Synonymous and non-synonymous changes in the avpr1a gene. A: Schematic overview of the structure of V1a receptor adapted from a model of Mus musculus . The functionally important receptor regions (ligand binding and G-protein binding domains) are shown in red, while six out of seven transmembrane regions are displayed in black (label TM1-TM6). B: Non-synonymous (grey bars) and synonymous (black lines) substitutions in Eutherian mammals and the marsupial Monodelphis domestica (one DNA sequence per species, see text). Highest numbers of non-synonymous substitutions are present in the ligand binding and the G-protein binding domains, while synonymous substitutions are scattered along the whole gene. C: Non-synonymous (grey bars) and synonymous (black lines) changes for 24 species of the Microtus genus (one sequence per species, see text). High numbers of AA variants are found in the ligand binding domain only, while the G-protein binding domain is relatively conserved. Similar to the pattern in higher mammalian taxa, synonymous substitutions are equally frequent along the exon.