Seven Wnt homologues in Drosophila: a case study of the developing tracheae.
Sequencing of the Drosophila genome has revealed that there are "silent" homologues of many important genes-family members that were not detected by classic genetic approaches. Why have so many homologues been conserved during evolution? Perhaps each one has a different but important function in every 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 family 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 trachea but is not responsible for all Wnt functions there. We test each one of the seven DWnts in several ways and find evidence that wg and DWnt2 can function in the developing trachea: when both genes are removed together, the phenotype is identical or very similar to that observed when the Wnt pathway is shut down. DWnt2 is expressed near the tracheal cells in the embryo in a different pattern to wg but is also transduced through the canonical Wnt pathway. We find that the seven DWnt genes vary in their effectiveness in specific tissues, such as the tracheae, and, moreover, the epidermis and the tracheae respond to DWnt2 and Wg differently. We suggest that the main advantage of retaining a number of similar genes is that it allows more subtle forms of control and more flexibility during evolution.