Limb development in a "nonmodel" vertebrate, the direct-developing frog Eleutherodactylus coqui

Mechanisms that mediate limb development are regarded as highly conserved a mong vertebrates, especially tetrapods. Yet, this assumption is based on th e study of relatively few species, and virtually none of those that display any of a large number of specialized life-history or reproductive modes, w hich might be expected to affect developmental pattern or process. Direct d evelopment is an alternative life history found in many anuran amphibians. Many adult features that form after hatching in metamorphic frogs, such as limbs, appear during embryogenesis in direct-developing species. Limb devel opment in the direct-developing frog Eleutherodactylus coqui presents a mos aic of apparently conserved and novel features. The former include the basi c sequence and pattern of limb chondrogenesis, which are typical of anurans generally and appear largely unaffected by the gross shift in developmenta l timing; expression of Distal-less protein (D1x) in the distal ectoderm; e xpression of the gene Sonic hedgehog (Shh) in the zone of polarizing activi ty (ZPA); and the ability of the ZPA to induce supernumerary digits when tr ansplanted to the anterior region of an early host limb bud. Novel features include the absence of a morphologically distinct apical ectodermal ridge, the ability of the limb to continue distal outgrowth and differentiation f ollowing removal of the distal ectoderm, and earlier cessation of the induc tive ability of the ZPA. Attempts to represent tetrapod limb development as a developmental "module" must allow for this kind of evolutionary variatio n among species. (C) 2001 Wiley-Liss, Inc.