The Dynamic Ontogenetic Structure of Mandibular Morphological Integration

Mammalian crania demonstrate conserved patterns of morphological integration and simultaneously possess varying magnitudes (degrees) of integration. Thus, it has been posited that plasticity in magnitudes of integration allow for adaptive diversification, while conserved patterns of integration reflect stabilizing selection within the population. This has not been widely tested in the mandible but could have profound implications for mandibular diversity. Here, we test these relationships in the mandible using a mouse model with a mutation that disturbs bone growth in the skull (Fgfr2 C342Y ). Pattern and magnitude of integration were compared between age‐matched wild‐type and mutant mice. The effect of growth was tested by comparing age groups within each genotype. Micro‐CT scans were collected from wild‐type and mutant mouse skulls at three developmental stages: embryonic, peri‐weaning and adult (n=152). Three‐dimensional landmark coordinate data were collected from each mandible. Procrustes superimposition was applied to coordinate data to produce shape variables. Covariance matrices and scaled variance of eigenvalues were generated from shape variables and used to compare patterns and magnitudes of covariance, respectively. Significance was determined via permutation tests. Patterns of integration in the mandible were conserved between genotype at each developmental stage ( R‐ obs <0.5; p<0.0001), while magnitude of integration was significantly higher in the mutant mouse model (p<0.001). Interestingly, when pattern was compared among age groups, integration was no longer conserved ( R‐ obs <0.2; p>0.05). Magnitude of integration decreased markedly with age in the wild‐type mice but increased as mutants aged (p>0.01). The way in which traits covaried (pattern) did not alter despite a perturbation in mandibular growth. The Fgfr2 C342Y mutation did however, result in a significant change in magnitude of integration. Importantly, the only instance where pattern of integration altered was between developmental periods. These results support the suggestion that pattern of integration stabilizes shape covariance in a population, and different magnitudes of integration allow for plasticity in shape. Furthermore, both pattern and magnitude change throughout ontogeny, as separate functional and developmental demands occur. Future research should consider the complex influence of pattern, magnitude and development on the diversification of mandibular shape. Support or Funding Information Supported by NIH #R03DE019816 and Louisiana Board of Regents Traditional Enhancement Grant