Phylogenetic relationships of the OBP proteins. The unrooted tree includes odorant-binding protein (OBP) sequences from Drosophila (for clarity the tree only includes sequences from Drosophila melanogaster, D. pseudoobscura, D. virilis, and D. willistoni), Anopheles gambiae (green rectangles), Apis mellifera (blue triangles), and Tribolium castaneum (orange circle). The OBP (I to VII) and chemosensory protein (CSP) phylogenetic subfamilies are shadowed by different colors. The scale bar represents 1 amino acid substitution per site. The tree was displayed using the iTOL web server . ABPX, antennal-binding protein X.