Genetic integration of molar cusp size variation in baboons

Abstract

Many studies of primate diversity and evolution rely on dental morphology for insight into diet, behavior, and phylogenetic relationships. Consequently, variation in molar cusp size has increasingly become a phenotype of interest. In 2007 we published a quantitative genetic analysis of mandibular molar cusp size variation in baboons. Those results provided more questions than answers, as the pattern of genetic integration did not fit predictions from odontogenesis. To follow up, we expanded our study to include data from the maxillary molar cusps. Here we report on these later analyses, as well as inter-arch comparisons with the mandibular data. We analyzed variation in two-dimensional maxillary molar cusp size using data collected from a captive pedigreed breeding colony of baboons, Papio hamadryas, housed at the Southwest National Primate Research Center. These analyses show that variation in maxillary molar cusp size is heritable and sexually dimorphic. We also estimated additive genetic correlations between cusps on the same crown, homologous cusps along the tooth row, and maxillary and mandibular cusps. The pattern for maxillary molars yields genetic correlations of one between the paracone-metacone and protocone-hypocone. Bivariate analyses of cuspal homologues on adjacent teeth yield correlations that are high or not significantly different from one. Between dental arcades, the nonoccluding cusps consistently yield high genetic correlations, especially the metaconid-paracone and metaconid-metacone. This pattern of genetic correlation does not immediately accord with the pattern of development and/or calcification, however these results do follow predictions that can be made from the evolutionary history of the tribosphenic molar.

Figure 1

These drawings represent the evolution of the primate molar from a triconodont early mammal ancestor. From top: 1 = Morganucodon; 2, 3, 4 = various “Symmetrodontans” (Kuehneotherium, Spalacotherium, and Spalacolestes, respectively); 5 = Pappotherium (a “tribotherian”/therian of metatherian-eutherian grade); 6 = Homo sapiens. Line drawings modified from Kielan-Jaworowska et al. (2004), Figure 9.1, and Hillson (1996), Figures 2.2 and 2.29.

Figure 2

Mesial is to the top and lingual to the center. This figure shows how the cusps were defined for data collection. See text for more details regarding protocol specifics. This figure also shows a schematic for the genetic correlations between cusps on the same molar crown. The solid arrows indicate a genetic correlation that is not significantly different from one. The dotted arrows indicate partial genetic correlations (incomplete pleiotropy). Significance is at p ≤ 0.01. Note the correlation present between the metacone and paracone on all three left maxillary molars. *Correlations using combined data in which right and left sides were averaged when possible, and right or left data were used when only one side was measured. The UM2 hypocone for the combined sample did not return a significant heritability estimate and therefore not analyzed for any genetic correlations.

Figure 3

Dorsal view through the skull where mesial is to the top and lingual is to the right. A. This figure shows a view of occluding cusps displaying the most distinct intra-molar cuspal correlations down the left molar row. Orange shading and arrows indicate correlation between maxillary cusps on the same crown. Blue shading and arrows indicate correlation between mandibular cusps on the same crown. Note the significant linear buccal correlation on the maxilla and the diagonal correlation on the mandible. B. This figure shows a view of occluding cusps displaying the most distinct inter-cuspal correlations between the dental arcade on the left side. The yellow shading and arrows indicate the interrelated cusps. Note the significant correlations involving the metaconid, metacone, and paracone.