Experimental Character Displacement in the Adzuki Bean Weevil, Callosobruchus Chinensis

Abstract

Laboratory populations of C. chinensis in the presence of interspecific competition evolve differently from populations in its absence. A strain of C. chinensis with high genetic variation (hereafter hvC) was reared for nine generations in competition with C. maculatus (Fujii lab aQ strain). Two types of beans were used as resources, mungbean (Vigna radiata) and lentil (Lens culinaris). Both beans were novel to both strains. Initially both beetle strains grew extremely well on mung. Lentil, on the other hand, was essentially lethal to aQ and severely reduced the emergence of hvC and retarded its development. Control treatments included hvC alone on both lentil and mung, hvC alone on lentil, hvC alone on mung, and a hard selection treatment in which hvC was allowed to oviposit on both mung and lentil but all mung were discarded before beetle emergence. At the end of the experiment, no difference was detectable in oviposition preference among the hvC from any of treatments. On the other hand, in all treatments where it was exposed to lentil, hvC increased its physiological adaptation to lentil, as expressed by changes in the probability of emergence, development time, and size at emergence (size is strongly correlated to fecundity). The increase in adaptation in the competition treatment was significantly greater than that in the treatment where hvC was raised on lentil and mung without interspecific competition. Furthermore, there was little difference between the competition treatment and the hard selection or lentil alone treatments, indicating the potency of competition as an evolutionary force.