A physical map of chromosome 4 from Drosophila melanogaster. J. Locke , L. Podemski , N. Aippersbach , H. Kemp , R. Hodgetts. Dept. Biological Sciences, University of Alberta, Edmonton, Alberta, CANADA.

   Drosophila melanogaster has two sex chromosomes and three autosomes. Chromosome 4 (the "dot" chromosome) is the smallest autosome (~5 Mb in length) and contains two major regions. The centromeric domain is heterochromatic and consists primarily of about 4 Mb of short, satellite repeats. The remaining ~1.2 Mb, which constitutes the banded region (101E to 102F) on salivary gland polytene chromosomes and contains the identified genes, is the region mapped in this study. Chromosome walking was hindered by the abundance of moderately repeated sequences dispersed along the chromosome and we used many entry points to recover overlapping cosmid and BAC clones. In situ hybridization of probes from the two ends of the map to polytene chromosomes confirmed that the contig had spanned the 101E 102F region. Our BAC contig contains two gaps. One gap, positioned distally in 102EF, is about one BAC clone in length (100 kbp). The other, located proximally at 102B, is less than 1 kb. Twenty-three genes, representing about half of our revised estimate of the total number of genes on chromosome 4, have been mapped to the contig. A minimal tiling set of clones from the contig will facilitate both the assembly of the DNA sequence of the chromosome and a functional analysis of its genes.