Genetic Analysis of Axon Guidance in Drosophila melanogaster

Due to its genetic manipulability and relatively short reproductive cycle, genetic screens are often carried out in the fruit fly, Drosophila melanogaster. Deficiency “kits” that cover the Drosophila genome with a minimum number of lines have been established by other groups to facilitate gene mapping. These kits cannot be systematically analyzed for many phenotypes, however, because embryos homozygous for many deficiencies fail to develop due to the loss of key gene products. To create new kits that can be screened for more phenotypes, we have examined the development of the nervous system in embryos homozygous for more than 700 distinct deficiency mutations. A kit of ~400 deficiency lines for which homozygotes have a recognizable nervous system and intact body walls encompasses >80% of the genome. Here we show examples of screens of this kit for orphan receptor ligands and neuronal antigen expression. Screens of this kit can also be used to find genes involved in expression, patterning, and subcellular localization of any protein that can be visualized by antibody staining. A subset kit of 233 deficiency lines, for which homozygotes develop relatively normally to late stage 16 (thus allowing for central nervous system development), covers ~50% of the genome. We have screened this smaller kit for motor axon guidance phenotypes, and we present examples of new axon guidance phenotypes in the central nervous system and neuromuscular system. Through screening of these kits, we also found deficiencies that fail to stain with monoclonal antibody BP102, which recognizes an unknown epitope on the proximal segments of central nervous system axons. In addition, we have found a deficiency that exhibits ectopic BP102 staining on peripheral sensory neurons. By defining the single genes under these deficiencies, we have obtained evidence that BP102 may recognize a chondroitin sulfate proteoglycan and that BP102 epitope expression is regulated by matrix metalloproteinase 1. Thus, in addition to this screen providing information about motor axon guidance in the embryo, we have also been able to further characterize an antibody that is frequently used by the Drosophila community