Ecologically dependent postmating isolation between sympatric host forms of Neochlamisus bebbianae leaf beetles

Ecological speciation is the promotion of reproductive isolation via the divergent adaptation of populations to alternative environments. A prediction peculiar to ecological speciation is that hybrids between such populations should be adapted poorly to parental environments, yielding reduced fitness and postmating isolation. However, F-1 analyses alone cannot demonstrate that ecological ("extrinsic') factors contribute to such isolation. Rather, this requires documenting a "switch" in the relative fitnesses of reciprocal backcrosses between environments. Specifically, each backcross should exhibit higher fitness in the environment of its pure parent, with which it shares the most genes, including environment- specific ones. In contrast, because genetic proportions are expected to be similar for all backcrosses (approximate to 3/4 from one parental type and approximate to 1/4 from the other), the more general genetic incompatibilities responsible for "intrinsic" isolation predict no such environment- specific fitness switches. Thus, although intrinsic isolation may contribute to the fitness reduction and variation underlying such patterns, it offers an insufficient explanation for them. Here, we present a quantitative genetic "backcross" analysis of sympatric Neochlamisus bebbianae leaf beetle populations adapted to maple versus willow host plants. Results statistically supported ecological speciation predictions, notably the switch in relative fitness for backcross types, the expected rank order of cross type fitnesses, and appreciable extrinsic isolation. We additionally documented genetic variation in host-associated fitness, ruled out nongenetic maternal effects, and discuss the maintenance of ecological differentiation in sympatry. In summary, our study provides a rare and strongly supported demonstration of genetically based, ecologically dependent postmating isolation during ecological speciation.