Karyotypic studies in the genus Echinocereus (Cactaceae) and their taxonomic significance

Mitotic chromosomes of 11 diploid (2n = 22) species of Echinocereus and five varieties of the polyploid (2n = 4x = 44) E. engelmannii included in the seven taxonomic sections of the genus were studied. The genus exhibits relatively symmetric karyotypes, having mostly metacentric chromosomes. Likewise, interspecific and intraspecific variability was observed in terms of genome length, chromosome length, and karyotipic formula. There was also variability in the number of satellites, and they were observed on the short arms of the chromosomes. There is an apparent correlation in decreasing chromosome and genome length in species with more derived morphological characters. In addition, karyotype asymmetry increases as the chromosome and genome length increases. Polyploid taxa investigated had larger chromosomes and genomes than diploid taxa. Robertsonian changes and centric fusion of telocentric chromosomes, are thought to be involved in the existence of homogeneous karyotypes. No evidence of structural chromosomal rearrangements was detected. Overall, the high morphological plasticity that characterizes the genus Echinocereus parallels the relatively large karyotypic variability found in the species investigated.