Investigating the specification and evolution of organ size in Drosophila

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The organs of animals have to be the correct size so that they can function properly and yet during the course of evolution organ size can change between species. We have growing knowledge of how organ growth is controlled but a poor understanding of how final size is specified. Furthermore, in some case we understand how changes in cell number, size or shape result in evolutionary changes in organ size between species but the causative genes are not known or vice versa. We have found that the gene tartan (trn) helps to specify the size of Drosophila claspers, male genital organs of these flies with important roles in copulation, but that differences in trn expression have also evolved to change clasper size between D. mauritiana and D. simulans. This represents an excellent opportunity to understand the genetic and cellular bases of organ size on one hand and how these mechanisms evolve to result in differences in organ size on the other.Trn protein is expressed in cell membranes. On the outside of cells, Trn marks their identity and signals this information to neighbouring cells to respond. On the inside of cells, Trn interacts with other proteins to regulate cell identity and position, and transmits information from its binding to other cells. Through these molecular functions, Trn helps to form boundaries within and between developing organs, including body segments, eyes, wings, and legs as well as genitalia, to ensure cells are correctly located based on their identity. Our preliminary data suggest that Trn and another protein Tenascin-major (Ten-m) form a boundary that helps to determine clasper size. We have also found that clasper size is regulated by the transcription factor C15 potentially restricting Trn expression within the clasper and by Cpr66D in the extra-cellular matrix. However, we don't know how trn gene expression and Trn protein interactions with other proteins combine to control how many cells, of what size and in what location make claspers of the correct size nor how these mechanisms evolve to change cells and therefore organ size between species. Therefore, we will study and quantify cell boundaries, number, size, division and movement during clasper formation and how this is influenced by Trn in D. melanogaster. We will also explore how this is regulated through potential interactions between Trn, hairy, ten-M, C15 and Cpr66D, and identify new proteins that directly interact with Trn to better understand its function. These experiments will provide new knowledge about how clasper size is regulated by trn and other genes, inform how Trn functions in developing Drosophila genitalia as well as other tissues and other animals, and further our understanding of organ size specification more generally. Studying differences in traits between species can often provide new insights into how these traits develop as well the basis for their evolution. Therefore, to complement our experiments investigating clasper development in D. melanogaster, we will study how natural variation in Trn expression changes cells to cause evolutionary differences in clasper size between D. mauritiana and D. simulans. Furthermore, since our developmental candidate genes hairy, ten-M, C15 and Cpr66D all map to regions of the genome that underlie differences in clasper size between D. mauritiana and D. simulans, we will also directly test if and how they contribute to clasper size evolution to more fully understand the genetic and cellular bases of this organ size difference. Taken together, we anticipate that our synthesis of development and evolution will provide important new insights into organ size regulation and transform understanding of how the molecular mechanisms that control organ size evolve to change cells to build organs of different sizes between species.