Dr. Bier and colleagues will analyze the genetic mechanisms required for forming sharp boundaries in developing wings of the fruit fly where fluid carrying veins form. They will compare the mechanisms for defining these borders for two veins with the goal of extracting general principles for how such linear structures form during development. They will also ask whether similar mechanisms are likely to be used in a distantly related insect, the mosquito. These evolutionary studies should shed light on whether this process of boundary formation has changed or remained fixed during evolution. Since formation of boundaries is a general feature of development and includes processes such as induction of mesodermal (e.g., muscle) tissue and the formation of digits in vertebrate embryos, the researchers expect that many of the principles they uncover will be broadly relevant. In terms of Broader Impacts, the project includes an educational component and undergraduate students will actively participate in this research. In addition, Dr. Bier has used wing vein development as an example illustrating the principles of boundary formation in his textbook, "The Coiled Spring: How Life Begins", which is used by Dr. Bier in a course taught at the University of California-San Diego.
The goal of this project was to understand how the identities of distinct body structures are determined during development. For example, how are differences in the thumb and pinky on the human hand determined? The system we chose to use to address this larger question was the fruit fly in which we asked how are the differences between two wing veins are established. One gene knirps (kni) determines the identity of the second wing vein (L2), while another gene abrupt (ab) specifies the fifth wing vein (L5). These two genes become active (that is DNA becomes expressed as mRNA) in cells giving rise to their respective veins (e.g., kni in L2 and ab in L5). Our strategy was to switch the positions in which these two genes became active and then ask whether they would make an L5 vein in place of L2 and vice versa. We were able to complete half of this experiment namely activating the L5 specifying gene ab gene in the position of the L2 vein in a fly lacking activity of kni (effectively substituting ab for kni in L2). The result was that the ab gene could make a vein in the position of the L2 vein but it was intermediate in nature between L2 and L5 in that in had qualities of both veins in that they expressed a hybrid mix of genes that normally distinguish the L2 and L5 veins from each other. We are currently poised to address the second half of this experiment (substituting kni for ab in L5), which for a various technical reasons was experimentally more involved to accomplish. These studies reveal that multiple genetic inputs act in combination to determine the identity of the L2 vein, including the gene most responsible for its forming in the right location (kni) and other genes determining features such as whether a cell is in the anterior (L2 vein) or posterior (L5 vein) region of the wing. This result is likely to be typical for a variety of structures in many organisms (e.g., the digits in the human hand). We also developed several sophisticated genetic tools to allow us to precisely swap the kni and ab genes for each other in the two veins which in principle should enable others to perform similar experiments in a variety of experimental systems, thus broadening the potential realm of such comparisons.
