CMD logo, 26K

SATELLITE SYMPOSIUM #2 2007

Developmental Biology and Evolutionary Transformations

 

ABSTRACTS


Evolutionary developmental biology and the ancestry of modern amphibians

BOB CARROLL, NADIA FROLISCH, AND RAINER SCHOCH
Redpath Museum, McGill University, Montreal, Quebec

The ancestry and interrelationships of modern amphibian orders remain among the greatest unsolved problems in vertebrate phylogeny. The are two reason for these problems--a 35 million year gap in the fossil record between essentially modern frogs, salamanders, and caecilians from the Jurassic and the much more primitive fossils from the Paleozoic, and the division of research on amphibians between herpetologists, who can study all aspects of the hard and soft anatomy, physiology, ecology, behavior and molecular systematics of living species, and paleontologists who have little to work with except skeletal anatomy. Solutions to these problems can be gained through the use of the modern concepts of Evolutionary-Developmental Biology. Not only do we have an every increasing knowledge of how developmental processes observed in living taxa are related to evolutionary changes, but also increasing data from fossils of Jurassic and Palaeozoic larvae that document the early evolution of unique patterns and sequences of skeletal development. Thousands of specimens from the Upper Carboniferous and Lower Permian illustrate the timing of ossification of all elements of the skeleton from hatchlings to near metamorphosis. They document derived sequences of ossification of the skull, vertebrae, and limbs that are in common with modern salamanders but no other tetrapods. The derived configuration of the hyoid apparatus is also a synapomorphy shared with urodeles. This information demonstrates the early divergence of the lineages leading to salamanders and frogs. Recently described Lower Jurassic fossils of caecilians demonstrate their very early divergence from the lineages leading to frogs and salamanders.

 


From ancestral appendages to tetrapod limbs: A Hox story

BASILE TARCHINI, DENIS DUBOULE, MARIE KMITA
Institut de Recherches Cliniques de Montreal, Montreal, Quebec

Genes belonging to HoxA and HoxD clusters are required for vertebrate limb development. Mice lacking all, or a subset of, HoxA and HoxD genes have helped us unravel aspects of Hox functions during limb development. Our studies suggest that the evolutionary recruitment of Hox functions was crucial to ensure distal outgrowth of tetrapod limbs and concomitant implementation of Sonic hedgehog (Shh) signaling. We also found that the establishment of the gradient of Shh signaling, and thereby the anterior-posterior polarity of limbs, is linked to the ancestral strategy underlying the transcriptional regulation of Hox genes (co-linearity). We propose that the co-option of Hox genes together with their mode of regulation led to the emergence of limbs as polarized structures.

 


Molecular evolution and correlation of Hoxa11 and Hoxa13 genes to skeletal patterning changes across the fin to limb transition

LUKE HARRISON, HANS LARSSON
Redpath Museum, McGill University, Montreal, Quebec

Hoxa11 and Hoxa13 are important genes in autopodial development. Here we examine the molecular evolution of these genes and correlate them to morphological changes, particularly the skeletal patterning changes involved with the fin to limb transition. Changes in nucleotide sequences, amino acid sequences, and protein structure were examined across selected vertebrate species to best resolve this evolutionary transition. Rates of evolution vary considerably along branches and at some levels appear to correlate with periods of morphological change. Tests were also applied for the type of selection across branches. A number of branches were shown to be under positive selection, while others, purifying.

 


A model for the developmental genetic origin of the wing polyphenism in ants

MARCOS HAGMAN, LEON GLASS, EHAB ABOUHEIF
Department of Biology, McGill University, Montreal, Quebec

Wing polyphenism in ants, which originated once 125 million years ago, was a key developmental transformation underlying the remarkable evolutionary success of ants. In most of the 12000 described ant species, the queen and male caste possess fully functional wings, while the worker caste is completely wingless. We present a mathematical model to explain the possible origin and evolution of the gene network that underlies the wing-polyphenism in ants. Our model demonstrates that the wing polyphenism could have originated by tinkering with a gene of major effect. This model holds important implications for the evolutionary dynamics of this gene network.

 


Comparative Morphology & Development home page
(revised March 28, 2007)