Fig. P1.

(A) Transcriptional response by members of four gene families implicated in host response (bean vs. tomato, horizontal) and pesticide response (susceptible vs. resistant strains, vertical). Abbreviations: ID-RCDs, intradiol ring-cleavage dioxygenases; MFS, major facilitator superfamily; P450s, P450 monooxygenases. (B) Model for rapid evolution of pesticide resistance in generalist herbivores compared with specialists (adapted from ref. 5). Preadaptation to multiple host plants is postulated to increase polymorphism in environmental responses, leading to several subsets of alleles. The initial stages of selection by a pesticide mimic those of a host plant shift, rapidly selecting the best-adapted subset of environmental response alleles and providing a larger population from which a rare (high-) resistance allele can be selected, thus accelerating the development of agriculturally significant resistance. A similar transcriptome signature occurs after both types of selection (i.e., host plant shift and pesticide) because it is drawn from a similar subset of genotypes.