Seven Wnt homologues in Drosophila: A case study of the developing tracheae

Sequencing of the Drosophila genome has revealed that there are "silent" ho mologues of many important genes-family members that were not detected by c lassic genetic approaches. Why have so many homologues been conserved durin g evolution? Perhaps each one has a different but important function in eve ry system. Perhaps each one works independently in a different part of the body. Or, perhaps some are redundant. Here, we take one well known gene fam ily and analyze how the individual members contribute to the making of one system, the tracheae. There are seven DWnt genes in the Drosophila genome, including wingless (wg). The wg gene helps to pattern the developing trache a but is not responsible for all Writ functions there. We test each one of the seven DWnts in several ways and find evidence that wg and DWnt2 can fun ction in the developing trachea: when both genes are removed together, the phenotype is identical or very similar to that observed when the Writ pathw ay is shut down. DWnt2 is expressed near the tracheal cells in the embryo i n a different pattern to wg but is also transduced through the canonical Wn t pathway. We find that the seven DWnt genes vary in their effectiveness in specific tissues, such as the tracheae, and, moreover, the epidermis and t he tracheae respond to DWnt2 and Wg differently. We suggest that the main a dvantage of retaining a number of similar genes is that it allows more subt le forms of control and more flexibility during evolution.