Selfish Mitochondrial DNA Proliferates and Diversifies in Small, but not Large, Experimental Populations of Caenorhabditis briggsae

Evolutionary interactions across levels of biological organization contribute to a variety of fundamental processes including genome evolution, reproductive mode transitions, species diversification, and extinction. Evolutionary theory predicts that so-called "selfish" genetic elements will proliferate when the host effective population size (Ne) is small, but direct tests of this prediction remain few. We analyzed the evolutionary dynamics of deletion-containing mitochondrial DNA (ΔmtDNA) molecules, previously characterized as selfish elements, in six different natural strains of the nematode Caenorhabditis briggsae allowed to undergo experimental evolution in a range of population sizes (N = 1, 10, 100, and 1,000) for a maximum of 50 generations. Mitochondrial DNA (mtDNA) was analyzed for replicate lineages at each five-generation time point. Ten different ΔmtDNA molecule types were observed and characterized across generations in the experimental populations. Consistent with predictions from evolutionary theory, lab lines evolved in small-population sizes (e.g., nematode N = 1) were more susceptible to accumulation of high levels of preexisting ΔmtDNA compared with those evolved in larger populations. New ΔmtDNA elements were observed to increase in frequency and persist across time points, but almost exclusively at small population sizes. In some cases, ΔmtDNA levels decreased across generations when population size was large (nematode N = 1,000). Different natural strains of C. briggsae varied in their susceptibilities to ΔmtDNA accumulation, owing in part to preexisting compensatory mtDNA alleles in some strains that prevent deletion formation. This analysis directly demonstrates that the evolutionary trajectories of ΔmtDNA elements depend upon the population-genetic environments and molecular-genetic features of their hosts.