Craniofacial Integrity: Co-variation between Facial Flatness and Craniofacial Shape in Humans from Lapps(Saami) and Japanese

摘要

The cranium is often regarded as a composite of different functional or developmental modules. Thus, morphological variation can be interpreted as a product of interactions of these modules during growth. This study aims to identify the degree and pattern of these interactions as seen in craniofacial traits among genetically distant populations. To assess this pattern of craniofacial integration, the skulls of males from Skolt Lapps in Finland and from modern Japanese were measured. Three manually-obtained indices of horizontal facial flatness were regressed on shape variation as measured from nine anatomical landmarks of cranial face and base digitized from lateral cephalograms. Extraction of this shape variation was attained through a process of geometric morphometrics; specifically, with the use of Procrustes superimposition and thin-plate spline transformations (TPS). The TPS-derived face-base variation displayed the inter-population differences between the Lapps and the Japanese populations. However, it did not co-vary with the degree of horizontal facial flatness. On the other hand, multiple regression analyses (MANCOVA) applied to the intra-population variation revealed significant correlations between the TPS-derived face-base shape variables and the facial flatness indices. Their covariate patterns were similar, and could be considered as a common trend among the two populations, while the three facial flatness indices exhibited different patterns with each other, indicating a different interaction of the three facial regions with the sagittal craniofacial profde. The frontal flatness is related to the combined position of the cranial base and the face, and also to the degree of cranial base flexion. The zygomaxillary flatness is related to the antero-posterior length of the palate. The results suggest that a common covariate pattern is detectable in the within-population variation and can be considered a consequence of developmental plasticity which can be controlled by both genes as well as functional demands.