Interspecific variation of nuclear DNA and structural changes in meiotic and mitotic chromosome in some species of Mammillaria (Cactaceae)

Mitotic and meiotic chromosome number, karyotype, chiasma frequency in poll en mother cells, Interphase Nuclear Volume (INV) and nuclear DNA amounts in 12 species of Mammillaria revealed significant interspecific variations. S ignificant variations in chromosome structure, chiasma number per nucleus, pollen sterility, INV and 4C DNA were recorded among the species. Chromosom e number 2n=22 and n=11 were recorded in all the species like M. armillata, M. asteriflora, M. bocasana, M. booli, M. bombycina, M. decipiens, M. gemi nispina, M. grandiflora, M. hahniana, M. plumosa, M sempervivi and M. woods i Chiasma frequency varied from 1.722 in M. woodsi to 2.618 in M. plumosa. The formation of univalents in some of the cells and/or spindle anomalies l ead to the formation of pentads, hexads or octads. Pollen sterility varied between 9.02% (M. decipiens) and 20.83% (M, bocasana). Cytophotometric esti mation of 4C DNA content showed significant variation among the species (18 .42 pg in M. booli to 26.46 pg in M. plumosa). INV varied significantly fro m 380.00 mu m(3) in M. grandiflora to 840.20 mu m(3) in M. booli. Chiasma f requency was positively correlated with chiasma per bivalent and 4C DNA con tent, moreover, the total chromosome length also correlated with chromosome volume. These results suggest evolution of Mammillaria through structural alterations of chromosome, chromosome duplication and translocations betwee n the chromosomes. Significant variations in the DNA amount with gross or m inor alteration of chisma frequency and karyotype variation leads to geneti c drift between the species.