DE and signature of positive selection plotted on a V2R phylogeny of five Xiphophorus spp. Because all orthologues are monophyletic for the five species, the tips are pruned to leave a single representative gene. *CodeML dN/dS M8-M8a model p < = 0.01; BM, differentially expressed between birchmanni–malinche exposures; CB, differentially expressed between control and birchmanni-exposed. The phylogeny is rooted at a clade containing V2R_71, corresponding to OlfCc1 (v2r1l) in zebrafish, a previously known basal V2R. (Online version in colour.)