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Abstract
Seven species of Mammillaria were studied, all diploid: 2n=2x=22, x=11. Genome size was determined by flow cytometry, varied from 2C DNA =3.20 pg, 1570 Mbp (1 Cx) in M. crucigera, to 2C DNA = 3.04 pg, 1490 Mbp in M. flavicentra. The variation of these species in the nuclear content of DNA was 5% and was not significant (P = 0.3469). This indicates that they are small, very stable genomes, in which changes in DNA content are not very evident. The variation among species, however, was clearly evident in the relative length (L%) and arm ratio (r), the proportion of metacentrics and submetacentrics, and the position of satellites. The karyotype for M. albilanata, M. dixanthocentron and M. flavicentra was 11m, and in M. huitzilopochtli, 10m + 1 sm. Only one pair of chromosomes was observed with satellite in the four species. M. dixanthocentron and M. flavicentra, species considered synonymous, exhibited the satellite on different chromosomes. The interspecific variation observed among the species of Mammillaria is possibly due to spontaneous structural changes in their chromosomes. These mechanisms of restructuring in the genome of these species have not involved significant changes in nuclear DNA content. The Mammillaria species exhibited an endopolyploidy pattern with 2–16 C DNA content in the stem parenchyma, which may give them alternative strategies for adaptation in arid environments.
