Trait performance correlations across life stages under environmental stress conditions in the common frog, Rana temporaria

Larval life history and anti-predator strategies are affected by breeding phenology in an amphibian

Seasonal time constraints can pose strong selection on life histories. Time-constrained animals should prioritise fast development over predation risk to avoid unfavourable growing conditions. However, changes in phenology could alter the balance between anti-predator and developmental needs. We studied variation of anti-predator strategies in common frog (Rana temporaria) tadpoles in four populations from the two extremes of a latitudinal gradient across Sweden. We examined, under common conditions in the laboratory, the anti-predator responses and life histories of tadpoles raised with predatory Aeshna dragonfly larvae in two consecutive years with a difference of 20 days in breeding time in the north, but no difference in breeding time in the nouth. In a year with late breeding, northern tadpoles did not modify their behaviour and morphology in the presence of predators, and metamorphosed faster and smaller than tadpoles born in a year with early breeding. In the year with early breeding, northern tadpoles showed a completely different anti-predator strategy by reducing activity and developing morphological defences in the presence of predators. We discuss the possible mechanisms that could activate these responses (likely a form of environmentally-mediated parental effect). To our knowledge, this is the first study to show that a vertebrate modifies the anti-predator strategy of its offspring in response to natural variation in reproductive phenology, which highlights the need to consider phenology in studies of life-history evolution.

Potential for evolutionary coupling and decoupling of larval and adult immune gene expression

Almost all studies of the immune system of animals with metamorphosis have focused on either larval or on adult immunity, implicitly assuming that these traits are either perfectly correlated or evolutionarily independent. In this study, we use 80 crosses among 21 Drosophila melanogaster lines to investigate the degree and constancy of genetic correlation in immune system activity between larvae and adults. The constitutive transcription of Diptericin, a gene encoding a defensive antimicrobial peptide, was controlled by the same genetic factors in larvae and adults, with variation in expression determined exclusively by nonadditive genetic effects. This contrasted with another peptide-encoding gene, Drosomycin, in which larval transcription was highly variable and determined by additive effects but adult transcription genetically invariant. We found no evidence for a fitness cost to the transcription of these genes in our study. The shared genetic control of larval and adult Diptericin transcription stands in contrast to predictions of the adaptive decoupling hypothesis, which states that distinct life-stages should permit the independent evolution of larval and adult phenotypes. Importantly, genetic correlations between larval and adult immunities imply that parasite pressure on one life-stage can drive the evolution of immunity (and resistance) in the other life-stage.