Fig. 3

Maximum likelihood (ML) tree constructed using full-length intact α and γ group olfactory receptors from 10 birds (chicken, zebra finch, flycatcher, duck, turkey, chuck-will’s-widow, barn owl, ostrich, tinamou, and kiwi) and two reptile genomes (anole lizard and Chinese soft-shell turtle). The ML topology shown above was cross-verified using the neighbor joining (NJ) method. Three Class A (Rhodopsin) family GPCRs from chicken genome, dopamine receptor D1 (DRD1), dopamine receptor D2 (DRD2), and histamine receptor H1 (HRH1) were used as the out-group (shown as non-olfactory receptors). The red dot indicates confidence estimates (% bootstrap from 500 resamplings, >90 % bootstrap support from both ML and NJ methods) for the nodes that distinguish α and γ ORs. The scale bar represents the number of amino-acid substitutions per site. The topology supports lineage specific expansions of γ group olfactory genes in the bird and the reptile species. Note, a few of the γ group ORs in kiwi cluster with reptilian ORs (highlighted by orange arrowhead), while some cluster basal to the clade containing bird ORs (highlighted by green arrowhead). The topology supports contrasting evolutionary rates within the analyzed γ ORs, as indicated by short (blue arc with arrowheads) and long branch lengths (pale orange arc with arrowheads). The inset shows the number of intact olfactory receptors in each species that are analyzed using the ML tree topology