Abstracts (first author)
The maintenance of mitochondrial genetic variation by negative frequency-dependent selection
The processes that maintain biological and genetic diversity are not fully understood. Mitochondrial genes, which encode proteins involved in a key energetic pathway in eukaryotic cells, often show high levels of standing genetic variation. This observation is especially puzzling, given the accumulating evidence for life history and fitness effects of mitochondrial genetic variation, because selection should rapidly exhaust genetic variation in the mitochondrial genome. Negative frequency-dependent selection, where the relative fitness of a genotype is inversely related to its frequency in a population, provides a potent and potentially general process that can maintain mitochondrial genetic polymorphism. However, empirical tests of this possibility are lacking. Here, we present experimental evidence that negative frequency-dependent selection acts to maintain polymorphism in mitochondrial genes. We assessed the change in mitochondrial haplotype frequencies over 10 generations of experimental evolution in a large number of seed beetle populations, where haplotypes competed for propagation to subsequent generations. We found that haplotypes consistently increased in frequency when they were initially rare and decreased in frequency when initially common. Furthermore, the strength of frequency-dependent selection was contingent upon epistatic mitonuclear interactions, directly supporting the tenet that intergenomic epistasis is important in mtDNA evolution. Our results have important implications for the use of mtDNA haplotype frequency data to estimate population level phenomena and they revive the general hypothesis that negative frequency-dependent selection may commonly facilitate genetic polymorphism in life history traits.