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Biosystematics of a Western North American polyploid complex in the genus aster

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  • Species relationships were investigated in a western North American polyploid complex in the genus Aster (Asteraceae). A combination of cytological, genetical, chemical, morphological and numerical approaches was employed. Fifteen presently recognized species were included in the study, and populations were sampled throughout most of the range of the group. Living plants from these populations were maintained in greenhouse and garden culture for studies of reproductive behavior, crossability, and flavonoid chemistry. A total of 232 chromosome counts were made from the sampled populations. Chromosomes were observed at meiosis in flower buds and in mitotic divisions in root-tip cells. The principal base chromosome number in the group is x = 8; six ploidy levels occur, ranging from n = 8 to n = 48. A few aneuploid numbers based on x = 9 were also recorded, and one species was found to have a base number of x = 13. Measurements of 35 morphological characters on 78 diploid specimens, followed by cluster analysis and discriminant analysis of the clusters, resolved seven phenotypic groups at the diploid level. These correspond to seven currently recognized taxa in six species. Three other taxa in which a diploid chromosome number was found failed to separate from these clusters; this may reflect the choice of characters used in the analysis, or an incorrect placement of these species by contemporary taxonomists. Morphological variation among the polyploids was greater than at the diploid level, and this variability included intermediates between certain diploid groups as well as forms not found among the extant diploids. Crossing experiments between diploids showed that in most cases the species are only partially interfertile. An exception was Aster greatai, a morphologically distinctive endemic taxon of southern California, which crosses readily with most of the other taxa. Several of the crosses yielded hybrid progeny; these had normal chromosome pairing at meiosis but lowered pollen viability. A preliminary investigation of flavonoid chemistry in 30 populations (representing seven taxa) was carried out using two-dimensional thin-layer chromatograms. Considerable chemical variation was found, which was not closely correlated with variation in external morphology. Detailed analysis of a clone of Aster eatonii revealed 11 flavones and flavonols, which were characterized and identified. A new finding was the common occurrence of diploid and tetraploid chromosome numbers based on x = 13 in the widespread species Aster ascendens. The karyotype of A. ascendens is distinctly asymmetrical, in contrast to the symmetrical karyotypes of the species based on x = 8. In view of the asymmetrical karyotype reported earlier for the related species A. ericoides (with n = 5), it seems likely that the genome of A. ascendens evolved from hybridization between a taxon with n = 5 and one (perhaps A. occidentalis) with n = 8. The complex is considered to contain approximately 14 species, of which nine contain some diploid populations. The present study extends the known morphological and geographical ranges of the diploid taxa, and improves understanding of species relationships and probable evolutionary lines within the group.
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