Reproductive isolation following hybrid speciation in Mediterranean pipefish (Syngnathus spp.)

Abstract

The development of reproductive isolation is a crucial step in the speciation process. Premating isolation is often implicated in traditional models of divergence with gene flow, but the evolution of reproductive isolating mechanisms has been poorly explored in species resulting from hybrid speciation. We investigated the mechanisms of reproductive isolation between two closely related Adriatic pipefish species, Syngnathus taenionotus and Syngnathus typhle, that show a complex genetic history consistent with introgressive hybridization. We studied the genetic mating system of S. taenionotus in situ, quantified differences in the reproductive behaviour of the two species and carried out a series of behavioural experiments aimed at identifying the factors responsible for the maintenance of species integrity in natural populations. We identified subtle differences in courtship behaviour between the two species and evidence of a preference for large mating partners in reproductive males of both species. Individual preferences were equivocal in conspecific–heterospecific preference trials, and S. typhle males were the only group that showed a significant preference for conspecifics. Reciprocal no-choice mating experiments resulted in a low frequency of heterospecific matings between S. typhle males and S. taenionotus females, all of which failed to produce viable offspring, indicating the presence of both strong premating and postmating isolation in this system. Our results suggest that the development of reproductive isolating mechanisms between species produced by homoploid hybridization may differ from that expected under standard models of divergence with gene flow.