Reports of 68 new chromosome counts attributed to 53 species from 25 g
enera of Eupatorieae of the Asteraceae, based mostly on determinations
of mitotic materials, include first counts for 2 genera (Acanthostyle
s and Lepidesmia) and 14 species and new reports for 8 species. B chro
mosomes are reported for 4 genera and 12 species. Karyotype analyses m
ade on 20 species of Eupatorieae and one species of Heliantheae showed
that total karyotypic lengths of the taxa with n = 16-19 of heliantho
id and eupatorioid taxa are comparable to those of some eupatorioid ta
xa with n = 10. This is contrary to the previous hypothesis that the h
igher chromosome numbers n = 16-19 were derived from n = 10 by polyplo
idization followed by dysploid loss. Cytological data supplement and a
re consistent with the following conclusions predicted from molecular
phylogenetical and biochemical data: (1) The ultimate base number of E
upatorieae is 17, and the lower numbers are derived by successive dysp
loid reductions; (2) A reduction in chromosomal and total karyotypic l
ength accompanied by evolutionary advancement has been revealed for so
me genera and species within this tribe; (3) A high base number of x =
17 in Eupatorieae is considered to be derived directly from one of th
e members of Heliantheae with n = 17 to 19.