Chromosome numbers and evolutionary patterns in the Aster Occidentalis (Asteraceae) polyploid complex of western North America

The 15 species studied have chromosome numbers predominantly based onx=8, and intraspecific polyploidy is frequent. The numbersx=13 andx=18 in two species are hypothesized to represent dibasic alloploidy rather than aneuploidy. The group forms a hybrid complex, with 11 basic diploids thus far discovered and with much morphological variation observed among species at the hexaploid and higher polyploid levels. Both alloploidy and introgression probably contribute to this pattern of variability. The known diploids are assigned to eight species, with the suggestion that further taxonomic division may be warranted as the group becomes better known. Seven recognized species exist only as polyploids. Because of their reticulate relationships, the polyploid populations form an extensively intergading series of morphological types. It is cautioned that excessive taxonomic splitting, based on minor polyploid variants, might lead to an unwieldy and impractical classification for the group.