Fig. 1.

Location of the 37 species studied in the phylogenetic tree of mammals [19]. The lineage of Monotremata (platypus) is indicated with a dashed line as no MEPE sequence has been identified yet. However, an orthologue of MEPE (OC-116) is present in the chicken (Sauropsida). The five positions (amino acids) identified through analysis as positively selected are located in the tree (numbers refer to our alignment). The black line indicates the presence of MEPE. The light and dark gray lines represent exons 3 and 4, respectively; exon 3 and/or exon 4 were lost in several lineages during evolution