intersexual genetic architecture for life span revealed it was largely sex-limited in nature, consistent with a history of intralocus sexual conflict affecting the trait. There was also evidence, however, that mutations affecting life span might interact to reinforce their effects, and that the resulting costs were manifested in both sexes. Selection on mitochondrial genomes in isolation did not contribute to evolution of age-specific reproduction, but mito-nuclear coadaptation was clearly important to its evolution for males in particular. Together, these results expand our understanding of how age-specific selection shapes life history in a popular model organism. They can be used to guide future research in this system and allow us to pose important questions to be tested in other species as well.