Assortative Mating between European Corn Borer Pheromone Races: Beyond Assortative Meeting

Abstract

BackgroundSex pheromone communication systems may be a major force driving moth speciation by causing behavioral reproductive isolation via assortative meeting of conspecific individuals. The ‘E’ and ‘Z’ pheromone races of the European corn borer (ECB) are a textbook example in this respect. ‘Z’ females produce and ‘Z’ males preferentially respond to a ‘Z’ pheromone blend, while the ‘E’ race communicates via an ‘E’ blend. Both races do not freely hybridize in nature and their populations are genetically differentiated. A straightforward explanation would be that their reproductive isolation is a mere consequence of “assortative meeting” resulting from their different pheromones specifically attracting males towards same-race females at long range. However, previous laboratory experiments and those performed here show that even when moths are paired in a small box – i.e., when the meeting between sexual partners is forced – inter-race couples still have a lower mating success than intra-race ones. Hence, either the difference in attractivity of E vs. Z pheromones for males of either race still holds at short distance or the reproductive isolation between E and Z moths may not only be favoured by assortative meeting, but must also result from an additional mechanism ensuring significant assortative mating at close range. Here, we test whether this close-range mechanism is linked to the E/Z female sex pheromone communication system.Methodology/Principal FindingsUsing crosses and backcrosses of E and Z strains, we found no difference in mating success between full-sisters emitting different sex pheromones. Conversely, the mating success of females with identical pheromone types but different coefficients of relatedness to the two parental strains was significantly different, and was higher when their genetic background was closer to that of their male partner's pheromone race.Conclusions/SignificanceWe conclude that the close-range mechanism ensuring assortative mating between the E and Z ECB pheromone races is unrelated to the difference in female sex pheromone. Although the nature of this mechanism remains elusive, our results show that it is expressed in females, acts at close range, segregates independently of the autosome carrying Pher and of both sex chromosomes, and is widely distributed since it occurs both in France and in the USA.

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

“…This study demonstrated that a mating barrier exists between the ABB and ECB based on quantitative genetics and comparative analysis between intra-and inter-specific mating trials. Yet, by contrast with the comparative analysis based on multiple crosses in the study by Pélozuelo et al (2007), the present phenotype measurements do not distinguish between the reproductive isolation between the ABB and ECB, and other intrinsic individual propensities Mapping reproductive isolation in Ostrinia spp R Streiff et al to mate. We thus distinguish the mating isolation (mi) trait measured here from the Am trait observed in the study of Pélozuelo et al (2007).…”

“…It thus mainly target the 'Am' (assortative mating) trait described by Pélozuelo et al (2007). This study demonstrated that a mating barrier exists between the ABB and ECB based on quantitative genetics and comparative analysis between intra-and inter-specific mating trials.…”

“…The extract was then stored at À20 1C. We analyzed 3 ml of the frozen mixture on a gas chromatograph, as described by Pélozuelo et al (2007). Pure synthetic E and Z isomers of the female sex pheromone were used as internal standards.…”

“…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. By crosses and backcrosses of E and Z strains, Pélozuelo et al (2007) have also shown that the close-range mechanism ensuring assortative mating between ECB and ABB moths is unrelated to the type of female pheromone produced.…”

“…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. By crosses and backcrosses of E and Z strains, Pélozuelo et al (2007) have also shown that the close-range mechanism ensuring assortative mating between ECB and ABB moths is unrelated to the type of female pheromone produced. The nature of the trait underlying assortative mating remains elusive, but investigations of this trait may provide new insight into the reproductive isolation between these two sibling species.…”