Chromosomal polymorphism in 12 populations of Mikania micrantha (Compositae)

Mikania micrantha is a climbing perennial weed of the family Asteraceae, wi th a vast distribution from South America to south of the United States. Th is species is widely distributed throughout Brazil, where it shows little m orphological variation. Mitotic chromosomes of 12 populations of M. micrant ha derived from several Brazilian sites were studied using Feulgen staining and C-banding. The populations included eight diploid (2n = 36 and 42) and four tetraploid (2n = 72) cytotypes. Chromosome numbers of 2n = 36 and 2n = 42 are reported for the first time for M. micrantha. These populations ha d a secondary constriction in the middle of the larger arm of chromosome pa ir 1, following the same pattern described for all Mikania species analyzed so far. Numerical and structural variation of the chromosomes was quite co mmon among the karyotypes and nearly all cytotypes differed from each other in some aspect. Most of the chromosomal differentiation may be attributed to inversions and addition or deletion of DNA fragments. C-banding, applied to three of the 12 populations, also revealed polymorphism in the distribu tion of heterochromatin. Additionally, one to 14 supernumerary or B-chromos omes were observed. The Bs were detected in six of the 12 populations and v aried in size, number, and structure among karyotypes and also among cells of the same root meristem. The B chromosomes were also heterochromatic, sho wing a C-banding pattern similar to the A chromosomes, and suggesting that they may be derived from the chromosomes of the A complement.