Behav_popgen

Alex Aebi’s research page

Behavioural ecology and population genetics of tritrophic interactions:

Many species are distributed over populations that live in a variety of habitats in which they may experience different environmental conditions. It can be expected that natural selection operates differently in these different habitats and if gene flow among these populations is limited, this can lead to genetic divergence and local adaptation. For parasitoids of herbivores, the host species that they attack and the plant species that the host feeds upon are likely to be the two most important external factors that will influence their fitness. Variation in these two trophic levels due to habitat characteristics may largely determine the type and evolution of the interaction with the hosts. Evidence is accumulating that plant attributes can have major impacts on parasitoid behavior and performance, but it is still unclear how these may affect parasitoid population structure. At the University of Neuchatel, with Dr Betty Benrey I examined the influence of plants on the ecological genetics of the third trophic level using a model system that comprises wild beans of the genus Phaseolus (P. vulgaris, P. coccineus and P. lunatus), several bruchid species in the genera Acanthoscelides and Zabrotes and a complex of hymenopteran parasitoids that attack these bruchids. Combining ecological, behavioral and genetical approaches, I investigated the influence of biotic and abiotic factors on the genetic structure of parasitoid populations and determined the geographic scale at which genetic differentiation occurs. In Mexico, bruchids in these two genera are attacked by three species of the genus Horismenus (Hymenoptera: Eulophidae). A taxonomic key based on morphological characters was developed and used to determine which are the Horismenus species associated with Acanthoscelides spp. and Zabrotes spp. on Phaseolus beans. Subsequently, molecular markers (microsatellites and mitochondrial DNA) were used to determine the phylogenetic relationships and degree of genetic similarity between H. depressus Gahan, H. missourensis Ashmead and H. butcheri Hansson and Aebi, and to confirm the status of the third as a new and distinct species. These molecular markers were also used to determine the genetic population structure of parasitoids and to estimate the degree of gene flow among and within parasitoid populations. Despite their close taxonomic position, the three Horismenus species have specific ecological requirements allowing them to take advantage of different ecological niches scattered along the altitudinal gradient. H. depressus is the most abundant parasitoid below 1700 m and the only one found on P. lunatus, a bean common at low altitudes. H. missouriensis is the most abundant above 1700 m and the only parasitoid reared on P. coccineus, a bean growing at high altitudes. Finally, H. butcheri is the only species displaying strict host plant specialization as it was only collected on P. vulgaris seeds, with the notable exception of samples collected on P. coccineus in sympatry with P. vulgaris. The strength of the parasitoid-host plant association is also very different among Horismenus species. Although all species displayed preference for their host plant of origin, host plants only had an effect on the population genetic structure of one species of parasitoid. Two very distinct genetic groups were detected for H. butcheri individuals that emerged from different plant species in sympatry, strongly suggesting that plants can mediate the formation of host races at the third trophic level in this system.

This work not only has an importance for a fundamental understanding of parasitoid ecology and evolution, but also has an applied significance. The understanding of how herbivore and parasitoid populations are spatially and temporally structured has broad implications for the theory and application of biological control. A natural enemy's long-term success in controlling a pest species is contingent with its ability to adapt to novel conditions.