Development and evolution occlude: evolution of development in mammalian teeth.

Abstract

According to Roy Lewis in The Evolution Man, an evolving mammal worries about nothing more than it does its teeth (1). And to a paleontologist, nothing about a mammal matters more than its teeth. To paleontologists, teeth are the population markers that microsatellite sequences are to population biologists. Even when represented by only a single tooth, a fossil mammal can often be identified by cusp number, relative cusp positions, and cusp heights, all features that are unfathomable to the nonpaleontologist. But in this issue of PNAS, Jernvall et al. (2) penetrate the fascinating “lost world” of fossil mammals, blazing a new path connecting molecular developmental biology with paleontology. Recent work, much of it by these same researchers, has given us a general framework for understanding the molecular basis of tooth crown formation (3–5), integrating genes and proteins into our previous histology-based knowledge of tooth development (6–8). It is now well known that enamel knots—transient structures that appear first at the apex of the developing tooth bud and later at the apices of growing cusps—are important molecular signaling centers that control the placement and size of individual cusps on the crown (4, 9). Enamel knots, via a cascade of gene activity, regulate their own integrity, stimulate growth in surrounding epithelium, induce the formation of subsequent knots (in some cases), and signal their own demise via apoptosis (Fig. 1). As molecular signaling centers, enamel knots seem to be a universal aspect of mammalian tooth crown patterning. They have been found in all species examined to date, regardless of tooth morphology. Their discovery has helped explain the mechanism of crown patterning but does not, by itself, explain why mammals have such a diversity of teeth. As common features, a shared gene cascade can explain only the similarities …