Evolution of vertebrate V1R and V2R gene repertoires. The tree shows the well established phylogeny of the 11 vertebrates considered here. The numbers of intact V1R and V2R genes are given with the numbers of disrupted genes in parentheses. A cross with a circle indicates the loss of the entire V1R repertoire, whereas a star indicates the loss of the entire V2R repertoire. Although humans have five intact V1R genes, they are likely to be relics of an ongoing pseudogenization process (Zhang and Webb 2003). Thus, the entire V1R repertoire is considered to be lost in humans. Based on the parsimony principle, we infer that the loss of the V2R repertoire in the cow and dog occurred in their common ancestor.