Radiation, biological diversity and host-parasite interactions in wild roses and insects

The insect fauna associated with a particular plant species depends on the available species pool of phytophages, the distribution and abundance of the host species, the number of feeding niches as well as the host’s taxonomic isolation and biochemical make-up. Variations in plant traits can influence the abundance of herbivorous insects as well as the structure and dynamics of associated herbivore communities. Even plant traits encoded by few genes may have important effects on the community of plant exploiters, as suggested by the concept of the “extended phenotype”. Radiation processes in plant species provide a variety of new plant traits in a short time period and morphological differences in closely related species. Differences in morphological traits could result in host preferences of herbivorous insects and, as a consequence, in adaptations to certain host species. These adaptations may lead to genetically differentiated populations of exploiters living in sympatry. Therefore, the most important question in interacting systems is: How do radiation and diversity of the hosts translate into the radiation and diversity of the exploiters? Similarly important for the understanding of interactions between species is the understanding of geographical structuring of host and exploiter species. This includes to which extent interacting species like parasitoid and host populations are structured on similar spatial scales and whether such structures are influenced or generated by the distribution of specific food availability like host plants or other features of their environment. Dog roses and their exploiters were chosen as model system to study the effects of a rapid radiation of hosts on dependent herbivores because they underwent a recent radiation process during the Pleistocene and are widespread in Europe. The available information on phytophages on roses shows that there is a large number of generalists attacking roses, but also many specialists for example the cynipid wasp Diplolepis rosae L. (Hym.) and the tephritid flies Rhagoletis alternata (Fallén 1814) and Carpomya schineri (Loew 1856). Within the dog rose system I analysed, host preferences of D. rosae and its associated community, host-associated genetic differentiation of D. rosae, comparative phylogeographies of D. rosae and two of its most common parasitoids, and genetic differentiation of Rh. alternata and C. schineri. Overall, the genetic variability and diversity of dog roses is not translated into a host-specific radiation process of the dependent insect fauna. Furthermore, the geographical distribution of the analysed insect species showed discordant patterns. We conclude that the ongoing hybridisation within the genus Rosa may prevent preferences and adaptations of exploiters to one specific host plant.