Molecular Genetic Analyses of Polytene Chromosome Region 72A-D in Drosophila melanogaster Reveal a Gene Desert in 72D

We have investigated a region of ∼310 kb of genomic DNA within polytene chromosome subdivisions 72A to 72D of Drosophila melanogaster. This region includes 57 predicted protein-coding genes. Seventeen of these genes are in six clusters that appear to have arisen by tandem duplication. Within this region we found 23 complementation groups that are essential for zygotic viability, and we have identified the transcription units for 18 of the 23. We also found a 55 kb region in 72D that is nonessential. Flies deficient for this region are viable and fertile. Within this nonessential region are 48 DNA sequences of 12 to 33 base pairs that are completely conserved among 12 distantly related Drosophila species. These sequences do not have the evolutionary signature of conserved protein-coding DNA sequences, nor do they appear to encode microRNAs, however, the strong selection suggests functions in wild populations that are not apparent in laboratory cultures. This region resembles dispensable gene deserts previously characterized in the mouse genome.