PLEIOTROPIC EFFECTS OF INDIVIDUAL GENE LOCI ON MANDIBULAR MORPHOLOGY

The genotypic basis of morphological variation is largely unknown. In this study we examine patterns of pleiotropic effects on mandibular morphology at individual gene loci to determine whether the pleiotropic effects of individual genes are restricted to functionally and developmentally related traits. Mandibular measurements were obtained from 480 mice from the F₂ generation of an intercross between the LG/J and SM/J mouse strains. DNA was also extracted from these animals, and 76 microsatellite loci covering the autosomes were scored. Interval mapping was used to detect chromosomal locations with significant effects on various mandibular measurements. Sets of traits mapping to a common chromosomal region were considered as being affected by a single quantitative trait locus (QTL) for mandibular morphology. Thirty-seven such chromosomal regions were identified spread throughout the autosomes. Gene effects were small to moderate with the allele derived from the LG/J strain typically leading to larger size. When dominance was present, the LG/J allele was typically dominant to the SM/J allele. Most loci affected restricted functional and developmental regions of the mandible. Of the 26 chromosomal regions affecting more than two traits, 50% affect the muscular processes of the ascending ramus, 27% affect the alveolar processes carrying the teeth, and 23% affect the whole mandible. Four additional locations affecting two traits had effects significantly associated with alveolar regions. Pleiotropic effects are typically restricted to morphologically integrated complexes.