Molecular organization of the Drosophila melanogaster Adh chromosomal region in D-repleta and D-buzzatii, two distantly related species of the Drosophila subgenus

The molecular organization of a 1.944-Mb chromosomal region of Drosophila m elanogaster around the Adh locus has been analyzed in two repleta group spe cies: D. repleta and D. buzzatii. The extensive genetic and molecular infor mation about this region in D. melanogaster makes it a prime choice for com parative studies of genomic organization among distantly related species. A set of 26 P1 phages from D. melanogaster were successfully hybridized usin g fluorescence in-situ hybridization (FISH) to the salivary gland chromosom es of both repleta group species. The results show that the Adh region is d istributed in D. repleta and D. buzzatii over six distant sites of chromoso me 3, homologous to chromosomal arm 2L of D. melanogaster (Muller's element B). This observation implies a density of 2.57 fixed breakpoints per Mb in the Adh region and suggests a considerable reorganization of this chromoso mal element via the fixation of paracentric inversions. Nevertheless, break point density in the Adh region is three times lower than that estimated fo r D. repleta chromosome 2, homologous to D. melanogaster 3R (Muller's eleme nt E). Differences in the rate of evolution among chromosomal elements are seemingly persistent in the Drosophila genus over long phylogenetic distanc es.