| Relation of ploidy to radiation effect M. NEZU National Institute of Genetics, Misima, Japan The effect of radiation is influenced by various factors, Polyploidy is one of them. The author has examined in this connection meiotic irregularities and seed fertility in three groups-di-, tetra- and hexaploid-of wheats. As material 6 species were used, namely, Triticum aegilopoides, T. monococcum, T. dicoccum, T. durum, T. Spelta, and T. vulgare. Dormant seeds were exposed to gamma-rays of 20, 30, or 40 kr for 18 hours. The percentage of PMC's with meiotic irregularities, mostly reciprocal translocations, increased with increasing ploidy. In 4x species the great majority (83.4%) of PMC's showed aberrations at 30 kr and in 6x species 88.7% cells showed similar irregularities at 20 kr., while in 2x species only 38.3% had aberrations at 20 kr. At higher dosages (30-40 kr) all cells were abnornal. A similar increase of multivalents ((6), (8), (10)) was observed, especially in 6x species (Table 1). As to other abnormalities, asynapsis and deficiency were observed, and rarely heteroploidy. The number of breaks per cell increased almost linearly with the dosage within each group. Comparison between the three groups-2x, 4x and 6x-showed an increase in breakages in a ratio of 1:2:4. Seed fertility decreased with increasing dosage. The 2x species were most susceptible in this respect, next to them the 6x, followed by the 4x. The fertility of the 6x species decreased suddenly at dosages between 20 and 30 kr. The higher the ploidy, the more aberrations arose. The highest number of multivalents was found in the 6x plants. |