1. Department of Biology, University of Missouri, St. Louis, Missouri 63121
2. Donald Danforth Plant Science Center, St. Louis, Missouri 63132
3. Division of Biology and Biomedical Sciences, Washington University in St. Louis, St. Louis, Missouri 63110
4. Section of Cell and Developmental Biology, Division of Biological Sciences, University of California, San Diego, La Jolla, California 92093