Fig. 6.

A model for the evolution of the three Drosophila melanogaster ab3A phenotypes. The ab3A-3 phenotype is likely the ancestral response profile and is conferred by the Or22a³ and Or22b³ genotype. We therefore propose a model whereby the ab3A-1 and ab3A-2 phenotypes are the result of two distinct molecular genetic mechanisms beginning with the ab3A-3 ancestral state. From here, the ab3A-1 phenotype has arisen via mutations that activated expression of Or22a¹ and a SNP that caused the M194R amino acid substitution in Or22b¹ to render it nonfunctional. The ab3A-2 phenotype has arisen via the formation of the chimeric gene Or22ab, resulting from an in-frame deletion between the first introns of Or22a³ and Or22b³. Two additional SNPs in Or22ab were further required for the two amino acid substitutions, N92D and D201A, to produce the ab3A-2 phenotype.