Graduate Thesis Or Dissertation
 

A biosystematic study in the genus Aster, section Aster, in western North America

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2227mr885

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  • A biosystematic investigation was undertaken in 15 species and three subspecies of the genus Aster, in western North America. Previous studies had been based entirely on external morphological features, and it was believed that much could be learned about the natural affinities of the taxa by exploring their cytological and genetical characteristics. More than 160 accessions were made during the course of the study. In the majority of cases, living rhizomes were transplanted in the greenhouse to provide material for making chromosome counts and hybridization experiments. Whenever possible, chromosomes were observed during meiotic divisions in microsporogenesis. One or more chromosome counts were made in 17 of the 18 taxa studied, 12 of which had not previously been examined cytologically. Ten taxa were found to include plants at two or more chromosome levels. The entire assemblage of species contains chromosome numbers on the basis of X = 8, with counts ranging from n = 8 to n = 48. Examples of aneuploidy, B-chromosomes, and meiotic irregularities were also discovered. A total of 383 artificial hybridizations was made among the species studied, resulting in the production of at least one fruit from 198 crosses. Most of the attempted crosses were interspecific, with the results indicating a striking lack of genetic incompatibility between the species at all polyploid levels. However, nearly all crosses involving diploid plants produced no fruits. First-generation hybrids of most species combinations were grown to maturity, and their potential fertility was estimated by observing the stainability of their pollen. The percent of pollen stainability in most of the hybrids was high enough to be judged partly or fully fertile. The polyploid complex composed of the species of Aster included in this study is comparable to certain other known complexes. Such complexes are similar in that the diploids are rather distinct morphologically and genetically, but they have contributed to the formation of a vast array of potentially interbreeding polyploids which lack sharp morphological definition. Most of the polyploids can be assigned to morphological categories that are more similar to related diploids than to other polyploids. Consequently, certain of the species have been delineated along vertical lines of relationship which may include several chromosome levels, In addition, some entities are retained as species which are known only as polypoids. which are relatively distinct and cannot be definitely donnested to a diploid species. This method is believed to be more indicative of relationship than would a system of grouping the entire polyploid assemblage in one highly polymorphic species, A phylogenetic chart of putative relationships among the taxa is presented, based largely on chromosome counts and morphology. The diploid entities are judged to be ancestral to, and thus more primitive than the polypoids, It was not possible to establish particular morphological traits as being more primitive or more advanced, Bathe it appears that evolution of morphological features has occurred at the diploid level, with subsequent polyploidy serving primarily to recombine the distinctive diploid features into various combinations. The information gained from the hybridization experiments contributed relative little to an understanding of natural affinities among these species, owing to the free crossability between the polyploids., Too few crosses were made involving diploids to determine accurately the degree of genetic incompatibility. Over 8000 herbarium specimens were examined in the process of delimiting species. The taxonomy section includes a key to the species, plus lists of synonyms, technical descriptions, citation of representative specimens, distribution maps, and brief discussions of the species. One new species, Aster idahoensis, is described in the thesis.
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