Testing the predictions of reinforcement: long-term empirical data from a damselfly mosaic hybrid zone

Theoretical work suggests that reinforcement can cause the strengthening of prezygotic isolation in sympatry by mitigating the costs of maladaptive hybridization. However, only a handful of studies have tested all predictions of this theory in natural populations. We investigated reinforcement in a mosaic hybrid zone of the damselflies Ischnura elegans and I. graellsii. Firstly, we tested if the conditions of reinforcement were fulfilled by quantifying whether hybridization was costly, and prezygotic isolation was strengthening in sympatry compared with in allopatry. Secondly, we investigated three specific predictions of reinforcement: rarer female effect, presence of concordant prezygotic and postzygotic isolation asymmetries in sympatry, and greater premating asymmetries associated with weaker postzygotic isolation in sympatry. We found that reinforcement has strengthened mechanical isolation in one cross direction between species in sympatry. Our study details a case of reinforcement and heterospecific gene flow causing opposite effects between reciprocal heterospecific crosses and describes a natural model in which reproductive isolation is built by the simultaneous effects of reinforcement, the lock-and-key model, Bateson-Dobzhansky-Müller incompatibilities and Haldane’s rule.