Proposed evolution of rhodopsin- and glutamate-family chemosensory receptor genes in chordate and vertebrate lineages. All representative vertebrate genomes listed have been analyzed. Trace amine-associated receptors were present in the common ancestor of chordates. Olfactory- and V1R pheromone receptor genes arose before the divergence of gnathostome and agnathan vertebrate lineages, but are absent from the genomes of those lower chordates described to date. Previous studies show that V2R pheromone receptors are widely diversified in higher vertebrates but have not been identified in the genome or olfactory organ transcriptome of the representative agnathan Petromyzon. Chemosensory receptor subfamilies in echinoderms have derived from biogenic amine receptors and have expanded in a lineage-specific manner. Arrows indicate nodes where gene families are first detected.