Meeting Abstract
Despite their homologous function of fertilizing eggs, sperm exhibit incredible morphological diversity across the animal kingdom. There is ample evidence that post-mating sexual selection (sperm competition and cryptic female choice) drives sperm diversity at the family- or genus-level. However, the vast majority of variation in sperm morphology exists among taxonomic groups, and likely has deep evolutionary roots. Yet, we know surprisingly little about how sperm diversification is influenced by selective agents at the macroevolutionary scale of the animal tree of life, such as variation in the fertilization environment (internal vs. external fertilization) and reproductive mode (egg-laying vs. live-bearing). Here, we examine sperm morphology from >4000 species, spanning all major phyla of animals, to generate a macroevolutionary view of sperm evolution across 400 million years of animal evolution. We show that transitions in both fertilization and reproductive mode prompt evolutionary responses in sperm size. Specifically, externally fertilizing species have significantly smaller sperm than internal fertilizers. We argue that sperm dilution associated with external fertilization imposes limits on the evolution of sperm size, while sperm-sperm and sperm-female interactions within the female reproductive tract in internal fertilization drives evolutionary increases in sperm size. Within internal fertilizers, oviparous species have significantly larger sperm than both ovoviviparous and viviparous species. This suggests that because the cost of reproduction differs for ovi-, ovovivi-, and viviparous species, post-mating selection on sperm morphology may also vary. Our results highlight the potential for large-scale databases and analyses spanning the animal tree of life to shed light on the macroevolutionary drivers of evolutionary diversification in animals.
