INSECT BEHAVIOR & EVOLUTION
We conduct experiments in insect behavior, genetics, and population biology. We are especially interested in the evolution of host range among herbivorous insects. For example, which traits are most likely to mediate shifts to new plant species, especially in cases where novel hosts are distantly related to ancestral ones? Are there trade-offs associated with adaptation to a new host, and do such trade-offs account for the relatively narrow diets of most insect herbivores? Greater understanding of the processes underlying insect host shifts can help explain the occasional rapid colonization of new crops by pest species. Host shifts are also thought to mediate the process of speciation in many groups of insect herbivores.
We use the seed beetle Callosobruchus maculatus as a model organism. The laboratory environment is an unusually close approximation of the "natural" habitat of C. maculatus, which has infested human stores of grain legumes for thousands of years. Because of their short generation time and ease of rearing, seed beetles are excellent subjects for performing experimental evolution. We carry out selection experiments in which replicate populations are shifted to novel environments, and beetle responses are measured with respect to a suite of behavioral, morphological, physiological, and life-history traits. Line crosses can then establish the genetic architecture underlying recently diverged traits.
Given their cosmopolitan distribution, seed beetles also provide a good system for studying population differentiation. We have been especially interested in the evolution of larval competitive behavior, which can be strikingly different between interfertile populations. Female oviposition behavior also varies widely, including the tendency of egg-laying females to avoid occupied seeds (and thus minimize the degree of competition experienced by their offspring). Seed beetles also provide a good system for investigating the magnitude of inbreeding depression in insect populations. Few studies have examined the effects of inbreeding on insect behavior outside the context of mating success. We investigated the effect of moderate inbreeding on female oviposition choices, along with other fitness-related traits (body mass, fecundity, and egg hatch) in the same population. Finally, we have examined the mating compatibility of divergent beetle populations in order to detect incipient stages of pre- or post-copulatory isolating mechanisms.
Seed-beetle experiments make everyone happy; maybe it's because we work in such diverse plant communities!
A current project examines beetle adaptation to an extremely poor host - lentil. Initial assays indicated that larval survival in lentil was about 1%, and a substantial number of females did not recognize lentil as an oviposition host. Nevertheless, three independent bouts of mass selection produced lines with >60% larval survival within only 5-8 generations, and >90% survival in 25-30 generations. We performed crosses between lentil lines (>90% survival) and a line maintained on the ancestral host. Analyses of F1, F2, and backcross hybrids indicated both additive and non-additive genetic components to the ability to survive in lentil. In addition, both artificial and quasi-natural selection produced significant increases in the tendency to oviposit on lentil. We are using a variety of approaches to understand the specific changes in larval physiology and adult behavior that mediated this radical host shift.
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