Numerical analyses of karyotypic diversity in the genus Eupatorium (Compositae, Eupatorieae)

Somatic metaphase karyotypes were analyzed for 22 diploid species of Eupatorium. The karyotypic comparisons were made using two indices: minimal chromosomal distance (MCD), measuring overall dissimilarities, and chromosomal identity (CI), measuring number of morphologically identical chromosomes between species. The resulting phenograms from these indices are largely compatible. The 22 species cluster into four groups in the phenogram using MCD, and the grouping corresponds well with morphology or geographic distribution into the three N. American groups Eutrochium, Uncasia, Traganthes, and the E. Asian group. These results suggest that karyotypes in perennial Eupatorium have been considerably conservative and changed not through large chromosomal mutations but through small chromosomal mutations gradually fixed.