Comparison of genomic location and orientation of Or genes in Drosophila melanogaster and 3 other species. (A) D. melanogaster versus Drosophila simulans. (B) D. melanogaster versus Drosophila yakuba. (C). D. melanogaster versus Drosophila pseudoobscura. Dotted boxes are the 5 Muller elements. Each Muller element is normalized to have unit length. The x-y coordinates of an Or gene are its relative positions in the corresponding Muller element. Orthologous Or genes with different orientation are represented by solid squares, whereas those with the same orientation by open circles. Labeled Or genes outside the boxes are translocation between Muller elements. Translocations and inversions within a Muller element can be identified by the off-diagonal squares and circles. For example, a paracentric inversion is obvious in chromosome 3R in D. simulans. In D. pseudoobscura, there are several inparaologs of Or67b and Or98a. Only one for each was represented in the graph.