Genetic Screens for Genes Essential in Morphogen Signaling Pathways
Drosophila provides a powerful model system to conduct genetic screens for genes essential for development and pattern formation. Formation of Drosophila wing is controlled by three morphogen molecules (Wg, Hh and Dpp). Mutations interfering with the functions of these morphogens will produce specific defects in the wing. Traditional genetic screen requires two generations to examine the effects of specific mutants in development. Therefore, it is very laborious and not efficient. We have developed and conducted a large scale F1 genetic screen using the 'direct mosaic' system to isolate genes involved in wing patterning (Belenkaya et al., 2002; Han et al., 2004). This screen utilizes GAL4-UAS system to control the expression of the yeast recombinase, flipase (Flp). A Vg-Gal4 line that is specifically expressed in wing imaginal disc is used to drive a high level of expression of Flp in the wing imaginal disc. We have examined about a total of 400,000 mutant chromosomes and have finished screens on both 2nd and 3rd chromosomes, which contain about 80% of genes in Drosophila genome. This screen led to identification of many important genes involved in wing patterning including pygopus (Pygo) which encodes a nuclear protein specifically involved in Wg signaling (Belenkaya et al., 2002). We have also identified brother of ttv (botv) and sister of ttv (sotv), two Drosophila members of human EXT tumor suppressor (Han et al., 2004). Both Botv and sotv are required for the distribution of Wg, Dpp and Hh morphogen molecules (Han et al., 2004).
Currently, we are further characterizing mutants isolated from this screen. It is anticipated that many of them encode molecules involved in Wg, Hh and Dpp signaling pathways. Analyses of these mutants will further help us to elucidate molecular mechanisms of morphogen gradient formation and their signal transduction processes.
