Adaptive decoupling in the evolution of complex life-histories

Dartmouth College, Hanover NH

Investigators

Abstract

All living things change and grow through time. Some of these changes are more dramatic than others. The changes that amphibians (e.g., frogs and salamanders) undergo during development are some of the most dramatic of any of the vertebrates. Metamorphosis is the process by which an amphibian transforms from an animal that lives under water, to one that lives on land. This incredible transition involves reorganizing almost the entire animal's body plan. Many species of amphibians absorb their tails, grow legs for moving on land, and their external gills disappear as they begin breathing air. All of these amazing changes are controlled by a single set of genes. This project will explore the various ways that natural selection has sculpted the amphibian genome to build these two vastly different types of animal bodies with the same sets of genes. Using the wood frog (Rana sylvatica) as a study system, the researchers will explore the role of different environmental factors that can influence the development of both tadpoles and adult frogs. In doing so, the research will determine how the environment interacts with gene expression to influence survival and mating success of wood frogs. Metamorphosis is a fascinating process to watch and provides opportunities for study at multiple age levels. The researchers will involve elementary school students, high school students, and both graduate and undergraduate researchers in the studies. Ultimately, this project will make important contributions to understanding how natural selection influences genes and genomes to make animals that are well suited to multiple environments. A central problem in the study of life history evolution is to understand how tradeoffs play a role in structuring natural populations. This project investigates potential tradeoffs between traits expressed during alternative life stages (i.e., tadpoles versus adults) in wood frogs, Rana sylvatica. For example, tradeoffs may arise across the life history of anurans because their complex life history requires dramatically different forms of adaptation to match changes in ecology, morphology, and behavior following metamorphosis. However, such tradeoffs could be alleviated if genetic correlations are broken apart during metamorphosis (so called "adaptive-decoupling"), or if traits are subject to similar forms of natural selection throughout the life history. Although the idea of adaptive-decoupling is not new, we still understand surprisingly little about the degree to which anuran life-stages are coupled and how this might influence adaptation in each life stage. This project will test for differences in the forms of natural selection between tadpoles and frogs using a combination of semi-natural mesocosms (tadpoles) and field enclosures (frogs). The degree of adaptive-decoupling will be measured using a half-sib breeding design to quantify genetic correlations within and between life-stages. Field surveys of natural populations that vary in predation intensity and timing (e.g., predation at larval and/or adult life stages) will test the prediction that natural selection and adaptive-decoupling interact to influence local adaptation. Further, the researchers will test whether shifts in gene expression and natural selection interact to influence traits important for mating success.