Abstract
BackgroundWhen simple sequence repeats are integrated into functional genes, they can potentially act as evolutionary ‘tuning knobs’, supplying abundant genetic variation with minimal risk of pleiotropic deleterious effects. The genetic basis of variation in facial shape and length represents a possible example of this phenomenon. Runt-related transcription factor 2 (RUNX2), which is involved in osteoblast differentiation, contains a functionally-important tandem repeat of glutamine and alanine amino acids. The ratio of glutamines to alanines (the QA ratio) in this protein seemingly influences the regulation of bone development. Notably, in domestic breeds of dog, and in carnivorans in general, the ratio of glutamines to alanines is strongly correlated with facial length.ResultsIn this study we examine whether this correlation holds true across placental mammals, particularly those mammals for which facial length is highly variable and related to adaptive behavior and lifestyle (e.g., primates, afrotherians, xenarthrans). We obtained relative facial length measurements and RUNX2 sequences for 41 mammalian species representing 12 orders. Using both a phylogenetic generalized least squares model and a recently-developed Bayesian comparative method, we tested for a correlation between genetic and morphometric data while controlling for phylogeny, evolutionary rates, and divergence times. Non-carnivoran taxa generally had substantially lower glutamine-alanine ratios than carnivorans (primates and xenarthrans with means of 1.34 and 1.25, respectively, compared to a mean of 3.1 for carnivorans), and we found no correlation between RUNX2 sequence and face length across placental mammals.ConclusionsResults of our diverse comparative phylogenetic analyses indicate that QA ratio does not consistently correlate with face length across the 41 mammalian taxa considered. Thus, although RUNX2 might function as a ‘tuning knob’ modifying face length in carnivorans, this relationship is not conserved across mammals in general.
Highlights
When simple sequence repeats are integrated into functional genes, they can potentially act as evolutionary ‘tuning knobs’, supplying abundant genetic variation with minimal risk of pleiotropic deleterious effects
We focus on xenarthrans, afrotherians, and primates as these orders show marked variations in face length, which are thought to be adaptively associated with diet and ecology [23], and for these orders we have access to morphometric and genetic data
In analyzing data for 41 placental mammal species representing 12 orders (Table 1), we find no correlation between Runt-related transcription factor 2 (RUNX2) sequence and face length (Figures 2, and 3; Tables 2, and 3) across our sample as a whole
Summary
When simple sequence repeats are integrated into functional genes, they can potentially act as evolutionary ‘tuning knobs’, supplying abundant genetic variation with minimal risk of pleiotropic deleterious effects. These repeats are common in mammalian genomes, often occurring within coding exons, potentially translating into An example of this is the correlation between specific tandem repeats and variation in midfacial length (i.e., the degree of prognathism, or the jutting of the face and jaw) in carnivorans [11,12,13]. Variation in this striking morphological trait appears to be causatively associated with variation in coding sequence repeats within the gene RUNX2 (Runt-related transcription factor 2). For RUNX2, the ratio of glutamines to alanines (QA ratio) appears to be positively correlated with the transcriptional activity of the protein [13,16]
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