Recent advances in molecular genetic techniques have made it possible to conduct broad-scale comparisons of genome organization, structure and function among taxa. Here, we test for synteny (conservation of linkage relationships) between Drosophila melanogaster (Meigen) and Rhagoletis pomonella (Walsh) flies in different superfamilies of Schizophora estimated to have diverged ≈50–55 mya. Our strategy involved searching GenBank to identify Drosophila homologues to sequenced and mapped Rhagoletis clones isolated from a cDNA library. Linkage maps for the homologues were compared between the flies using release I (CD-ROM version) of the D. melanogaster genome project. We report evidence for broad-scale synteny between Drosophila and Rhagoletis. The entire right arm of chromosome 3 in Drosophila was comprised of genes mapping to linkage group II in Rhagoletis, while the left arm contained only Rhagoletis linkage group I markers. In addition, five of the six homologues identified on the Drosophila X chromosome were genes from linkage group III in Rhagoletis, while the left arm of Drosophila chromosome 2 was mainly composed of Rhagoletis linkage group V loci. Although gene order was not entirely conserved within linkage blocks displaying synteny, there were instances of possible co-linearity, some of which involved regions of the genome associated with sympatric host race formation and speciation in R. pomonella. Much more detailed mapping studies of R. pomonella are needed to confirm this, however, as comparisons among Drosophila subgenera have revealed extensive differences in gene order due to fixed paracentric inversions. Nevertheless, our finding of broad-scale synteny between R. pomonella and D. melanogaster suggests that a comparative genomics approach could be useful for addressing a number of basic and applied issues in Diptera, including speciation, systematics and biocontrol.
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Vol. 94 • No. 6