Identifying the genes that confer evolutionary adaptation to environmental temperature in an ectotherm

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Many animals grow to be larger under cold conditions. For instance, warm-blooded animals such as species of birds and mammals often have larger body size in the colder parts of their geographical range. This pattern makes sense because it is easier for a large animal to keep warm, since it has a larger volume of heat-producing tissue in relation to the area of the body surface over which heat is lost to the environment. However, it has become apparent that cold-blooded creatures too become large in the cold, and the aim of this project is to find out why. This is an important topic, because global climate change will expose animals to changing temperature and we need to understand how they will respond. Food animals such as fish are cold-blooded and our work will be directly relevant to understanding how their growth efficiency is altered by temperature. To do this, we shall focus on a small animal, the fruit fly Drosophila, which has a wide geographic range. Fruit flies evolve to be larger at higher latitudes; experiments where flies from different populations are grown in the same environment shows that they differ genetically in size. Temperature seems to be a critical variable in the evolution of larger body size at higher latitudes, because experiments where flies are allowed to evolve for several years at different temperatures in the laboratory also result in genetically larger body size at lower culture temperatures. In addition, flies from the same population that grow up to adulthood at different temperatures produce larger adults with growth at lower temperatures, showing that there is a direct, environmental effect of temperature on size. We shall discover the reasons for larger size in the cold. First, we shall identify the genes that produce larger body size at higher latitudes in nature. We shall do this by mapping their location in the genetic material and by looking for altered patterns of gene expression. Critically, we shall then confirm our identification by experimentally inducing the genetic variants that we think are critical and checking their effects on body size. We shall test the importance of temperature in the evolution of body size, by measuring the performance of flies with the 'hot' and 'cold' genetic variants at different temperatures. We shall also find out which genes show altered expression during growth at different temperatures. By identifying the genes and pathways that control changes in body size in response to temperature, we shall start to understand the reasons why cold-blooded animals become larger in the cold.