Asymmetric evolution of egg laying behavior following reciprocal host shifts by a seed-feeding beetle

Colonization of new environments can lead to rapid changes in fitness-related traits. For herbivorous insects, switching to a new host plant can be comparable to invading a new habitat. Behavioral, physiological, and life-history traits commonly vary among insect populations associated with different plants, but how host shifts cause trait divergence is often unclear. We investigated whether experimental host shifts would modify a key insect trait, egg-laying behavior, in a seed beetle. Beetle populations associated long-term with either a small-seeded host (mung bean) or a large-seeded host (cowpea) were switched to each other’s host. After 36–55 generations, we assayed three aspects of oviposition behavior known to differ between the mung bean- and cowpea-adapted populations. Responses to the host shifts were asymmetrical. Females from lines transferred from mung bean to cowpea produced less uniform distributions of eggs among seeds, were more likely to add an egg to an occupied seed, and were more likely to “dump” eggs when seeds were absent. These lines thus converged toward the cowpea-adapted population. In contrast, the reciprocal host shift had no effect; oviposition behavior was unchanged in lines transferred from cowpea to mung bean. We suggest that these results reflect an asymmetry in the fitness consequences of each host shift, which in turn depended on differences in larval competitiveness in the original populations. Interactions among multiple fitness components are likely to make evolutionary responses less predictable in novel environments.