Repetitious Production of Similar Karyotypes in Different Plants of Haplopappus gracilis, an Annual Asteraceae, Following Exposure to Ionizing Radiation

Abstract

Following exposure to X-rays or reactor radiation at the early germinating stage, seedlings of Haplopappus gracilis (Nutt.) Gray were inspected for karyotypic changes in the meristems of lateral roots 2 months later. Among 210 plants studied in total, 12 were carriers of aberrant karyotypes in the root systems. One of them was a whole-body variant of spontaneous origin, showing a complete change of karyotype not only in the root system but also in the shoot system. The remainders were mosaics, showing a complete karyotypic change in one or more lateral roots but not in the whole root system. Of the 11 cases of aberrant karyotypes detected as inter-root mosaics, 5 were unique to the respective plants, each characterized by a specific kind of non-reciprocal translocation, reciprocal translocation or complex exchange; 2 were commonly characterized by a complex exchange; 3 were by a non-reciprocal translocation; one was characterized by a fragment chromosome accompanied with a shortened chromosome and was not distinguishable from the aberrant karyotype of spontaneous origin. These results support the conclusion that the chromosomes of H. gracilis possess the plasticity to rearrangements after irradiation. The repetitious occurrence of specific rearrangements in different plants and the recovery of fragment-bearing karyotype are discussed as suggestive evidence for the existence of preferential sites for breakage and rearrangement in H. gracilis genome.