Résumé
BACKGROUND: 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. 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. RESULTS: In 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 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. CONCLUSIONS: Results 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.