Evolving isolation mechanisms between two incipient species of the endoparasitic wasp, Cotesia congregata

Understanding the mechanisms of speciation and reproductive isolation has important implications for studies in biodiversity, evolution, and ecology. Insect parasitoids in particular display rapid speciation that may be associated with differential host species or chemically diverse host plants. Investigations of host-associated differentiation of parasitoids have largely focused on the degree of molecular genetic differentiation, but a true test of species status must consider whether differentiated populations are capable of interbreeding and producing viable offspring. We tested for mechanisms of isolation between two genetically distinct groups of the parasitoid, Cotesia congregata Say (Hymenoptera: Braconidae), originating from sphingid hosts, Manduca sexta on tobacco (MsT) or Ceratomia catalpae on catalpa (CcC). To evaluate potential pre- and post-zygotic isolation mechanisms, we compared male responses to female pheromones, elements of male acoustic courtship signals, and mating and reproductive success of F₁ hybrid and F₂ sibling crosses. Males displayed a ~30% decreased response to female pheromone from the reciprocal source and male courtship songs differed significantly in duration, frequency, and amplitude. Wasps from both sources mated and produced F₁ hybrid offspring in the laboratory; however, 90% of hybrid females resulting from one of the reciprocal crosses failed to produce offspring due to encapsulation and melanization of wasp eggs. Results demonstrate that MsT and CcC wasps are incipient species in the process of evolving both pre- and post-zygotic reproductive barriers.