Molecular Analysis of the Evolution and Development of the Chordates

Our collective goal is to understand the evolution of the chordate body plan, a complex problem that requires interdisciplinary research. We combine methods and approaches in phylogenetics, development, ecology and evolution to study the evolution of this unique body plan from a deuterostome ancestor. We believe that the deuterostome ancestor was a burrowing worm, with gill slits and a cartilaginous skeleton (Swalla and Smith, 2008).

The major project in my laboratory is aimed at understanding the evolution of chordates. One part of this project is to figure out the phylogeny of tunicates (urochordates) and hemichordates in order to understand phylogenetic relationships of the different families in these groups. Because the phylogenetic analysis suggests that enteropneust worms may be basal hemichordates, we are studying the development of direct developing saccoglossid hemichordates and also ptychoderid worms that have a larvae similar to echinoderm larvae. We first focused on the gill slits and cartilage of the gill bars in hemichordates and cephalochordates and found that the gill slits between hemichordates and chordates are homologous, but the cartilage gill bars are not (Rychel and Swalla, 2007). We are now looking at anterior structures, including the stomochord, central nervous system, and heart-kidney complex in an effort to understand possible homologies with either chordates or echinoderms.

The second major research project is aimed at understanding the phylogeny and evolutionary diversity of hemichordates, in collaboration with Dr. Ken Halanych at Auburn University. We are specifically interested in the evolution of the nervous system and the anterior structures and how they vary morphologically between the different families of hemichordates. We are comparing solitary and colonial hemichordates to see how gene expression changes influences adult morphology. This project has just been funded by NSF.

Finally, our third major research project to study the evolution of coloniality in ascidians. We have identified a clade of stolidobranch ascidians that contains species with solitary, social and colonial lifestyles (Zeng et al. 2006). We are looking at the molecular basis of coloniality by examining genes that are turned on during metamorphosis, especially in germ cells and somatic stem cells. One set of genes that is activated at metamorphosis are the innate immunity genes. The innate immune system appears to be critical for remodeling the body plan during metamorphosis. We are also examining the role of stem cells and cell division in bud formation in colonial larvae. Collectively, this research may allow insight into the amazing regenerative powers of colonial ascidians.