REACTION NORMS FOR METAMORPHIC TRAITS IN NATTERJACK TOADS TO LARVAL DENSITY AND POND DURATION

The evolution of environmentally-induced changes in phenotype or react ion norm implies both the existence at some time of genetic variation within a population for that plasticity measured by the presence of ge notype x environment interaction (G x E), and that phenotypic variatio n affects fitness. Otherwise, the genetic structure of polygenic trait s may restrict the evolution of the reaction norm by the lack of indep endent evolution of a given trait in different environments or by gene tic trade-offs with other traits that affect fitness. In this paper, w e analyze the existence of G x E in metamorphic traits to two environm ental factors, larval density and pond duration in a factorial experim ent with Bufo calamita tadpoles in semi-natural conditions and in the laboratory. Results showed no plastic temporal response in metamorphos is to pond durability at low larval density. The rank of genotypes did not change across different hydroperiods, implying a high genetic cor relation that may constrain the evolution of the reaction norm. At hig h larval density a significant G x E interaction was found, suggesting the potential for the evolution of the reaction norm. A sibship (#1) attained the presumed ''optimal'' reaction norm by accelerating develo pmental rate in short duration ponds and delaying it in longer ponds. This could be translated in fitness by an increment in metamorphic sur vival and size at metamorphosis in short and long ponds respectively w ith respect to non-plastic sibships. However, genetic variability for plasticity suggests that optimal reaction norm for developmental rates may be variable and hard to achieve in the heterogeneous pond environ ment. Mass at metamorphosis was not plastic across different pond dura tions but decreased at high larval density. Significant adaptive plast icity for growth rates appeared in environments that differed drastica lly in level of crowding conditions, both in the field and in the labo ratory. The fact that survival of juveniles metamorphosed at high dens ity ponds was a monotonic function of metamorphic size, implies that r esponse to selection may occur in this population of natterjacks and t hat genetic variability in plasticity may be a reliable mechanism main taining adaptive genetic Variation in growth rates in the highly varia ble pond environment.