High variability and non-neutral evolution of the mammalian avpr1a gene

Abstract

Background: The arginine-vasopressin 1a receptor has been identified as a key determinant for social behaviour in Microtus voles, humans and other mammals. Nevertheless, the genetic bases of complex phenotypic traits like differences in social and mating behaviour among species and individuals remain largely unknown. Contrary to previous studies focusing on differences in the promotor region of the gene, we investigate here the level of functional variation in the coding region (exon 1) of this locus.

Results: We detected high sequence diversity between higher mammalian taxa as well as between species of the genus Microtus. This includes length variation and radical amino acid changes, as well as the presence of distinct protein variants within individuals. Additionally, negative selection prevails on most parts of the first exon of the arginine-vasopressin receptor 1a (avpr1a) gene but it contains regions with higher rates of change that harbour positively selected sites. Synonymous and non-synonymous substitution rates in the avpr1a gene are not exceptional compared to other genes, but they exceed those found in related hormone receptors with similar functions.

Discussion: These results stress the importance of considering variation in the coding sequence of avpr1a in regards to associations with life history traits (e.g. social behaviour, mating system, habitat requirements) of voles, other mammals and humans in particular.

Figure 1

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 [95]. 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.

Figure 2

Structural model of the V1a receptor with amino acid substitutions in the genus Microtus. AA substitutions are spread over the whole protein, but largest numbers of changes are found in the functionally important ligand binding domain. Position of changes and type of changes are marked as: black circle = radical change; white circle = conservative change; grey circle = conservative and radical changes at the same position; white square = deletion; black square = insertion; black triangle = radical change and deletion at the same position. Changes between protein types within an individual occur in the functionally important regions (ligand and G-protein binding domains) and are marked as red diamond for a radical change, and as orange diamond for a conservative change.

Figure 3

Amino acid alterations of the avpr1a gene plotted onto a mitochondrial cytochrome b phylogeny of the genus Microtus. Positions and types of changes are labelled as in Figure 2. Bootstrap values > 50 (10'000 replicates) of the maximum likelihood method are shown on the branches. AA alterations in Microtus segregate generally independently of the phylogenetic background except for the closely related sister species M. arvalis and M. rossiaemeridionalis which show two identical changes at the same positions (58, 85). Additionally, a two AA long insertion together with an alteration at position 42 appear in the cluster of M. agrestis together with M. montebelli and M. kikuchii, where the changes seem to have been subsequently lost in M. oeconomus.

Figure 4

Maximum likelihood tree inferred from the nucleotide sequences of exon 1 of the arginine-vasopressin 1a receptor gene for various mammalian taxa. Bootstrap values > 50 are shown for the maximum likelihood method above branches and for neighbour-joining below branches. Positively selected sites (ω > 1) are shown in black circles. Note that most of these sites are found in the G-protein binding domain (231–274). Only two positively selected sites (191; 228) were detected outside this domain in two species (O. aries and C. familiaris).

Figure 5

Sliding window analysis of the ratio of non-synonymous substitutions (d_N) over synonymous substitutions (d_S) along the avpr1a gene of mammals compared to the marsupial Monodelphis domestica (see text). The ratio is drawn over the midpoint window position (window size 30, step size 10) from nucleotide position 50 to 800 from the start codon (due to primer selection). d_N/d_S exceeds 1 in the ligand binding domain, which indicates positive selection in this region. A second peak of d_N/d_S close to 1 is found around 750 bp corresponding to the G-protein binding domain of the AVP 1a receptor.

Figure 6

Comparison of synonymous and non-synonymous substitutions per site for orthologous nuclear genes in Microtus, mouse and rat. Genes are ranked according to non-synonymous substitutions (black bars) per site. Synonymous substitutions per site are shown as white bars.