Summary Fluorescence in situ hybridization (FISH) using an Arabidopsis-type telomeric sequence (TTTAGGG) n probe to the mitotic chromosomes of Haplopappus gracilis (nϭ2) revealed the presence of Interstitial Telomere-like Repeats (ITRs) in at the subdistal position of the long arm of both chromosome pairs (1g and 2g). The H. gracilis genome (nϭ2) is generally thought to reconstitute from the nϭ4 complement of the allied species, H. ravenii. The sites we identified by FISH are in close proximity to the chromosomal rearrangement fusion points. This evidence supports the hypothesis that the karyotype of H. garacilis evolved from that of H. ravenii due to chromosome breakage and the subsequent end-to-end chromosome fusion.

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.

Summary Rearranged karyotypes of 2nϭ5 present in three plants of Haplopappus gracilis (2nϭ4) were cytogenetically analyzed to clarify the origin of an additional centromere in the chromosome complement. One of the plants was a progeny of the homozygote with normal chromosome 1 crossed with the heterozygote with normal and centromere-shifted chromosome 1. The other two plants were derived from seedlings irradiated with X-rays during early germination stage. The chromosome complement of 2nϭ5 found in the first plant was comprised of 3 normal chromosomes (one chromosome 1 and two chromosome 2) and two rearranged chromosomes each possessing a centromere at the subterminal position. All the chromosome complements of the irradiated plants were similar to each other and each consisted of three normal chromosomes (one chromosome 1 and two chromosome 2) and 2 rearranged chromosomes, one large and metacentric and the other, fragmentlike. The origin of the additional centromere in each complement of 2nϭ5 is discussed from the point of chromosomal evolution in the genus Haplopappus.