A comparison of reptilian and avian olfactory receptor gene repertoires: Species-specific expansion of group γ genes in birds

We identified 156 green anole OR genes, including 42 pseudogenes. The OR gene repertoire of the two bird species was substantially larger with 479 and 553 OR gene homologs in the chicken and zebra finch, respectively (including 111 and 221 pseudogenes, respectively). We show that the green anole has a higher fraction of intact OR genes (~72%) compared with the chicken (~66%) and the zebra finch (~38%). We identified a larger number and a substantially higher proportion of intact OR gene homologs in the chicken genome than previously reported (214 versus 82 genes and 66% versus 15%, respectively). Phylogenetic analysis showed that lizard and bird OR gene repertoires consist of group α, θ and γ genes. Interestingly, the vast majority of the avian OR genes are confined to a large expansion of a single branch (the so called γ-c clade). An analysis of the selective pressure on the paralogous genes of each γ-c clade revealed that they have been subjected to adaptive evolution. This expansion appears to be bird-specific and not sauropsid-specific, as it is lacking from the lizard genome. The γ-c expansions of the two birds do not intermix, i.e., they are lineage-specific. Almost all (group γ-c) OR genes mapped to the unknown chromosome. The remaining OR genes mapped to six homologous chromosomes plus three to four additional chromosomes in the zebra finch and chicken.We identified a surprisingly large number of potentially functional avian OR genes. Our data supports recent evidence that avian olfactory ability may be better developed than previously thought. We hypothesize that the radiation of the group γ-c OR genes in each bird lineage parallels the evolution of specific olfactory sensory functions.Olfaction plays a pivotal role for many organisms. It is used to detect food, conspecifics, mates, as well as threats. However, the sense of smell is obviously more important to certain animals than others. Birds (with the exception of tube-nosed seabirds, Procellariiformes,