Genetic Isolation Between Two Sympatric Host-Plant Races of the European Corn Borer, Ostrinia Nubilalis Hübner. I. Sex Pheromone, Moth Emergence Timing, and Parasitism

Abstract

Adaptation to different environments may be a powerful source of genetic differentiation between populations. The biological traits selected in each environment can pleiotropically induce assortative mating between individuals of these genetically differentiated populations. This situation may facilitate sympatric speciation. Successful host shifts in phytophagous insects provide some of the best evidence for the ecological speciation that occurs, or has occurred, in sympatry. The European corn borer, Ostrinia nubilalis (Lepidoptera: Crambidae), colonized maize after its introduction into Europe by humans about 500 years ago. In northern France, two sympatric host races feed on maize (Zea mays) and mugwort (Artemisia vulgaris), respectively. We investigated the factors involved in the genetic isolation of these two races at a field site near Paris, France. We identified two biological differences that might make a significant contribution to the genetic divergence between sympatric populations feeding on the two host plants. First, assortative mating may be due to differences in the moth emergence pattern between the two races: mugwort-race moths emerged on average 10 days earlier than maize-race moths. In addition, the males emerged earlier than females in both races. Hence, the likelihood of mating between maize-race males and mugwort-race females was higher than that of mating between mugwort-race males and maize-race females. Second, the females feeding on mugwort and maize produced sex pheromones with different E/Z isomeric ratios of ⌬-11-tetradecenyl acetate. This difference in mate recognition systems reinforces the potential for assortative mating in the two races. During the experiment, overwintering mortality was much lower on maize than on mugwort. This difference was due to a braconid parasitoid wasp, Macrocentrus cingulum, that killed more than 50% of the larvae overwintering on mugwort but did not infest larvae diapausing on maize. Hence, by colonizing maize, European corn borer populations probably escaped from numerous predators, competitors, and parasitoids, such as M. cingulum. This decrease in host-associated selection may have favored the colonization of this new host. Finally, throughout this experiment we observed selection at two allozyme loci (or at linked loci): Tpi and Mpi. The Tpi locus is tightly linked with the genes involved in the response of the male to the sex pheromone and in developmental timing. The location of these traits on the Z chromosome may play a role in shortening the time required for the evolution of premating barriers.

Divergence in behaviour between the European corn borer,Ostrinia nubilalis, and its sibling speciesOstrinia scapulalis: adaptation to human harvesting?

“…Accordingly, ECB larvae were found to perform better than O. scapulalis larvae on maize, and conversely to perform less well on mugwort, indicative of physiological specialization (Calcagno et al 2007). In addition, the two Ostrinia sibling species differ with respect to development time (Thomas et al (2003), but see Malausa et al 2005). Natural enemies could also have promoted the divergence, since both the identity of parasitoids and their prevalence differ between maize and mugwort (Thomas et al 2003;Pélissié et al 2010).…”

Partial island submergence and speciation in an adaptive radiation: a multilocus analysis of the Cuban green anoles

“…On Cuba, where anole diversity is highest, more than 60 species occur over an area of only 110 000 km 2 (Rodríguez-Schettino 1999). Most anole species on Cuba and other Greater Antillean islands result from within-island processes (Losos & Schluter 2000), leading some authors to suggest that sympatric speciation plays a part in anole diversification (Shaw et al 2000; Thomas et al 2003). However, several lines of evidence suggest that geographical processes play the dominant role in intraisland anole speciation.…”

Genetic mapping of two components of reproductive isolation between two sibling species of moths, Ostrinia nubilalis and O. scapulalis

“…Genetic, ecological and biological studies in western Europe (Bourguet et al, 2000;Martel et al, 2003;Thomas et al, 2003;Malausa et al, 2007a;Malausa et al, 2007b) have shown that the ECB and ABB are sibling species (Frolov et al, 2007). Each is adapted to its own principal host plants (maize for the ECB and hop, mugwort and hemp for the ABB), as shown by host choice for oviposition and larval feeding performance in reciprocal infestation experiments Calcagno et al, 2007;Malausa et al, 2008).…”

“…The pheromone communication system is a key trait involved in reproductive isolation within the genus Ostrinia (Cardé et al, 1975;Huang et al, 1997;Huang et al, 2002;Thomas et al, 2003;Bontemps et al, 2004;Takanashi et al, 2005). Natural populations of both the ECB and ABB have a polymorphic communication system with two different blends of the female sex pheromone.…”

“…Indeed, in Europe, sympatric ECB and ABB females produce Z and E blends of pheromones, respectively . One straightforward explanation for this would be that the reproductive isolation between these two sibling species is a mere consequence of long-range assortative mating due to differences in female sex pheromones, resulting in the specific attraction of males to females of the same type (Thomas et al, 2003;Pélozuelo et al, 2004). However, experiments in controlled conditions (Pélozuelo et al, 2007) have shown that even when moths are paired in a small box, forcing short-range encounters and, thus, mating between sexual partners, the mating success of E Â Z and Z Â E crosses remains lower than that of E Â E and Z Â Z crosses.…”