Phenotypic plasticity, genotype-by-environment interaction and the analysis of generalism and specialization in Callosobruchus maculatus

The inter-relationships between three approaches used to characterize phenotypic plasticity are analysed, and it is shown how these relate to the evolution of specialization. Specialization cannot evolve if there is no genotype-by-environment interaction, but specialization necessarily evolves if there is a negative between-environment genetic correlation. In the absence of a negative between-environment correlation specialization may evolve if there is some genotype-by-environment interaction, depending on the details of the genotype reaction norms — namely that those genotypes that do best in one environment do not do best in the other. These points are illustrated by analysing full-sib/half-sib data obtained from a population of Callosobruchus maculatus in two environments (30°C, 70% r.h. and 25°C, 45% r.h.). Phenotypic plasticity occurred in both body weight and development period. Genotype-by-environment interaction occurred in development period, but not body weight, producing some crossing-over in reaction norms between environments. However, the between-environment correlations of family means were positive for both body weight and development period. These results suggest that, with respect to body weight, the Callosobruchus genotypes are generalists, but the development period reaction norms have some characteristics of specialization (crossing-over of reaction norms and genotype-by-environment interaction) and also some of generalism (the between-environment correlation).