Abstract
A host race is a population which is partially reproductively isolated as a direct consequence of adaptation to a certain host. For host race formation to occur five conditions should be met. First of all, the populations should occur in sympatry, which means that they co-occur within the normal cruising range of the herbivore species. The second condition is that individuals with a different phenotype use different resources. Thirdly, some degree of host preference should be shown. The fourth condition is that fitness consequences are associated with the host preference. Finally, positive assortative mating is the fifth condition for host race formation. The main question addressed in this thesis is whether sympatric speciation via host race formation occurs in nature. This question was addressed by testing these five conditions in part of the Galerucella nymphaeae-complex, the water lily leaf beetle, living on Yellow and White waterlily (Nuphar lutea and Nymphaea alba, both Nymphaeaceae) and Amphibious bistort and Great waterdock (Polygonum ampbibium and Rumex hydrolapathum, both Polygonaceae). Furthermore, it was investigated whether morphological and ecological differences have led to reduced gene flow between sympatric populations living on different hosts. Finally, the taxonomic status of the host-associated populations was examined, using molecular DNA techniques. All five conditions for host race formation are met in G. nymphaeae. Furthermore, RAPD analysis revealed that gene flow is indeed limited between sympatric beetle populations living on Nymphaeaceae and Polygonaceae. Breeding experiments and ITS-1 sequence data show that all beetles studied belong to one biological species. Thus, G. nymphaeae provides an excellent example of host race formation in nature, in which all conditions were tested and met. Hence, host race formation, and eventually sympatric species, is possible in nature, even when none of the hosts is recently introduced
