Abstract
Chromosome data from PMCs and/or root tip cells in 16 species belonging to four genera (Struchium, Centratherum, Vernonia, Elephantopus) of the tribe Vernonieae, and nine species belonging to four genera (Adenostemma, Ageratum, Eupatorium, Mikania) of the tribe Eupatorieae from South India are reported. Karyomorphology was studied in most of them.
The basic chromosome constitution and their evolution in the two tribes are discussed in the light of available chromosome data. In both tribes, x=10 is considered to be the original or earlier evolved condition from which the lower ones evolved by progressive reduction, and the higher ones by polyploidy followed by reduction. The consistently less specialised karyotypes in the x=10 taxa is shown to support this. In the Eupatorieae, the x=10 condition is postulated to have originated from an ancestral n=5. The occurrence of chromosome numbers in a regular series in multiples of 5 in the type genus Eupatorium is shown to support this.
The systematic relationships of the Vernonieae and Eupatorieae are considered. The predominant occurrence of x=10 in both the tribes is pointed out to provide cytological support for their close relationship, which in turn are most related to the Heliantheae.
