Genomic Consequences of Outcrossing and Selfing in Plants

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Abstract
Evolutionary transitions from outcrossing to selfing are expected to cause a reduction in the effective population size and a corresponding increase in fixation rates of slightly deleterious mutations and decrease in fixation of advantageous mutations. Despite these predictions, evidence from genomic data does not suggest a significant reduction in the efficacy of selection associated with high levels of self‐fertilization. Here, we discuss opportunities for selfing populations to avoid an irreversible decline in fitness toward extinction and the implications for genome evolution. Most directly, large population sizes and the purging of deleterious recessive mutations can reduce genetic loads and slow the effects of genetic drift. Theory suggests that recombination rates may also evolve in response to the evolution of mating system, which can offset the harmful effects of inbreeding. Cytological data supporting the evolution of higher recombination rates in selfing species should be supplemented with genetic and molecular methods for estimating this parameter. Mutation rates may also evolve to be higher in selfing plants as a result of hitchhiking with advantageous mutations, although this is unlikely to lead to increased fitness. Finally, the abundance and activity of selfish genetic elements may also be reduced in selfing lineages, reducing the accumulation of transposable elements, B chromosomes, biased gene conversion, and the spread of cytoplasmic male sterility mutations. This reduction in genomic conflict can increase mean fitness, reduce deleterious mutation rates, and reduce genome size. We show, using comparative data, that highly selfing plants have genomes significantly smaller than those of outcrossing relatives, consistent with reduced activity and spread of repetitive elements in inbred plants. We discuss opportunities for tests of theory as plant genomic data accumulate and argue that a genomic perspective on reproductive transitions in a phylogenetic context should provide important insights into the diversity of reproductive systems in flowering plants.