Günter Wagner


Digit loss and re-evolution in the Philippine skink genus Brachymeles

Yale University and University of Vienna


In tetrapods the loss of digits and limbs is a major mode of body shape evolution. In recent years it became clear that re-evolution of digits is also a distinct possibility, violating the model of irreversible evolution of complex characters (a.k.a. “Dollo’s law”). In order to investigate the functional, ecological and developmental processes of digit re-evolution a group of colleagues (Cameron Siler, University of Oklahoma, Philip Bergman, University of Connecticut, Dunkan Ischik, University of Massachusetts) and I studied the skink genus Brachymeles, a group (almost) exclusive to the Philippines. Phylogenetic analysis reveals that large five digited forms are deeply nested in a paraphyletic group of digit and limb reduced species, and likelihood models confirm that digit re-evolution is the most likely scenario. The origin of the pentadactyl sub-clade coincides with the Miocene warming period and the transition from a dry loose soil habitat to a wet forest environment. Functional morphological tests show that the pentadactyl animals are better in both, surface as well as subterranean locomotion in coarse forrest soils, and thus have found a way to overcome the ancestral tradeoff between snake like and quadrupedal forms of locomotion. In terms of developmental mechanisms we found that a digit reduced species, B. bicolandia, retain mesenchymal condensations for at last four and possibly even five digits. This is a pattern that is also seen in other digit reduced limbs such as bird wings and hoofed animals. This embryological fact explains why digit re-evolution in Brachymeles is developmentally plausible as it seems to only require the loss of an inhibition of the development of preserved digit anlagen.