Evolution of sperm conjugation in the arthropod tree of life
We investigated the evolution of sperm conjugation in phylum Arthropoda by reconstructing its diversification along a time-calibrated phylogeny for 621 arthropods and related ecdysozoans. Phylogenetic modeling reveals conjugation has had a dynamic evolution history with multiple origins of all five observed categorical types of sperm conjugates. Contrary to prior predictions that conjugation is taxonomically widespread but relatively rare within the clades in which it is observed, our reconstructions suggest conjugation is pervasive in the arthropods, whose tree is estimated to have had conjugated sperm for 40% of its time. Conjugation has likely evolved between 39-56 times independently, and our results indicate aggregates were likely present in an early hexapod ancestor nearly 500 million years before the present. Specialized conjugate types such as spermatostyles are estimated to have been derived independently multiple times, including in both the true bugs (Order Hemiptera) and beetles (Order Coleoptera). We leverage spermatostyles as distinct structures with well supported independent origins in bugs and beetles to investigate the nature of molecular convergence in ejaculate structures using comparative proteomics. Results from comparisons with previously characterized whirligig beetle (Gyrinidae) proteomes reveals true bugs have converged in their use of sperm leucyl aminopeptidases (S-LAPs) to construct spermatostyles. The SLAP protein families are found in high abundance in all speramtostyle proteomes excluding the cicada and the independent recruitment of S-LAPs in both taxa may represent evidence of a common underlying toolkit available to insects when structuring their ejaculates.