University of California, Berkeley
Dept. of Integrative Biology
3060 VLSB #3140
Berkeley, CA 94720-3140
Lab phone: 510/643-4201
Fax: 510/643-5022
Email: vanhook@gmail.com
The segmental pattern of the notostracan crustacean Triops longicaudatus is unusual among segmented animals in that there are an unequal number of dorsal and ventral segments, and the segments do not align along the dorsoventral (DV) axis. This segmental pattern is interesting for two reasons, the first of which is that it has not been observed in any other living animal outside the family Triopsidae. However, it has been observed in specimens of the lower Cambrian arthropod Branchiocaris (540 mya) from three different lower Cambrian sites (the Burgess shale, Utah and China). Therefore such an asymmetrical segmentation pattern has been around nearly as long as the crustaceans themselves.
The Triops segmentation pattern is interesting secondly because the anatomy of Triops itself has not changed in about 300 million years. Fossil Triops from the Carboniferous period (360 to 280 mya) have been assigned to the extant European species Triops cancriformis, indicating that this unique segmentation pattern is not a derived character. While it may not represent an ancestral character, it has certainly been around for a long time, so it is a useful model for understanding how early crustaceans may have organized segments along the anteroposterior (AP) axis.
The study of crustacean patterning systems, especially a basal crustacean like Triops, will give us insights into the evolutionary origins of segmentation, whether segmentation is likely to have arisen once in evolution or developed independently in each of the three lineages. It is not known whether segmentation arose independently in the three groups that have a segmented body plan today (arthropods, annelids and vertebrates) or was present in a common ancestor. Some components of AP patterning systems are conserved between arthropods and vertebrates, but entire systems are not. Most of what we know about AP patterning in the arthropods comes from work in the long-germ insect Drosophila melanogaster. Even amongst arthropods, the long-germ mode of development is unusual. Most arthropods, including crustaceans, develop by adding body segments from a growth zone, termed the “short-germ” mode in insects and similar to the manner vertebrates add somites from a posterior region of presomitic mesoderm, rather than patterning the entire axis onto a blastoderm, like the long-germ insects.
Transcrption factors belonging to the Hairy/Enhancer of Split (HES) family play important roles in segmentation in diverse organisms from the fruit fly to the mouse. I was able to identify only a single HES family member in Triops by degenerate PCR (Tlhes1) and am currently completing the assembly of its sequence by RACE and chromosomal walking methods. RNA in situ analysis followed by functional studies, if warranted by the Tlhes1 expression pattern, are the next step in analysis of the Triops HES family member. I am also currently working on an analysis of Pax Group 3 expression during Triops segmentation.
Triops’ unique segmentation appears to be accomplished by differential regulation of the cell cycle in dorsal and ventral regions of the embryo. It is curious that cell cycle rates appear to be differentially controlled across a single tissue. In the future, I would like to investigate what role cell cycle plays in uncoupling the dorsal and ventral sides of the animal by altering cell cycle rates pharmacologically and/or by RNAi. If differing cell cycle rates are responsible for the discrepancy between dorsal and ventral segments, the really interesting question then becomes: How do the cell cycle rates become different in the first place? Is there a clock, established early in development, that triggers the change at a predetermined time? Is each side regulated by its own clock so that uncoupling occurs when one or both clocks switch from a regulated mode to a free-running mode? It is curious that the uncoupling of the dorsal and ventral sides occurs posterior to the genital segments. Once the animal has made feeding and reproduction appendages, perhaps the posterior portion of the animal is simply not so important and thus is not constrained in its development. The ultimate cause of the segmentation irregularities in Triops is likely to be found in its dorsevental patterning system, but we are far from knowing where or how early in development the asymmetry is determined.
Publications:
VanHook, A., and A. Letsou. 2003. Bully for bugs. Developmental Dynamics.
VanHook, A., and A. Letsou. 2003. Closing the gap: signaling cell shape changes in dorsal closure. Recent Research Developments in Genetics 3:85-106.
VanHook. A., and A. Letsou. 1998. Signaling cell shape changes and motility in the Drosophila embryo. Proceedings of the 17th International Cancer Congress pp.77-82.
Education:
Ph.D. Human Genetics
University of Utah
Department of Human Genetics
Anthea Letsou, advisor
September 1994 - September 2001
B.A., M.A. Biology
Kenyon College
David, Marcey, advisor
Cum Laude, Biology High Honors with Thesis
September 1990 - June 1994
Additional Research Experience:
Associate Online Editor. AAAS Science Signalling.
Postdoctoral Research. Department of Integrative Biology / Howard Hughes Medical Institute. University of California at Berkeley. Advisor: Nipam Patel. Project Title: Segmentation In Branchiopod Crustaceans. November 2002 to Present.
Postdoctoral Research. Department of Human Genetics, University of Utah, Salt Lake City, Utah. Advisor: Anthea Letsou, Ph.D. Project Title: The role of Beclin in Drosophila development. January-September 2002.
Doctoral Research. Department of Human Genetics, University of Utah, Salt Lake City, Utah. Advisor: Anthea Letsou, Ph.D. Project Title: The role of Keph in TGF-b-mediated signal transduction. June 1995 - September 2001
Undergraduate Honors research. Department of Biology, Kenyon College, Gambier, Ohio. Advisor: David Marcey, Ph.D. Thesis Title: Electron microscopic analysis of Exu protein function during Drosophila oogenesis. September 1992 – June 1994.
Undergraduate Research. Department of Biology, Kenyon College, Gambier, Ohio. Advisor: David Marcey, Ph.D. September 1990 – May 1992.
