Developmental Morphospace of Embryonic Brain and Face Growth in the Chicken (Gallus gallus)

Abstract

Normal facial morphogenesis involves the precise spatiotemporal choreography of independent facial prominences that must grow, contact each other, and fuse to form a functional upper jaw. We previously identified in reptiles, birds, and mammals (amniotes) a significant shift in the trajectory of facial growth at the time of primary palatal prominence fusion that is associated with reduced variance in shape across species. We speculated that this period is associated with developmental constraints derived from a shared embryonic bauplan, while reduced variance represents selection to minimize the likelihood of palatal clefts. We further hypothesized that these constraints operate within species and so should observe a similar pattern not only across but within species. In addition, because facial growth occurs in the context of underlying brain growth, we hypothesized that it is a significant co‐factor in establishing this pattern. To begin to test these ideas we collected a sample of iodine‐contrast chicken (Gallus gallus) embryos (N=200) spanning the period of facial prominence formation, fusion, and outgrowth (HH16‐30), imaged them using micro‐computed tomography, and used three‐dimensional geometric morphometrics and principal components analysis (PCA) constructed a “developmental morphospace”. Our results confirm that chickens undergo a U‐shaped shift in developmental trajectory and reduced shape variance at the time of primary palatal fusion, that this occurs in both the face and brain, and it is highly coordinated (partial least squares (PLS) analysis: RV=0.995, P<0.0001). Specifically, after contact and fusion the relative the frontonasal and maxillary growth rates (i.e., allometry) switch, as does the relative growth of the forebrain and eyes. Future analyses will focus on identifying key regulators of this switch in growth profile and whether a similar pattern occur in other species (mouse, human). Ultimately, this developmental morphospace approach may provide a generalized model for predicting how perturbations to facial prominence shape variability, growth trajectory, and brain size can combine to impact a range of contact and fusion events, both normal and abnormal.