Limb Development of a Limbless Skink
New Haven, CT
Nobel Museum of Natural History
Department of Biology
University of Oklahoma
Reduced limbs and limblessness have evolved multiple independent times in different lizard clades (Leal and Cohn, 2017). Skinks exhibit a wide range of limb-reduced morphologies, but only some species have been used to study different embryological aspects of limb reduction; i.e., digit reduction in Chalcides (Young et al., 2009), limb reduction in Scelotes (Raynaud, 1985). We studied embryonic sequences of members of the genus Brachymeles, a Southeast Asian clade of skinks including pentadactyl to limbless species obtained from collecting trips to the Philippines, focusing in the development of a limbless species. The elongated snake-like Brachymeles lukbani shows almost no sign of external limbs in the adult except for a small depression where the limb could be expected to be found (Siler et al., 2010) , while the early embryos develop a truncated but well-developed limb with a stylopod and a zeugopod, but no signs of an autopod. In later stages, the limb seems to retain its younger small size through time even if the embryo lengthens and acquires a more elongated body shape. We used florescent whole-mount immunofluorescence to visualize the morphology of skeletal elements and muscles in the embryos. The early stages show the presence of a humerus and separated ulna and radius cartilages, and associated with them dorsal and ventral muscle masses as those found in the embryos of other limbed species. While the limb is small, the pectoral girdle seems of normal size in proportion to the rest of the body, with well-developed skeletal elements and their associated muscles. In later stages, the small limb is almost impossible to find, except for the morphology of the scale covering it, the ulna and radius fuse and the arm muscles reduce or degenerate, as they are no longer visible. The pattern of ontogenetic reduction observed, leading to a functionally limbless adult, bears some similarities to those described for other limb-reduced skinks (Scelotes (Raynaud, 1985)), but differs from patterns seen in other clades (Anguis, Ophisaurus (Raynaud, 1985)), suggesting there are some skink-specific ways of becoming limbless. Future work might help understanding more clade-specific patterns of limb reduction, and the repeated evolution of seemingly similar phenotypes through very different developmental processes.