CHANGES IN CHROMOSOME NUMBER IN MICROSPORE CALLUS OF RICE DURING SUCCESSIVE SUBCULTURES

Forty-six callus cultures of rice (Oryza sativa L., 2n = 24), each presumably originating from a single microspore, were established and maintained on a medium containing 2,4-D. At the end of the first transfer 24% of the cultures were nonhaploid consisting of only diploid or polyploid cells, or of cells of two ploidy levels. Nuclear fusion and endomitosis occurring during the initial stages of in vitro microspore development were postulated to account for the formation of nonhaploid callus. Seventeen cultures were studied cytologically through 19 transfers. Only in one tetraploid and one hexaploid callus did the ploidy levels of cells remain unchanged during culture. Chromosome numbers in 13 cultures fell into a geometric series. Since no diplo- and quadruplochromosomes were observed, it was inferred that endomitosis rather than endoreduplication was responsible for the changes. In spite of the tendency for chromosome doubling, the proportions of cells of different ploidy levels were fixed in the 13 cultures at later transfers. Haploid cells were eliminated from all cultures. Diploid cells became predominant in eight cultures and tetraploid cells in five, suggesting a selection for either cell type. Triploids appeared in two cultures which initially did not contain this type of cell. Limited cytological information indicated that triploid cells might have originated from tetraploid cells through reductional grouping of chromosomes accompanied by multipolar formation.