Pervasive convergent evolution of sperm conjugation across the Arthropoda tree of life

Theory suggests that ejaculates should evolve heightened functionality through integration of their component parts: spermatozoa, seminal fluid, and ejaculate structures. Here, we exhaustively review the vast literature on sperm ultrastructure in arthropods to examine sperm conjugation - a form of social cooperation among sperm - and its relationship with an ostensible antecedent: membrane-bound, extracellular sperm-associated material (SAM). Our reconstructions suggest that sperm conjugation first arose on the branch leading to insects and remipedes during the Cambrian or Ordovician Periods (i.e., 452.6 – 508.5 million years ago). Since then, arthropods and related ecdysozoans have spent an estimated two-thirds of their time with sperm conjugation, which has been evolutionarily lost and gained approximately 45 times each. We show that most evolutionary derivations of conjugation occurred following the origin of SAM, with SAM subsequent diversifying to facilitate the evolution of multiple conjugate types. Finally, comparative analyses of proteomes of the convergently-derived spermatostyles (specialized rods of SAM to which sperm attach) of true bugs and beetles indicate parallel utilization of a common genetic toolkit that may draw upon deep homology for the evolution of complex ejaculate adaptations. Our analyses reveal an ancient, pervasive and dynamic history of evolutionary experimentation with ejaculate form and function. Sperm cells evolve rapidly and have unique life histories. This study uses phylogenetic modelling to show that sperm conjugation, a type of social cooperation among sperm, has a deep and pervasive evolutionary history, including repeated convergence of both structures and their underlying proteins.