Identification of the gene responsible for host-plant selection in Drosophila sechellia

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Inter-specific hybrids between Drosophila sechellia and deficiency mutants of D. melanogaster were tested for their preference to caproic acid. Three deficiency strains showed caproic acid preference in this test. Available cytological data, however, did not support the existence of overlapping region among these three deficiencies. Since cytological data are not precise enough, we examined the break points of these deficiencies at molecular level. PCR markers revealed the existence of the overlapping region, and it sizes 80kb. According to the genome project data on D. melanogaster, this region contains 22 predicted genes. Using RT-PCR, we confirmed that all of these genes are expressed in D. sechellia, suggesting that the responsible genetic alteration in D. sechellia affects on the structure of the gene product rather than its expression pattern. Assuming synteny between D. sechellia and D. melanogaster in this region, we screened D. sechellia cosmid library to clone the corresponding genomic fragment, and obtained two independent clones that cover most of the target region. Complete sequence was determined by shotgun sequencing, and was compared with D. melanogaster genomic sequence to identify the responsible gene.