Asymmetric evolution of egg laying behavior following reciprocal host shifts by a seed-feeding beetle
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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.
KeywordsCallosobruchus maculatus Egg dispersion Experimental evolution Oviposition behavior Seed size
We thank Bill Wallin, Elliot Campbell, Fariba Kanga, Anna Muncy and Daniel Sullivan for help running experiments. Jacqueline Dillard, Melise Lecheta, Allyssa Kilanowski, Josiah Ritchey, and Boris Sauterey provided comments on an earlier version of this manuscript. Rachel Zitomer participated in this project as part of a 10-week NSF-funded Research Experience for Undergraduates summer program at the University of Kentucky (summer 2013; NSF DBI-1062890). This work was funded in part by the Kentucky Agricultural Experiment Station and the Utah Agricultural Experiment Station (paper no. 8985).
CWF managed the selection experiment, quantified egg dispersion at 36 generations, and analyzed the data. RZ quantified egg dispersion (two experiments) at 50 generations and commented on the manuscript. JBD quantified egg dumping and commented on the manuscript. CWF and FJM co-wrote the manuscript.
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