Karyotypic studies in Opuntia cochinera, O-hyptiacantha, and O-strepthacantha (Cactaceae)

Plants of Opuntia cochinera, O. hyptiacantha and O. streptacantha species, collected in the Valley of Mexico, were consistently octoploids with a chro mosome number 2n=8x=88, confirming a base chromosome number x=11 for the ge nus Opuntia. Karyotypes of the three species were homogeneous with 88 metacentric chromo somes. Interspecific variation of the genome was shown in chromosome size, genome length and chromosomes with satellites. The smallest chromosomes wer e found in O. hyptiacantha (1.07-2.50 mum), the intermediate size in O. coc hinera (1.13-2.71 mum) and the largest in O. streptacantha (1.55-3.57 mum). Genome length was as follow: O. hyptiacantha had the smallest value (152.4 9 mum), O. cochinera an intermediate value (166.79 mum), and O. streptacant ha the longest (210.39 mum). Opuntia cochinera presented 16 chromosomes wit h a satellite, this fact suggest ed an auto-octoploid origin. There were 6 chromosomes with a satellite in O. hyptidcantha and 20 in O). streptacantha , it indicated that both species had an allooctoploid origin; so it is poss ible to call these Opuntia species with genomes of partial homology.