Potted plants of Tradescantia ohiensis (=T. reflexa) were chronically exposed to 60Co gamma-rays at exposure rates of 2.3 to 18.5R/day (115 to 925mR/hr) in the gamma field of the National Institute of Radiation Breeding. Other potted plants were placed on the protection earth bank surrounding the gamma field (Fig. 1) and were exposed to a low level (280mR/day or 14mR/hr) of scattering radiation. The genetic effects of these radiation treatments were studied in the stamen hairs (essentially single-meristematic-cell system) at two different periods. Evident genetic responses to these radiation treatments were clearly observed. Somatic mutation rates increased roughly linearly with increasing daily exposure of gamma rays. The gamma-ray exposure given during hair development induced somatic pink mutations at a rate of 3.36×10-4 pink mutant events per hair per R effective exposure. It was estimated that the gamma-ray exposure given during hair development was almost twice as efficient as those given before hair development. The frequency of pink mutant cells in the hair cell population also increased roughly linearly with increasing daily exposure. The frequency per cell per R effective exposure reached the value of 8.78×10-5. Genetic effects more than expected were observed in the stamen hairs irradiated with scattering radiation at a very low exposure rate of 280mR/day at the top of the earth bank of the gamma field. Namely, the mutation rates by this treatment were almost as high as those by 2.3R/day gamma-ray irradiation in the gamma field. It is suspected that the scattering radiation may have seven to eight times more efficiency than the direct 60Co gamma rays. The stamen hair system of Tradescantia heterozygous for flower color was proved to be a sensitive biometrical system for detecting low levels of radiation exposures (e.g., ca. 2R exposure during hair development plus ca. 2.5R exposure before hair development at 280mR/day exposure rate). The present results obtained by using Tradescantia stamen hairs seem to throw doubt on the safety standards for radiation facilities.
Chromosomal inversions in natural populations of Drosophila bifasciata were analyzed in distribution and frequency. A total of 13 inversions were observed in eleven populations, with each population showing high polymorphism. In view of results of comparison of geographical distributions and frequencies for each inversion, D. bifasciata forms “flexible” type chromosomal polymorphism. Judging from differences of the genetic constitutions between populations, it is suggested that the marginal sourthwestern range of this species is Kujyusan, Kyushu, while the central range is the north-east territories of Japan.
A unified treatment of substitutional and mutational loads was presented assuming a steady flux of molecular mutations in a finite population. The case of genic selection for diploid populations (“no dominance”) was analysed in detail. The load is given by If(p) (formula 3) and several examples are illustrated in Fig. 2. An important point which emerged from the analysis is that mutations with very small effects (having selection coefficients of the order of the reciprocal of the effective population number) can create considerable genetic load. This suggests that slightly detrimental mutations constitute a real threat to eventual extinction for a species having small population number. The mutational load as illustrated in Fig. 3 may be used to define the “cline” between neutral and deleterious mutations. The load depends on |Nes| rather than |s| alone. The importance of mutations with very small effects for variation and evolution of the species was discussed.