Genetic mapping with SNP markers in Drosophila.

Map-based positional cloning of Drosophila melanogaster genes is hampered by both the time-consuming, error-prone nature of traditional methods for genetic mapping and the difficulties in aligning the genetic and cytological maps with the genome sequence. The identification of sequence polymorphisms in the Drosophila genome will make it possible to map mutations directly to the genome sequence with high accuracy and resolution. Here we report the identification of 7,223 single-nucleotide polymorphisms (SNPs) and 1,392 insertions/deletions (InDels) in common laboratory strains of Drosophila. These sequence polymorphisms define a map of 787 autosomal marker loci with a resolution of 114 kb. We have established PCR product-length polymorphism (PLP) or restriction fragment-length polymorphism (RFLP) assays for 215 of these markers. We demonstrate the use of this map by delimiting two mutations to intervals of 169 kb and 307 kb, respectively. Using a local high-density SNP map, we also mapped a third mutation to a resolution of approximately 2 kb, sufficient to localize the mutation within a single gene. These methods should accelerate the rate of positional cloning in Drosophila.

Dispatched, a novel sterol-sensing domain protein dedicated to the release of cholesterol-modified hedgehog from signaling cells.

Members of the Hedgehog (Hh) family of secreted signaling proteins function as potent short-range organizers in animal development. Their range of action is limited by a C-terminal cholesterol tether and the upregulation of Patched (Ptc) receptor levels. Here we identify a novel segment-polarity gene in Drosophila, dispatched (disp), and demonstrate that its product is required in sending cells for normal Hh function. In the absence of Disp, cholesterol-modified but not cholesterol-free Hh is retained in these cells, indicating that Disp functions to release cholesterol-anchored Hh. Despite their opposite roles, Disp and Ptc share structural homology in the form of a sterol-sensing domain, suggesting that release and sequestration of cholesterol-modified Hh may be based on related molecular pathways.