Abstract
Sperm are among the most variable cells in nature. Both within and between species, sperm display more diversity in form than one would expect of an ostensibly single-function cell. Morphologically, some of this variation within species has been demonstrated to be non-adaptive, but for many species that consistently produce multiple sperm morphs, the significance of variable sperm remains unknown. Here, we investigate the molecular evolution of dimorphic sperm in Lepidoptera, the butterflies and moths; males of this order produce both fertilizing eupyrene sperm and a secondary apyrene type that lacks DNA. Based on population genetic analyses in two species, the monandrous Carolina sphinx moth and the highly polyandrous monarch butterfly, we see evidence for increased selection in fertilizing sperm, but only in the polyandrous species. This signal comes primarily from a decrease in non-synonymous polymorphism in sperm proteins compared to the rest of the genome, indicative of strong purifying selection. Additionally, rates of non-synonymous divergence are comparable between sperm genes and the rest of the genome, suggesting that many alleles reach fixation owing to positive selection as well. Investigation of the distribution of fitness effects of new non-synonymous mutations in monarch sperm confirms stronger selection on sperm proteins in monarchs, with very few neutral variants and a preponderance of deleterious and beneficial variants. These results mirror findings on sperm evolution in other taxa; increased sperm competition decreases within-population morphological variation. Our results suggest that sperm competition can be a powerful selective force at the sequence level as well.
- Posted August 31, 2018.
