Morphological and Cytological Variation in the Western North American Aster occidentalis Complex (Asteraceae)

Abstract

The Aster occidentalis complex is a western North American group of rhizomatous perennials. The group shows considerable morphological variation and includes at least six ploidy levels with chromosome numbers ranging from n = 8 to n = 48. In this study, 283 chromosome counts are reported; most specimens sampled had euploid chromosome numbers based on x = 8, but a few aneuploid plants were also found. The relationship between morphological variation and chromosome number was assessed using a cluster analysis and principal components analysis of up to 35 characters in 172 specimens of known ploidy level. The diploid specimens fell into six morphological groups, corresponding to five of the approximately ten species included in the complex. The polyploids showed greater variability and more extensive intergradation between groups than did the diploids. The geographic distributions of chromosome numbers in the more widely distributed species are also discussed. Aster, taxonomically one of the most difficult of North American genera, has presented sys- tematists with numerous problems in delimit- ing species boundaries and determining rela- tionships. The genus is cytologically variable and contains chromosome numbers based mainly on x = 5, 8, and 9; both aneuploids and polyploids have been reported (Dean 1966; Dean and Chambers 1983; Jones 1980b; Semple