CHROMOSOME-NUMBER VARIATIONS IN MICROPROPAGATED TRUE-TO-TYPE AND OFF-TYPE BANANA PLANTS (MUSA-AAA-GRANDE-NAINE-CV)

The objective of the present study on banana plants (Musa AAA Grande N aine cv.), obtained by in vitro shoot tip culture, was to determine wh ether modifications in chromosome number could account for the appeara nce of the off-types with mosaiclike leaf defects or dwarf stature, th e most frequent off-types observed after micropropagation. Chromosome counts were conducted on shoot tip samples treated with 8-hydroxyquino line, digested in pectinase and stained with Schiff's reagent. On aver age, 160 counts were made for each treatment. Four types of plant mate rial were studied: phenotypically true-to-type plants, dwarf off types , mosaiclike off-types obtained by micropropagation, as well as true-t o-type plants obtained by standard propagation techniques of suckers w ith no micropropagation history. Some cells from all four types of pla nt material were found to have an abnormal chromosome number (i.e., 2n = 3x = 33), characteristic of triploid bananas. The percentages of an euploid cells were 14%, 22%, 35%, and 5%, respectively. Descending ane uploidy was noted in micropropagated plants derived from true-to-type and dwarf off-type suckers. The statistical analysis revealed that the two latter types of plant material had the same percentage of aneuplo id cells. Thus, the dwarfism could not be correlated with a change in the chromosome number. Conversely, ascending aneuploidy was observed i n the mosaiclike material, with 34 or 35 chromosomes in almost 28% of the cells. This percentage was significantly higher than in true-to-ty pe plants and highlight the genetic origin of the mosaiclike variation .