Supernumerary limbs in the axolotl

Abstract

THE polar coordinate model of French et al.¹, commonly called the clockface model, is a concept of great significance to developmental biology, as it presents a unified view of pattern regulation in Drosophila imaginal disks and insect and amphibian limbs. By postulating two rules, intercalation by the shortest route and the complete circle rule for distal transformation, the production of supernumerary limbs in insects^2–5 and amphibians^6,7 after axial reversal is elegantly explained. Following 180° rotation, the insect limb can produce two supernumeraries in various positions^2–4, whereas in the newt the extra limbs are in constant circumferential positions⁸. Because a complete circle is needed for distal transformation, the model clearly predicts that at no other angle of ipsilateral rotation apart from 180° will supernumeraries be induced. However, Wallace (personal communication), the first to test such a prediction on amphibian limbs, has demonstrated that after 90° or 270° rotation, supernumerary limbs are produced. We report here that, unlike newts and more like insects, axolotls can produce supernumerary limbs anywhere on the circumference after 180° rotation of blastemas. In addition, supernumeraries can be produced at a variety of angles of rotation between 45° and 315°. This work calls for a reappraisal, at least in axolotls, of the clockface model.