Genetic architecture of mandible shape in mice: Effects of quantitative trait loci analyzed by geometric morphometrics

This study introduces a new multivariate approach for analyzing the effects of quantitative trait loci (QTL) on shape and demonstrates this method for the mouse mandible. We quantified size and shape with the methods of geome tric morphometrics, based on Procrustes superimposition of fi ie morphologi cal landmarks recorded on each mandible. Interval mapping for F-2 mice orig inating from an intercross of the LG/J and SM/J inbred strains revealed 12 QTL for size, 25 QTL for shape, and 5 QTL for left-right asymmetry. Multiva riate ordination of QTL effects by principal component analysis identified two recurrent features of shape variation, which involved the positions of the coronoid and angular processes relative to each other and to the rest o f the mandible. These patterns are reminiscent of the knockout phenotypes o f a number of genes involved in mandible development although only a few of these are possible candidates for QTL in our study. The variation of shape effects among the QTL showed no evidence of clustering into distinct group s, as would be expected from theories of morphological integration. Fur the r, for most QTL, additive and dominance effects on shape were markedly diff erent, implying overdominance for specific features of shape. We conclude t hat geometric morphometrics offers a promising new approach to address prob lems at the interface of evolutionary and developmental genetics.