SHAPIRO, M.D.: Developmental morphology and gene expression in lizard digit reduction

Evolutionary digit loss in lizards has been attributed to truncations in ancestral developmental programs, while another hypothesis refutes this idea. However, neither hypothesis has been tested in a developmental framework. Among living tetrapods, Australian skinks may offer the best examples of graded limb reduction. Within its Western Australian species, the genus Hemiergis displays a graded morphocline of digit configurations, containing from 2 to 5 fully formed digits, on both sets of limbs. Growth series of embryos from taxa with different adult limb configurations were cleared and stained to elucidate comparative ontogenetic patterns of the limb skeleton. Digit loss among these taxa did not result from the truncation of an ancestral limb development program, which would have yielded only partial digits, but rather from localized differences in limb morphogenesis. Moreover, immunohistochemical studies suggest that differential temporal expression of the anteroposterior patterning gene Sonic hedgehog may play a role in digit loss in Hemiergis, but do not implicate other proteins as regulators of digit loss in these lizards.