© 1999 by The Society for Integrative and Comparative Biology
Out on a Limb Parallels in Vertebrate and Invertebrate Limb Patterning and the Origin of Appendages1
,2
*Department of Genetics, Harvard Medical School 200 Longwood Avenue, Boston, Massachusetts
Howard Hughes Medical Institute and Laboratory of Molecular Biology, University of Wisconsin Madison, Wisconsin 53706
Department of Anatomy, University of Wisconsin Medical School Madison, Wisconsin 53706
Correspondence: 2corresponding author: E-mail:gePangan{at}facstaff.
Recent discoveries of similarities in the developmental genetics underlying the formation of insect and vertebrate eyes, hearts, segments, and other structures have fueled new speculation and debate about the origins of these features and the morphological complexity of early bilaterians. The pivotal issue concerning these developmental similarities is whether they represent convergence of pattern-forming mechanisms or reveal developmental regulatory mechanisms or even physical characteristics derived from a common ancestor. Here, we set forth an explicit hierarchical set of criteria for assessing developmental genetic similarities among animals. We suggest that interpretations of convergence versus descent from common ancestors should be weighed by the number, type, and phylogenetic distribution of genetic regulatory similarities. We then apply these criteria to the analysis of appendage evolution. We conclude that there has been no continuity of any structure from which the insect and vertebrate appendages could be derived, i.e., they are not homologous structures. However, there is abundant evidence for continuity in the genetic information for building body wall outgrowths and/or appendages in several phyla which must date at least to the common, potential appendage-bearing pre-Cambrian ancestor of most protostomes and deuterostomes. In order to further trace the origin of this genetic information and of appendages, it will be essential to analyze more primitive taxa such as the Cnidaria and to obtain a much better fossil record of pre-Cambrian animals.