It's about time: studying gene regulatory programs across serial organs

In some serial organs, however, important differences are also manifested, as in the cases of our arms and legs, and our different types of teeth. [...]an important question is how variation across these organs is encoded in gene regulatory programs. [...]the requisite rewiring could be concentrated in the middle of development, leading to a model with an “inverse-hourglass-like” shape (Fig. 1a). The authors’ analysis indicated that variation is largest in the middle stages of the development of the two types of molar—which corresponds to the inverse hourglass model (Fig. 1a)—during which the differences in expression correspond to genes involved in cell migration and cell adhesion during crown morphogenesis. [...]they detected heterochrony in the expression programs of forelimb and hindlimb development: the forelimb was relatively more mature than was the hindlimb.