Genetic contributions to expression of the baboon cingular remnant

Primitive mammalian molar morphology is characterised in part by a ridge of enamel that encircles the entire base of the molar crown, the cingulum. Many higher primates have reduced the cingulum, but often retain remnant features on the lingual surface of maxillary molars and the labial surface of mandibular molars. Two of these remnants in cercopithecoid primates, the interconulus and interconulid, are morphologically similar though the interconulus is found on maxillary molars and the interconulid is located on mandibular molars. Here we present results from a quantitative genetic analysis of expression of these two traits in a sample of 479 modern savannah baboons from the Southwest Foundation for Biomedical Research (SFBR). We found that both traits are significantly heritable with little variance attributable to other factors, such as sex, age, and molar crown size. Bivariate analyses yielded point estimates for genetic correlations between left and right side expression that are either equal to or not significantly different from 1.0; meaning that 100% of their additive genetic variance is due to the effects of the same gene or suite of genes. By contrast, our estimates of the genetic correlations between maxillary and mandibular expression of this trait range from 0.52 to 0.72, suggesting that 28-52% of the additive genetic variance in the interconulus and interconulid is due to the effects of shared genes. These results demonstrate that intra-arch expression is characterised by complete pleiotropy whereas inter-arch expression is caused by incomplete pleiotropy. These results are relevant to dental developmental studies as well as paleontological analyses of the evolution of the primate dentition.