Host plant effects on the ecology and genetics of a tri-trophic interaction: The case of beans, bruchids, and their parasitoids

Classical examples of host race formation and possible sympatric speciation come from herbivorous insects, whereby an important role is attributed to the plants on which the herbivores feed.  In these examples, a host plant switch or specialization is presumed to have lead to reproductive isolation and restricted gene flow among the segregating groups.  An increasing number of studies show that plants also play a key role in mediating the interactions between herbivores and their natural enemies.  Hence, we propose that genetic population structure for instance of parasitoids of insect herbivores can be correlated with the plant genotypes on which the parasitoids find their hosts.

We investigated the role of host plants on the genetic population structure of the bruchids and three species of parasitoids in the genus Horismenus, which can be found on three bean species of the genus Phaseolus in Mexico.  Bruchids and parasitoids that emerged from beans were collected from different populations in Mexico and they were used for genetic analyses, using microsatellites and mitochondrial DNA. 

For bruchids we found that most of the between-population genetic differentiation was explained by geographic distance and not by variation in their host plant. However, isolation by distance was only evident for populations originated from wild beans and not from cultivated beans.

The three parasitoid species all showed some degree of isolation by distance.  Moreover, for one of the species it was found that individuals collected from one bean species were genetically distinguishable from individuals from another bean species, also within populations. In an olfactometer assay in which these parasitoids were allowed to choose between their plant of origin and alternative plants, parasitoids preferred their plant of origin. These results suggest that plants can contribute to the genetic structuring of the parasitoid populations, which may even lead to host race formation.