Homeotic effects, somitogenesis, and the evolution of vertebral numbers in recent and fossil amniotes

MULLER, J.; SCHEYER, T.M.; HEAD, J.J.; BARRETT, P.M.; WERNEBURG, I.; ERICSON, P.; POL, D.; SANCHEZ-VILLAGRA, M.

PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA

National Academy of Sciences (USA)

Lugar: Washington; Año: 2010 vol. 107 p. 2118 - 2123

0027-8424

The development of distinct regions in the amniote vertebralcolumn results from somite formation and Hox gene expression,with the adult morphology displaying remarkable variation amonglineages. Mammalian regionalization is reportedly very conserva-tive or even constrained, but there has been no study investigatingvertebral count variation across Amniota as a whole, underminingattempts to understand the phylogenetic, ecological, and develop-mental factors affecting vertebral column variation. Here, we showthat the mammalian (synapsid) and reptilian lineages show early intheir evolutionary histories clear divergences in axial developmen-tal plasticity, in terms of both regionalization and meristic change,with basal synapsids sharing the conserved axial configuration ofcrown mammals, and basal reptiles demonstrating the plasticity ofextant taxa. We conducted a comprehensive survey of presacralvertebral counts across 436 recent and extinct amniote taxa. Verte-bral counts were mapped onto a generalized amniote phylogeny aswell as individual ingroup trees, and ancestral states were recon-structed by using squared-change parsimony. We also calculated therelationship between presacral and cervical numbers to infer therelative influence of homeotic effects and meristic changes andfound no correlation between somitogenesis and Hox-mediatedregionalization. Although conservatism in presacral numbers char-acterized early synapsid lineages, in some cases reptiles and synap-sids exhibit the same developmental innovations in response tosimilar selective pressures. Conversely, increases in body mass arenot coupled with meristic or homeotic changes, but mostly occur inconcert with postembryonic somatic growth. Our study highlightsthe importance of fossils in large-scale investigations of evolution-ary developmental processes.