Figure 4.

Evolutionary scenarios of V2Rs during vertebrate evolution proposed by previous (A) and this (B) studies. The presence/absence of the four major V2R clades was plotted on the phylogenetic tree of vertebrates from agnathans to mammals. The red circle with “2,” yellow with “a,” gray with “t,” and blue with “f” indicate V2R2, ancV2R, t-V2Rs, and f-V2Rs, respectively. (A) Except for V2R2, teleost fish and tetrapods possess only f-V2Rs or t-V2Rs, respectively, while the coelacanth possesses both V2Rs. Common ancestor of Osteichthyes was expected to possess V2R2 and f-V2Rs. (B) The large-scale analyses revealed the existence of t-V2Rs in basal ray-finned fish and identified a novel clade of ancV2R. In contrast to basal ray-finned fish with all four clades of V2Rs, teleost fish, mammals, and lizards possess only two of them. The reduction of specific V2R clades in these lineages would be due to adaptation to specific oceanic and terrestrial environments. Note that the origin of t-V2Rs dates back to the common ancestor of extant Osteichthyes, but its antiquity in the jawed vertebrate ancestor remains to be examined with complete genome sequences of more cartilaginous fish (dotted circle with “t” inside). Asterisks above the circles indicate clades with substantial copy numbers due to gene expansions in each extant species. The illustration of the animals was drawn by using free software Vectr (https://vectr.com).