Finding the genetic basis of prezygotic isolation barriers in the moth Spodoptera frugiperda
Final Report Abstract
Our main findings are the following. In terms of sexual communcation, corn- and rice-strain males in Florida were mostly attracted to a corn-strain pheromone blend, at least in corn fields. There may be synergistic effects of host plant volatiles and sex pheromone components in corn fields. In grass fields, we did not find a preference for a corn- or a rice-strain pheromone blend in either strain. Together, these data suggest that strain-specific differences in the sexual communication of both strains do not cause assortative mating in Florida and thus seem to be a weak prezygotic isolation barrier between the corn- and the rice-strain. In our field studies conducted in different geographic regions, we found that corn-strain males preferred the synthetic corn-strain blend over the rice-strain blend in South America (Peru, Argentina), but were equally attracted to both blends in North America (Canada, North Carolina) and exhibited differential attraction in Puerto Rico and Florida. Furthermore, corn-strain males showed geographic variation in response to different doses of the critical component (Z)- 7-dodecenyl acetate (Z7-12:OAc). In contrast, rice-strain males showed almost no geographic variation in response to different pheromone blends, suggesting that corn-strain males are more restricted in their response to pheromone blends than rice-strain males. In our population genetic studies we found that the two strains do still hybridize and that both types of hybrids occur in nature. RC hybrids (rice-strain mother, corn strain father) are mostly found in corn habitats, while other hybrids (CR, CI, RI) are mostly found in rice habitats. Overall, the two strains seem to be predominantly found in the habitats from where they were originally described, but we found significant exceptions. Therefore, our preliminary conclusion is that divergence between the strains is not likely due to host plant specialization, or at least not alone. We hypothesize that an interaction between ecological and behavioral mechanisms has contributed to reproductive isolation between the two strains. On the basis of the distribution of the two strains, particularly the distribution of the respective hybrids, and the behavioral differences between the two strains, we hypothesize that the rice strain is the ancestral strain and corn the derived strain. This is a new finding. Higher levels of genetic homogeneity observed in the corn- than the rice-strain suggests that the cornstrain went through a bottleneck, i.e. that the corn-strain arose from a few individuals. The observation that males of both strains are mostly attracted to a corn-strain sex pheromone blend in corn fields, while this preference is not found in rice fields, is consistent with these results. Differences in diel patterns of reproductive behaviors seems to be much more consistent than host-plant associations between the strains. We found that the genetic basis of this timing difference is governed by one major QTL and 7 minor QTL. The major QTL houses the circadian clock gene vrille, which cycles in a strain-specific manner. A shift in timing activity at night can immediately inhibit gene flow and thus may have been a driving force in the strain differentiation. Therefore, the strains may rather be called “timing strains” than “host strains.” In determining the genetic basis of strain-specific pheromone differences in both S. frugiperda strains, we found that multiple genomic regions on 10 different chromosomes were involved in the production of corn- and rice-strain specific female pheromone blends. Interestingly, we found one QTL that affected the production of the critical minor sex pheromone component Z7-12:OAc to be the same genomic regions as the major QTL of the timing difference to which vrille maps. Thus, genes involved in strain-specific Z7-12.OAc production and timing of mating in the night are located on the same chromosome, which suggests that these two prezygotic mating barriers may be genetically linked and influenced by the same set of genes and/or regulatory elements.
Publications
- 2009. Time-shifted reproductive behaviours among fall armyworm (Noctuidae: Spodoptera frugiperda) host strains: Evidence for differing modes of inheritance. Journal of Evolutionary Biology, 22: 1447-1459
Schöfl G, Heckel DG, Groot AT
- 2010. The roles and interactions of reproductive isolation mechanisms in fall armyworm (Lepidoptera: Noctuidae) host strains. Ecological Entomology, 35, 105-118
Groot AT, Marr M, Heckel DG, Schöfl G.
- 2011. Allochronic separation vs. mate-choice: nonrandom patterns of mating between fall armyworm host-strains. American Naturalist, 177(4), 470-485
Schöfl G, Dill A, Heckel DG, Groot AT
- 2012. Host association of Spodoptera frugiperda (Lepidoptera: Noctuidae) corn and rice strains in Argentina, Brazil and Paraguay. Journal of Economic Entomology 105: 418-428
Juárez ML, Murúa MG, García MG, Ontivero M, Vera MT, Vilardi JC, Groot AT, Castagnaro AP, Gastaminza G, Willink E
- 2013. Pheromonal divergence between two strains of Spodoptera frugiperda. Journal of Chemical Ecology
Unbehend M, Hänniger S, Meagher RL, Heckel DG, Groot AT
(See online at https://doi.org/10.1007/s10886-013-0263-6)
