The loss of female sperm storage ability as a potential driver for increased extinction in Chondrichthyes

Female sperm storage (FSS) has been reported in Chondrichthyans species, and involves the prolonged maintenance of viable sperm after mating events, prior to egg fertilization. Along with multiple paternity (MP–female producing offspring of multiple males within the same litter), FSS has been hypothesized to be related to the increased reproductive fitness of cartilaginous fish lineages. The present study aimed to investigate if: (1) are FSS and MP evolutionarily related and share the same evolutionary history in Chondrichthyes? (2) How is the presence of FSS implied by extinction and speciation rates and thus related to the current species diversity of the group? To answer these questions, we obtained FSS and MP records for Chondrichthyes species from the literature and performed ancestral reconstruction analyses for each character in the phylogenetic tree. We employed MEDUSA and MiSSE to determine if the shifts in diversification rates were related to the characters along the phylogeny. Finally, we utilized HiSSE to calculate the net diversity rates for observed and unobserved states. The ancestral reconstruction indicates that both characters are plesiomorphic for the group; FSS is suggested to be absent in Lamniformes and Rhinopristiformes, whereas MP may be absent in Galeocerdo cuvier. MEDUSA and MiSSE revealed that all clades lacking FSS showed no increase in rates, while there was a higher diversification rates in clades with FSS. HiSSE identified lower net diversity rates in clades lacking FSS associated with hidden states. Therefore, FSS absence seems to contribute to increased extinction rates by reducing diversity among the Chondrichthyes.