| Tetrapod limbs have become a model for the study of evolution and development. There is a surprising amount of morphological and molecular conservation in development of tetrapod limbs that allows for informative comparisons over large stretches of evolutionary distances. Simultaneously, significant variation exists in the development of limbs among the major lineages, especially between urodeles and amniotes. Indeed, much variation in limb and digit formation rests within the urodeles. In this dissertation, I investigate the differences among urodeles and between salamanders and amniotes using morphological, histological, and molecular techniques.; Urodeles can be classified into three major groups based on life history: Type A larval, Type B larval, and direct-developing---each associated with a unique set of developmental traits. Investigation of a Type A species, Ambystoma mexicanum, and a direct-developing species, Desmognathus aeneus, revealed that salamander limbs and digits form in a very different way than do amniote limbs, including the way digits extend from the limb bud, the order of digit condensation, and the condensation of elements along the proximal-distal axis.; Additionally, I found that two species of salamander, representing Type B and direct-developing life histories, as well as a direct-developing anuran species, Eleutherodactylus coqui, utilized cell death during limb development. Larval forms of both anurans and urodeles, including Type A urodeles species such as Ambystoma mexicanum, appear not to use apoptosis. Our findings suggest that the use of apoptosis during limb development is not an amniote-specific trait as was previously thought. However, the pattern of apoptosis in amphibian limbs is markedly different from patterns of amniote limb apoptosis.; In situ hybridization studies of fibroblast growth factors (FGFs) and bone morphogenetic proteins (BMPs) in Ambystoma mexicanum suggest significant conservation of function between salamanders and amniotes. However, there were important differences. Expression of Fgf-8 was located entirely in the mesoderm of the developing limb bud, yet may function in a manner similar to that of FGFs in the apical ectodermal ridge of amniotes. Also, the location of Bmp-2/4 expression was similar to what is seen in amniotes, except that there is no evidence of expression related to cell death. |
