15-P011 A typical relaxation of structural constraints in Hox gene clusters of squamates

rugae), rat (8) and golden hamster (7). We showed that palatal growth rate, spacing and addition rate in mouse/rat were remarkably similar (with respect to the embryo size difference), and that increase to 9 rugae in mouse is achieved by postponing the end of the addition process (hypermorphosis). Such a heterochronic shift may be typical of 1 variations observed among muroid rodents. In contrast, decrease to 7 rugae in golden hamster is attributed to early growth termination (progenesis) of the palate, which correlates with the severe shortening of gestation in this species. Our results provide an experimental support to the intuitive view that terminal heterochronies are especially relevant to meristic evolution of traits that rely on a sequential addition process.