THERE IS NO HIGHLY CONSERVED EMBRYONIC STAGE IN THE VERTEBRATES - IMPLICATIONS FOR CURRENT THEORIES OF EVOLUTION AND DEVELOPMENT

Embryos of different species of vertebrate share a common organisation and often look similar. Adult differences among species become more a pparent through divergence at later stages. Some authors have suggeste d that members of most or all vertebrate clades pass through a virtual ly identical, conserved stage. This idea was promoted by Haeckel, and has recently been revived in the context of claims regarding the unive rsality of developmental mechanisms. Thus embryonic resemblance at the tailbud stage has been linked with a conserved pattern of development al gene expression - the zootype. Haeckel's drawings of the external m orphology of various vertebrates remain the most comprehensive compara tive data purporting to show a conserved stage. However, their accurac y has been questioned and only a narrow range of species was illustrat ed. In view of the current widespread interest in evolutionary develop mental tal biology, and especially in the conservation of developmenta l mechanisms, re-examination of the extent of variation in vertebrate embryos is long overdue, We present here the first review of the exter nal morphology of tailbud embryos, illustrated with original specimens from a wide range of vertebrate groups, We find that embryos at the t ailbud stage - thought to correspond to a conserved stage - show varia tions in form due to allometry, heterochrony, and differences in body plan and somite number. These variations foreshadow important differen ces in adult body form. Contrary to recent claims that all vertebrate embryos pass through a stage when they are the same size, we find a gr eater than 10-fold variation in greatest length at the tailbud stage. Our survey seriously undermines the credibility of Haeckel's drawings, which depict not a conserved stage for vertebrates, but a stylised am niote embryo. In fact, the taxonomic level of greatest resemblance amo ng vertebrate embryos is below the subphylum. The wide variation in mo rphology among vertebrate embryos is difficult to reconcile with the i dea of a phyogenetically-conserved tailbud stage, and suggests that at least some developmental mechanisms are not highly constrained by the zootype, Our study also highlights the dangers of drawing general con clusions about vertebrate development from studies of gene expression in a small number of laboratory species.