A loss of sex is hypothesized to decrease the ability of hosts to evolve defenses against parasites, but no study of plants has tested how this affects genome-wide patterns of molecular evolution. Here, we test whether repeated losses of sex in the genus Oenothera alter the molecular evolution of defense genes against herbivores and pathogens. Using transcriptomes from 32 Oenothera species, we determined the function of 2431 orthologs. Phylogenetic Analysis by Maximum Likelihood was used to

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... if the relative rates of nonsynonymous to synonymous substitutions (ω = dN/dS) in 721 defense and 1710 non-defense genes were higher in sexual (16 spp.) than asexual (16 spp.) lineages. Based on our experimental design, we conducted all the analyses at the species-level. We detected variability in ω in >55% of genes and positive selection on 3% of genes. Although mean ω was similar between positively selected defense and non-defense genes, a higher proportion of amino acid sites were under positive selection in defense genes. Particularly, defense genes subject to positive and divergent selection were disproportionately associated with constitutive and induced defenses and various functions within the immune plant system that mediate detection, signaling and response to pathogens. Overall, average signature of positive and purifying selection were similar between sexual and asexual lineages for defense and non-defense genes. However, the frequency of positively selected genes was higher in sexual species for non-defense genes. Our results suggest that losing sex affects protein evolution, but these effects are small in young, rapidly diversifying lineages.