Chromosomal polymorphism in 12 populations of Mikania micrantha (Compositae)

Abstract

Mikania micrantha is a climbing perennial weed of the family Asteraceae, with a vast distribution from South America to south of the United States. This species is widely distributed throughout Brazil, where it shows little morphological variation. Mitotic chromosomes of 12 populations of M. micrantha 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 had a secondary constriction in the middle of the larger arm of chromosome pair 1, following the same pattern described for all Mikania species analyzed so far. Numerical and structural variation of the chromosomes was quite common 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 distribution of heterochromatin. Additionally, one to 14 supernumerary or B-chromosomes were observed. The Bs were detected in six of the 12 populations and varied in size, number, and structure among karyotypes and also among cells of the same root meristem. The B chromosomes were also heterochromatic, showing a C-banding pattern similar to the A chromosomes, and suggesting that they may be derived from the chromosomes of the A complement.

Chromosome studies in the genus Mikania (Asteraceae)

“…In all the species and cytotypes so far studied (Ruas and Ruas, 1987;Ruas, 1989;Maffei, 1996;Ruas and AguiarPerecin, 1997;Maffei et al, 1998Maffei et al, , 1999b Barroso (1958Barroso ( , 1986 (Table I).…”

“…Thirty-two taxa of Mikania have so far been examined, including 16 different species (Ruas and Ruas, 1987;Ruas, 1989;Ruas and Aguiar-Perecin, 1997), 14 populations of M. micrantha (Ruas and Ruas, 1987;Maffei, 1996;Maffei et al, 1998Maffei et al, , 1999b and two populations of M. cordifolia (Ruas and Ruas, 1987;Maffei et al, 1999a). All the karyotypes were characterized by the presence of a secondary constriction in the long arm of chromosome 1, and Ruas and Ruas (1987), in the analysis of six species, proposed that this secondary constriction should be considered a cytological marker for the genus.…”

“…Fourteen cytotypes of M. micrantha were studied, all from different localities in Brazil (Ruas and Ruas, 1987;Maffei, 1996, Maffei et al, 1998, 1999b; seven were diploid, six with 2n = 2x = 36 and one with 2n = 2x = 42, while six were tetraploid with 2n = 4x = 72. In all the cytotypes of M. micrantha chromosome pair 1 was the most inconstant, showing variation in both size and structure.…”

“…The population from Praia Grande (São Paulo State) had the smallest chromosomes and haploid length. This difference in chromosome size occurred in all the chromosome pairs, reflecting a gain or loss of genetic material (Maffei, 1996;Maffei et al, 1999b). Two other cytotypes (from Alfredo Guedes and Campinas), both with 2n = 36, showed the same karyotype formula and similar haploid sets but they differed slightly in their arm-ratio values, which were probably modified by inversions (Maffei, 1996;Maffei et al, 1999b).…”

“…This difference in chromosome size occurred in all the chromosome pairs, reflecting a gain or loss of genetic material (Maffei, 1996;Maffei et al, 1999b). Two other cytotypes (from Alfredo Guedes and Campinas), both with 2n = 36, showed the same karyotype formula and similar haploid sets but they differed slightly in their arm-ratio values, which were probably modified by inversions (Maffei, 1996;Maffei et al, 1999b). Similar conclusions can be reached for the analysis of the six tetraploids with 2n = 4x = 72 chromosomes, of which the population from Londrina showed the largest haploid set, with most of the variation between the tetraploids being represented by centromeric shifts.…”