A heat map of functional variation by vomeronasal receptor class and phylogeny. The amount of functional variation in all genes in our parsed dataset is represented by colour (black to yellow). The values are normalised between and within strain, as the number of non-synonymous SNPs in a kilobase of coding sequence. Each row represents a gene from the (A) V1R (B) V2R and (C) FPR sub-families, arranged by phylogeny (left). Where receptor sub-families have been further divided into recognised phylogenetic clades, those are indicated (right) in alternating shade of grey. The clade nomenclature is as described [10, 11, 19, 20]. Each column represents a strain, divided into lab-derived (left, black text) and wild-derived (right, blue text). Genes removed from the dataset because of evidence of duplication (green), or deletion (white), are also indicated.