Genotype-environment interaction and the ontogeny of diet-induced phenotypic plasticity in size and shape of Melanoplus femurrubrum (Orthoptera : Acrididae)

The developmental origin of phenotypic plasticity in morphological shape ca n be attributed to environment-specific changes in growth of overall body s ize, localized growth of a morphological structure or a combination of both . I monitored morphological development in the first four nymphal instars o f grasshoppers (Melanoplus femurrubrum) raised on two different plant diets to determine the ontogenetic origins of diet-induced phenotypic plasticity and to quantify genetic variation for phenotypic plasticity. I measured di et-induced phenotypic plasticity in body size (tibia length), head size (ar ticular width and mandible depth) and head shape (residual articular width and residual mandible depth) for grasshoppers from 37 full-sib families rai sed on either a hard plant diet (Lolium perenne) or a soft plant diet (Trif olium repens). By the second to third nymphal instar, grasshoppers raised o n a hard plant diet had significantly smaller mean tibia length and greater mean residual articular width (distance between mandibles adjusted for bod y size) compared with full-sibs raised on a soft plant diet. However, there was no significant phenotypic plasticity in mean unadjusted articular widt h and mandible depth, and in mean residual mandible depth. At the populatio n level, development of diet-induced phenotypic plasticity in grasshopper h ead shape is mediated by plastic changes in allocation to tissue growth tha t maintain growth of head size on hard, low-nutrient diets while reducing g rowth of body size. Within the population, there was substantial variation in the plasticity of growth trajectories since different full-sib families developed phenotypic plasticity of residual articular width through differe nt combinations of head and body size growth. Genetic variation for diet-induced phenotypic plasticity of residual articu lar width, residual mandible depth and tibia length, as estimated by genoty pe-environment interaction, exhibited significant fluctuation through ontog eny (repeated measures MANOVA, family x plant x instar, P < 0.01). For exam ple, there was significant genetic variation for phenotypic plasticity of r esidual articular width in the third nymphal instar, but not earlier or lat er in ontogeny. The observed patterns of genetic variation are discussed wi th reference to short-term constraints and the evolution of phenotypic plas ticity.