The cytogeography of Aster ageratoides var. laticorymbus (Asteraceae), a polyploid complex endemic to China

Abstract

Aster ageratoides var. laticorymbus is distrubuted from southwestern China to eastern China. Population sampling across its whole distribution area was made in its twenty-six populations. Chromosome numbers of all the populations were investigated, and six populations, representative of various ploidy levels, were analyzed karyotypically for the first time. The results show that this variety is a polyploid complex comprised of 2x, 4x and 6x. The populations are mostly hexaploid (2n=6x=54) and occupy an extensive area from southwestern China to eastern China while the diploid (2n=2x=18) and the tetraploid (2n=4x=36) are less frequent and limited to narrower regions of the Yunnan-Guizhou Plateau and the transitional belts from the plateau to adjacent areas. Based on the cytogeographical distribution of the complex, the analysis of its karyotypes, its morphological variations related to geography and a previous report, some hypothesis are made: (1) the polyploids of the complex might be autopolyploids, and the tetraploid might have originated independently twice; (2) in terms of some karyotypical parameters, different ploidy levels of the complex evolved at different speeds; (3) the Yunnan-Guizhou Plateau is the diversity center and the origin center of the complex, and the place from which the hexaploid dispersed eastwards to eastern China; (4) the distribution pattern that the hexaploid is dominating might have been formed by the competitive exclusion and its high capability to disperse and occupy new habits; and (5) the variety is a relative young complex.