Somatic Chromosome Reduction in Sorghum After Colchicine Treatment

Diploid mutant plants that breed true in subsequent generations arise following colchicine treatment of sorghum seedlings of the variety Experimental 3. The production of diploid mutants was found to be dependent upon the genotype as well as the presence of certain environmental conditions after colchicine treatment. To explain the homozygosity of the mutants, it has been proposed that a reduction of chromosomes to a haploid condition occurred followed by doubling to restore the diploid number. To test this hypothesis, seedlings heterozygous for chromosome markets (one and two reciprocal translocations) were treated. After treatment of 320 seedlings heterozygous four one reciprocal translocation (two marked chromosome pairs) four immediately true-breeding diploid mutants were observed among the 188 survivors. Two true-breeding mutants among 90 surviving plants were obtained following treatment of 124 seedlings heterozygous for two reciprocal translocations (four marked chromosome pairs). Examination of the chromosomes of the six mutants at diakinesis showed no configuration of four chromosomes indicating that they were homozygous for the marked chromosomes. The occurrence of mutants with homozygous normal chromosome structure arising from colchicine-treated seedlings heterozygous for two reciprocal translocations removes the possibility of selfing and androgenesis, since in this case the mutants had a different structure than either parent. The homozygosity of the chromosomes in only the true-breeding mutant plants and not in any of the unmutated plants may indicate that the somatic reduction phenomenon is associated in some way with the mutational phenomenon