Comparing mating systems and the strength of sexual selection across three congeneric species of pipefish spanning the continuum of sexual dimorphism

The strength of sexual selection is often linked to genetic mating system and is thought to be a driving force in the evolution of sexual dimorphism. Here, we compared selection between males and females across three closely related species: the dusky (Syngnathus floridae), Northern (S. fuscus), and Gulf (S. scovelli) pipefishes, using new and previously published datasets. These three species exhibit male pregnancy but vary in mating systems and sexual dimorphism, enabling us to test if (i) the species differ in their opportunities for pre- and post-copulatory selection, (ii) females demonstrate a steeper Bateman gradient in polyandrous species, and (iii) sexually dimorphic traits are targets of selection. We quantified pre- and post-copulatory selection using a variance decomposition of the opportunity of selection, Bateman gradients, and selection differentials on snout-vent length. Pre-copulatory fitness was estimated with genetic mating success and post-copulatory fitness with the number of eggs stored and proportion of developed eggs. We found polygynandrous species had higher opportunities for selection in both sexes than the polyandrous species, S. scovelli. Despite S. scovelli showing large sex differences in the opportunity for selection and selection differentials, Bateman gradients varied little between sexes and across species. Pre-copulatory selection accounted for the largest opportunity for selection across all three species; however, we identified potential sexually antagonistic selection acting on body size in S. floridae and S. fuscus, and a trade-off between pre- and post-copulatory selection for female S. fuscus and S. scovelli. Overall, the largest differences emerged between polygynandrous and polyandrous species, with potential trade-offs across episodes of selection.