Evolution of Vertebrate Limb Development

Abstract

Abstract The origin and diversification of fins and limbs have long been a focus of interest to both palaeontologists and developmental biologists. Studies conducted in recent decades have resulted in enormous progress in the understanding of the genetic and developmental bases of the evolution of paired appendages in vertebrates. These discoveries in the areas of genetics and developmental biology have shed light on the mechanisms underlying the evolution of this key morphological innovation in vertebrates. In this article, recent advances in these fields and how they can provide a mechanistic explanation for the origin and evolution of paired appendages have been discussed. Key Concepts Step-wise changes seem to be involved in the acquisition of paired fins, such as regionalisation of the lateral plate mesoderm (LPM) into the anterior and posterior LPM; sub-division of the LPM into somatic and splanchnic layers; acquisition of expression of genes that initiate limb formation, such as Tbx4/5; and dorso-ventral compartmentalisation of ectoderm. Morphological changes during the fin-to-limb transition include the acquisition of the autopod and the evolutionary modification of skeletal patterns along the anterior–posterior axis. The fin-to-limb transition seems to involve changes in transcriptional regulation of HoxA and HoxD clusters, changes in the expression of Gli3 and Shh, loss of the fin fold, and modification of the BMP-SOX9-WNT Turing network. Changes in the activity of regulatory elements of genes known to play pivotal roles in limb development seem to be related to the morphological diversification of limbs and the loss of paired fins and limbs.