Quantitative genetics of signal evolution: a comparison of the pheromonal signal in two populations of the cabbage looper, Trichoplusia ni.

Pheromones are important in reproductive isolation among populations of moths, but the genetics associated with diversification of pheromonal signals is poorly understood. To gain insight into processes that may lead to diversification we examined the genetic architecture underlying the production of the sex pheromone of the cabbage looper moth, Trichoplusia ni. We compared genetic parameters of two populations; one with a wild-type pheromone phenotype (N) and one where a single-gene mutation affecting the pheromone blend produced by females had been established (M). Using a half-sib breeding design we estimated heritabilities, coefficients of additive genetic variation, and phenotypic, genetic, and environmental correlations of the pheromone components. In both populations, narrow sense heritabilities were generally moderate and genetic correlations were mostly positive. Comparisons between the two populations showed that, while the pattern of phenotypic correlations showed significant agreement between populations, the patterns of genetic (co)variation (i.e. the shapes of the within population matrix) were dissimilar between the two populations. The presence of additive genetic variation in both populations indicates that there is the potential for further evolution of individual pheromone components. However, because of the differences between the populations in the pattern of genetic variation and covariation, the populations will evolve along different evolutionary trajectories even under identical selection pressures. These results suggest that single gene mutations, once established, can be associated with further alterations in the genetic architecture and this has implications for the evolution of pheromone communication.