PLASTICITY AND CANALIZATION OF INSECT REPRODUCTION: TESTING ALTERNATIVE MODELS OF LIFE HISTORY TRANSITIONS

Abstract

Life histories may show phases of both plasticity and canalization in response to feeding rate. Models for life history canalization and plasticity postulate a threshold for initiation of canalized developmental events. Some models postulate adaptive plasticity, whereas others postulate nonadaptive plasticity that results from environmental modulation of fixed development. These models have been tested by changing feeding rate at various times and determining when timing of life history events becomes unresponsive to those changes. This approach has been criticized because putative thresholds are usually not known. We use an alternative experimental design to test models of reproductive plasticity and canalization, and to estimate thresholds, in the grasshopper Romalea microptera. We develop mathematical models for published verbal models that predict how life history timing changes with feeding rate. Alternative models predict distinct relationships of time to oviposition vs. mean food intake that we test via experimental manipulation of food intake and nonlinear regressions. Regressions yield estimates of both the threshold and the duration of post-threshold development. A model postulating a fixed threshold and canalized post-threshold development provides the best, most parsimonious fit to data for this grasshopper. Thus, the simplest model, postulating no adaptive variation in development, is supported, a result that is consistent with previous experiments on this system using changing feeding rates. We use the estimate of the threshold (in units of food eaten) and measurements of hemolymph protein content to estimate the threshold in units of physiologically relevant storage. These results elucidate the structure of reproductive plasticity in this system and how this alternative experimental approach can provide testable predictions of developmental thresholds for further experiments on life history plasticity and canalization.