The ecology of diet expansion in a seed-feeding beetle: Pre-existing variation, rapid adaptation and maternal effects?

Abstract

Most studies of insect dietary evolution suffer from limited information on the history of host use patterns; historical patterns can only be inferred from modern patterns. We examine the ecology of a recent diet breadth expansion onto an introduced host by the seed beetle, Stator limbatus, to (1) determine if pre-existing variation for survival on Chloroleucon ebano is present in natural populations of S. limbatus, and (2) test whether natural selection has resulted in local adaptation to C. ebano in a population where this host is used. Our results indicate that variation in survival on C. ebano does exist in natural populations of S. limbatus that have historically never encountered this host, providing the variation necessary for adaptation to this host. However, we found no evidence that S. limbatus have locally adapted to C. ebano. Our most interesting discovery, however, was that the use of C. ebano by S. limbatus is facilitated by non-genetic effects of parental host plant on progeny survival; parents reared on Cercidium floridum produce progeny with substantially higher survivorship on C. ebano than parents reared on Acacia greggii due to an environmentally based parental effect. We argue that such host-plant-mediated maternal effects are likely to be common and thus important for our understanding of herbivorous insect evolution and population dynamics. This paper provides one example of how an understanding of environmentally based maternal effects can provide important information on the evolution of life-history patterns observed in nature.