Untangling the Components of Phenotypic Plasticity in Nonlinear Reaction Norms of Drosophila Mediopunctata Pigmentation

Abstract

AbstractPhenotypic plasticity may evolve as a generalist strategy to cope with environmental heterogeneity. Empirical studies, however, rarely find results confirming this prediction. This may be related to constraints imposed by the genetic architecture underlying plasticity variation. Three components of plasticity are central to characterize its variation: the intensity of response, the direction of response and the total amount of change. Reaction norm functions are a key analytical tool in plasticity studies. The more complex they are, the more plasticity components will vary independently, requiring more parameters to be described. Experimental studies are continuously collecting results showing that actual reaction norms are often nonlinear. This demands an analytical framework – yet to be developed – capable of straightforwardly untangling plasticity components. In Drosophila mediopunctata, the number of dark spots on the abdomen decreases as a response to increasing developmental temperatures. We have previously described a strong association between reaction norm curvature and across-environment mean values in homozygous strains. Here, we describe seven new reaction norms of heterozygous genotypes and further the investigation on the genetic architecture of this trait’s plasticity, testing three competing models from the literature – Overdominance, Epistasis and Pleiotropy. We use the curves of localized slopes of each reaction norm – Local Plasticity functions – to characterize the plastic response intensity and direction, and introduce a Global Plasticity parameter to quantify their total amount of change. Uncoupling plasticity components allowed us to discard the Overdominance model, weaken the Epistasis model and strengthen the support for the Pleiotropy model. Furthermore, this approach allows the elaboration of a coherent developmental model for the pigmentation of D. mediopunctata where genetic variation at one single feature explains the patterns of plasticity and overall expression of the trait. We claim that Global Plasticity and Local Plasticity may prove instrumental to the understanding of adaptive reaction norm evolution