Funds are requested for a study of how oviposition preference and specificity evolve in phytophagous insects during shifts onto new hosts. Current theory for host choice in insects holds that ovipositing females exhibit a preference hierarchy among potential host plants and according to the rank order in the hierarchy. Host shifts then begin either as (a) genetic shifts in the preference hierarchy, (b) genetic changes that decrease the degree of specificity to highly ranked hosts and thereby broaden the range of hosts, or (c) colonization of plants lower on the preference hierarchy in the absence of more highly ranked plants with little or no genetic change in preference. Recent work on two closely-related swallowtail butterfly species, Papilio zelicaon and P. oregonius, that feed on different plant families showed that these species exhibit quite different preference hierarchies. Preliminary studies of other populations within this species complex, however, indicate that some host shifts may not involve complete rearrangements in preference hierarchy but instead reflect changes in degree of specificity or behavioral responses to available hosts. This variation provides an opportunity to ask how much rearrangement of the preference hierarchy has occurred as populations in the P. machaon species complex have diversified onto different host plants. Therefore, the proposed work will ask if the conclusions on the evolution of oviposition preference drawn from interspecific differences between P. zelicaon and P. oregonius apply to population differentiation and speciation throughout the P. machaon species complex in North America. Objective 1 will ask how umbellifer-feeding species of the P. machaon complex differ in their preference hierarchy for plant species. That is, has colonization of new hosts always involved a reordering of the preference hierarchy and, if not, which hosts have required such a reordering? Objective 2 will ask if populations of P. zelicaon differ in their preference hierarchy. Papilio zelicaon populations differ geographically in their host plants. Consequently, it is possible to compare the interspecific results from objective 1 with intraspecific results in this section. Finally, Objective 3 will ask whether differences in preference hierarchy among populations of P. zelicaon are inherited as X- linked loci as are the interspecific difference between P. zelicaon and P. oregonius. The recent finding that the major interspecific differences are primarily X-linked provides for the first time the opportunity to compare the mode of inheritance of intraspecific divergence in host preference with that observed for interspecific divergence. Thompson Together, the results for the three objectives will aid in the development of the theory of insect/plant interactions by addressing the fundamental problem of how oviposition preference diverges as insect populations colonize new host plants. Since much of insect diversity appears to arise from divergence onto different host species, the proposed work therefore also addresses the problem of the origins of insect diversity.
