ZELDITCH, M.L.; LUNDRIGAN, B.L.; University of Michigan, Ann Arbor; Michigan State University, East Lansing: Ontogenetic dynamics of developmental regulation of skull shape

The developmental processes generating and regulating morphological variation are of fundamental importance to evolution. We might expect variance to diminish during development due to intrinsic regulatory mechanisms, a hypothesis supported in the few studies that examine the ontogenetic dynamics of variance. But it is not yet known whether variation diminishes gradually or instead drops abruptly, nor whether the structure of variation changes even as its level remains constant. To answer these questions, we use landmark-based morphometrics, comparing the level and spatial distribution of variance of postnatal skull shape in two species of rodents, the cotton rat (Sigmodon fulviventer) and house mouse (Mus domesticus). Skulls of S. fulviventer are sampled at 10-day intervals from birth to sexual maturity; those of M. domesticus are sampled at five day intervals beginning at 10 days of age (when the skulls first become sufficiently ossified to measure), until 30 days, after which they are sampled at 10-day intervals until sexual maturity. In both species, variance is halved between the first two ages sampled, and is subsequently constant. That same general pattern is found in the face and braincase as well as in the skull as a whole, although the drop in facial variance is subtle in M. domesticus compared to the braincase variance, and also more subtle than in S.fulviventer. The difference between species in the regional patterns of developmental regulation results from differences in the spatial distribution of variance over their skulls. That spatial distribution changes over developmental time, suggesting that regulation is spatially, temporally and historically dynamic.