A FRAMEWORK PHYSICAL MAP OF DROSOPHILA-VIRILIS BASED ON P1 CLONES - APPLICATIONS IN GENOME EVOLUTION

The analysis of patterns of genome evolution may help to evaluate the evolutionary forces that shape the composition and organization of the genome. Comparisons between the physical maps of divergent species ca n be used to identify conserved blocks of closely linked genes whose s ynteny is possibly under selective constraint. We have used in situ hy bridization to determine the genomic position of 732 randomly selected clones from a bacteriophage P1 library of Drosophila virilis. The res ulting map includes at least one clone in each of 69% of the subdivisi ons into which the D. virilis polytene chromosomes are divided. A subs et of these clones was used to carry out a comparative physical analys is of chromosome 2 from D. virilis and from Drosophila montana. A numb er of discrepancies with the classical scenario of chromosome evolutio n were noted. The D. virilis P1 clones were also used to determine the physical relations between ten genes that are located in the X chromo some of Drosophila melanogaster between the markers crn (2F1) and omb (4C5-6). In this region, which is approximately 2 Mb in length, there have been at least six breakpoints since the divergence of the species , and six of the genes are found at widely scattered locations in the D. virilis X chromosome. However, a block of four functionally unrelat ed genes, including white, roughest, Notch, and dunce, seems to be con served between the two species.