BODY PLAN COMPARISONS: How can we compare the body plan of hemichordates and chordates? What insights do these data give into the origin of chordates and the evolution of life history?
APPROACH: Large scale EST and genome sequencing. Broad expression study of developmental regulatory genes in each species. Functional analysis of selected genes.
CHOSEN SPECIES: The direct developing enteropneust hemichordate Saccoglossus kowalevskii, and the indirect- developing Schizocardium sp.
The phylum Hemichordata is composed of two extant classes: the solitary burrowing marine worms or enteropneusts, and the pterobranchs, which are small, colonial, sessile organisms. Bateson originally placed this group within the chordates due to many of the adult morphological affinities of the two groups, and it was not until much later that they were reclassified into their own phylum. Current molecular phylogenies place them as the sister group of the echinoderms. Despite the close relationship of these two groups, their respective adult body plans are highly divergent, and even gross axial morphological comparisons between the groups are problematic.
For any consideration of deuterostome evolution hemichordates are a critical component: their key phylogenetic placement and proposed morphological affinities with the chordates make them an intriguing but poorly described group. Our work, and that of other labs, is beginning to generate a substantial amount of both descriptive and functional molecular genetic data, allowing a more comprehensive phylogenetic sampling of developmental data within the deuterostome lineage.
These results are beginning to provide critical insights into early deuterostome evolution, the evolutionary origins of chordates, and larval evolution. Only by a comprehensive understanding of the body plans of all three of the deuterostome phyla can we hope to reconstruct the early evolutionary history of this major metazoan lineage, and understand the transitions that led to the unique and unusual body plans of this group
We work on two species of enteropneust; a direct-developing species Saccoglossus kowalevskii found up and down the eastern seaboard of the USA, and an indirect-developing species a Spengeliid with a true larval life history stage. Work on both species with contrasting developmental modes allows us to address hypotheses of adult and larval body plan evolution.