Extreme sexual size dimorphism correlates with proxies for postcopulatory sexual selection across selected Dolomedes fish spiders
Abstract
Sexual size dimorphism (SSD) and mating system diversity are key factors shaping reproductive evolution, but their relationship to postmating sexual selection remains incompletely understood. Here, we use a cross-species comparative framework of Dolomedes fishing spiders to examine how mating systems, sperm production strategies and proxies for postcopulatory selection relate to SSD. We quantified testis size and spermatogenesis in three closely related species – D. tenebrosus, D. okefinokensis and D. scriptus – that differ in mating system and SSD. Dolomedes tenebrosus, a monogynous species with extreme female-biased SSD (eSSD) and male self-sacrifice, showed evidence of termination of spermatogenesis at maturity, indicating a terminal investment strategy. Dolomedes okefinokensis showed a similar pattern, suggesting convergent evolution towards monogyny. In contrast, D. scriptus, with moderate SSD and a polygynous system, maintained continuous spermatogenesis, consistent with repeated mating opportunities. Extending to 13 Dolomedes species, we found a strong positive association between eSSD, monogyny and spermatogenesis termination, along with increased spermophore volume in eSSD species. These findings suggest that eSSD is associated with both a terminal male reproductive strategy and intensified investment in sperm transfer structures. Together, our results highlight an integral evolutionary link between sexual size dimorphism, mating system and male reproductive investment across Dolomedes spiders.
