Evolution of vertebrate limbs: Robust morphology and flexible development

Abstract

SYNOPSIS. New information about molecular mechanisms of development can be combined with existing knowledge about embryology, anatomy and paleontology to allow for an increased understanding of evolutionary biology. The tetrapod limb is appropriate for such an approach since much is known about both its structural variation and development. To this end we are investigating molecular regulatory mechanisms in urodele limb development and regeneration. Urodeles have unique patterns of limb development compared to other tetrapods. In addition they are able to regenerate their limbs as adults, thus providing the opportunity to conduct comparative studies of the molecular mechanisms involved in development and regeneration in an identical genetic background. We have investigated the role of several homeobox-containing genes in the control of growth and pattern formation during limb development and regeneration, and have found that although there can be considerable variation in the ways in which expression of these genes is regulated in time and space, their expression patterns relative to morphological landmarks is conserved. These results suggest that the function of these genes is a deeply conserved feature of all tetrapods, and may be the molecular manifestation of the homology between different limb types. These conserved similarities are overlaid with changes in the time at which genes are expressed and the sequence in which structures differentiate. It is these latter features that are most likely responsible for the wide variety of morphologies observed among tetrapod limbs.