Evolutionary dynamics of olfactory receptor genes in Drosophila species

Olfactory receptor (OR) genes are of vital importance for animals to find food, identify mates, and avoid dangers. In mammals, the number of OR genes is large and varies extensively among different orders, whereas, in insects, the extent of interspecific variation appears to be small, although only a few species have been studied. To understand the evolutionary changes of OR genes, we identified all OR genes from 12 Drosophila species, of which the evolutionary time is roughly equivalent to that of eutherian mammals. The results showed that all species examined have similar numbers ( approximately 60) of functional OR genes. Phylogenetic analysis indicated that the ancestral species also had similar numbers of genes, but there were frequent gains and losses of genes that occurred in each evolutionary lineage. It appears that tandem duplication and random inactivation of duplicate genes are the major factors of gene number change. However, chromosomal rearrangements have contributed to the establishment of genome-wide distribution of OR genes. These results suggest that the repertoire of OR genes in Drosophila has been quite stable compared with the mammalian genes. The difference in evolutionary pattern between Drosophila and mammals can be explained partly by the differences of gene expression mechanisms and partly by the environmental and behavioral differences.