Figure 1.

Arrangement of Muller's elements (chromosome arms) in D. melanogaster and D. pseudoobscura. The chromosomal arms (A-F) are highly conserved between the two species, but their organization into chromosomes differs. The chromosome number corresponding to each element is indicated. Gene content is conserved between elements, but gene order is not. The rearrangement of gene order is represented by shading within each chromosome arm.