Part II: Telling the Back from the Front
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Modern cell and developmental biology has a lot to contribute to our understanding of the deep history of animal origins, which until recently has been largely the province of paleontology. In this set of lectures, I hope to show how recent studies by a very small group of scientists on a virtually unknown phylum of marine organisms, the hemichordates, has helped explain some of the major mysteries of the origin of vertebrates. This is a tour of not only vertebrate origins but the contribution that modern molecular and genomic tools are making to developmental biology.
In the Introduction: Vertebrate body plans and the odd phylum of Hemichordates, I discuss the largely anatomical features that we use to identify the Vertebrates as a Subphylum or the chordates as a Phylum. These include such commonly perceived anatomical features, as the blocks of muscle around our trunk, called somites and tail. I also discuss some less obvious features, such as the notochord, a cartilaginous rod found in fish and found at least embryologically in every vertebrate. How did these originate from simpler organisms? I introduce a primitive related phylum, the hemichordates, and a particular animal, the acorn worm. In Part 1: The origin of the vertebrate nervous system: the Hemichordate perspective, I discuss why vertebrates ended up with a centralized nervous system that is highly organized from head to tail. It is surprising that the acorn worm has many of the patterning features of the vertebrate brain and centralized nervous system, although it has neither of these structures.
In Part 2: Telling the back from the front or what the chordates invented, I discuss why we look like invertebrate animals turned upside down, i.e. vertebrates have their central nervous system on their backs and invertebrates have it on their bellies.
In Part 3: How chordates got their chord, I discuss how the overall body plan of vertebrates, arose from the invertebrates based on knowledge of the commonalities in their developmental mechanisms. Here again, the acorn worm, offers the key comparison, being close enough to us to share some recognizable features, but far enough away to indicate the direction from whence we came.
Marc Kirschner is the founding chair of the Department of Systems Biology at Harvard Medical School. Prior to that, he Chaired the Department of Cell Biology at the same institution for ten years. Before moving to Harvard he was a Professor at the University of California in San Francisco for fifteen years. Prior to his tenure at UCSF, he was on the faculty at Princeton University. His PhD was with Howard Schachman at the University of California, Berkeley in 1971 in biochemistry.
Through his career Professor Kirschner has taken biochemical and molecular biological approaches to fundamental problems in cell and developmental biology, such as the dynamics and function of the cytoskeleton, the regulation of the eukaryotic cell cycle, and the process of signaling in embryos. In 1997 and in 2005 he collaborated with John Gerhart on two books aimed at explaining what cellular and developmental processes in organisms contribute to their evolution. He was elected to the National Academy of Sciences in 1989 and as a foreign member to the Royal Society of London in 1999. His current interests are cell growth, signaling and morphogenesis, and in particular quantitative approaches to these subjects.
- Nicole LeDouarin iBioSeminar: The Neural Crest in Vertebrate Development
- Marc Kirschner Short Clip: Spemann Organizer
- Nicole King iBioSeminar: Choanoflagellates and the Origin of Animal Multicellularity
Lowe, C.J., Wu, M., Salic, A., Evans, L., Lander, E., Stange-Thomann, N., Gruber, C.E., Gerhart, J., and Kirschner, M. (2003) Anteroposterior Patterning in Hemichordates and the Origins of the Chordate Nervous System. Cell 113 (7): 853-865.
Lowe, C.J., Terasaki, M., Wu, M., Freeman, R.M. Jr., Runft, L., Kwan, K., Haigo, S., Aronowicz, J., Lander, E., Gruber, C., Smith, M., Kirschner, M., Gerhart, J. (2006)
Dorsoventral patterning in hemichordates: Insights into Early Chordate Evolution. Public Library of Science Biol. 4(9):e291.
Gerhart, J., Lowe, C., Kirschner, M. (2005) Hemichordates and the origin of chordates. Current Opinion in Genetics and Development 15 (4): 461-467.
Gerhart J and Kirschner M. (2007) The theory of facilitated variation. Proc Natl Acad Sci USA. 104 Suppl 1:8582-9.