Chromosome analysis of Mammillaria supertexta, M. crucigera and M. haageana and their comparison with M. san-angelensis (Cactaceae)

This is the first report of chromosome number in Mammillaria supertexta, M. crucigera and M. haageana, which was the same in the three species: 2n=22, x=11. This number coincides with the number reported for the genus Mammilaria and for the Cactaceae family. However, interspecific variation was observed in the karyotypes: M. supertexta exhibited longer chromosomes (1.79-3.21 μm) and those of M. haageana were shorter (1.51-2.69 μm); in M. crucigera the chromosomes were 1.63-2.74 μm. The chromosomes of M. san-angelensis (0.80-1.70 μm) are the shortest (Palomino et al. 1999). Likewise, variation in the length of the haploid genome (LG) was observed: 26.84 μm for M. supertexta; of 23.81 μm for M. crucigera; and 23.06 μm for M. haageana. The LG of M. san-angelensis was shorter than that of the above species (13.83 μm). The karyotypic formula for M. supertexta and M. crucigera was 10m+1sm, and in M. haageana it was 9m+2sm. A pair of chromosomes with satellites was observed in M. supertexta and M. haageana, and in M. crucigera two pairs of chromosomes were observed with satellites. Unlike these species, M. san-angelensis exhibited no sm chromosome; all were metacentric, and like M. crucigera, there were two pairs of chromosomes with satellites. The index of asymmetry calculated for M. supertexta was 43.44%, for M. crucigera it was 42.55%, and for M. haageana it was 42.71%, indicating that the karyotypes were symmetric. The karyotype of M. haageana (9m+2sm) was different from that determined for M. san-angelensis (11m), thus showing divergence in its genomes. Hunt (1987) considers them to be synonymous, and complementary studies on morphology, hybridization, and cytogenetics are needed to determine the taxonomic category of these plants. M. haageana exhibited a total of 11 bivalents, a chiasmata frequency (Fq) = 13.86 and a recombination index (RI) = 24.86. These results are different from those of M. san-angelensis whose Fq = 16.74 and RI = 27.74. The higher RI in M. san-angelensis indicates that this species has more possibilities of forming new genetic combinations in its progeny and more opportunities of adapting to environmental changes than M. haageana with a lower RI. The interspecific variation observed among the four species of Mammillaria is possibly due to spontaneous structural changes in their chromosomes.