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The following HTML text is provided to enhance online readability. Many aspects of typography translate only awkwardly to HTML. Please use the page image as the authoritative form to ensure accuracy. with high chromosome numbers of ancient polyploid origin? If so, what can we learn about gene silencing from these plants? How extensive has gene silencing been, and is there evidence for the cooption of duplicated genes for new function? The study of polyploidy is a dynamic and open area of research, ranging from molecular genetic comparisons to population genetics, with important implications for the biology and evolution of the majority of plant species. We thank Kent Holsinger and an anonymous reviewer for helpful comments on the manuscript. This research was supported, in part, by the National Science Foundation. This work is dedicated to the memory of G. Ledyard Stebbins. REFERENCESBarrett, S. C. H. & Shore, J. S. ( 1989) Isozyme variation in colonizing plants. In Isozymes in Plant Biology, eds. Soltis, D. E. & Soltis, P. S. (Dioscorides Press, Portland), pp. 106–206. Brehm, B. G. & Ownbey, M. ( 1965) Variation in the chromatographic patterns in the Tragopogon dubius-pratensis-porrifolius complex (Compositae). American Journal of Botany 52, 811–818. Charlesworth, D. & Charlesworth, B. ( 1987) Inbreeding depression and its evolutionary consequences. Annual Review of Ecology and Systematics 18, 237–268. Cook, L. M. ( 1998) Mating systems and multiple origins in the North American Tragopogon complex. Ph.D. dissertation. Washington State University. Cook, L. M. & Soltis, P. S. ( 1999) Mating systems of diploid and allotetraploid populations of Tragopogon (Asteraceae) I. Natural populations. Heredity 82: 237–244. Cook, L. M. & Soltis, P. S. ( 2000) Mating systems of diploid and allotetraploid populations of Tragopogon (Asteraceae) II. Artificial populations. Heredity: 84, 410–415. Cook, L. M., Soltis, P. S., Brunsfeld, S. J. & Soltis, D. E. ( 1998) Multiple independent formations of Tragopogon tetraploids (Asteraceae): Evidence from RAPD markers. Molecular Ecology 7, 1293–1302. Crawford, D. J. ( 1983) Phylogenetic and systematic inference from electrophoretic studies In Isozymes in Plant Genetics and Breeding, Part A, eds. Tanksley, S. D. & Orton, T. G. (Elsevier, Amsterdam), pp. 257–287. Ehrendorfer, F., Krendl, F., Habeler, E. & Sauer, W. ( 1968) Chromosome numbers and evolution in primitive angiosperms. Taxon 17, 337–468. Felber, F. ( 1991) Establishment of a tetraploid cytotype in a diploid poulation: effect of relative fitness of the cytotypes. Journal of Evolutionary Biology 4, 195–207. Federov, A. (ed.). ( 1969) Chromosome Numbers in Flowering Plants (Academy of Sciences of the U.S.S.R., Leningrad). Fowler, N. L. & Levin, D. A. ( 1984) Ecological constraints on the establishment of a novel polyploid in competition with its diploid progenitor. American Naturalist 124, 703–711. Goldblatt, P. ( 1980) Polyploidy in angiosperms: monocotyledons. In Polyploidy-Biological Relevance, ed. Lewis, W. H. (Plenum Press, New York), pp. 219–239. Gottlieb, L. D. ( 1982) Conservation and duplication of isozymes in plants. Science 216, 373–380. Grant, V. ( 1981) Plant Speciation. 2nd Edition (Columbia University Press, New York). Grant, V. ( 1982) Chromosome number patterns in primitive angiosperms. Botanical Gazette 143, 390–394.
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