| 2. Radiogenetic investigations In total, 278 chlorophyll mutations were obtained in our X-ray treatments. Furthermore, some other mutants were selected ; details can be seen in a previously published paper (GOTTSCHALK and IMAM 1965). It was not our intension to perform a quantitative analysis of the mutations induced, but to compare the mutation frequency of the different varieties using chlorophyll deficiencies as a well discernible criterion of gene action. The results are listed in Table 1. The values of the dosages between 17.5 and 32.5 kr are summarized and compared with the mutagenic action of 10 kr. The mutation frequency was related to the number of ear progenies of the X1-plants. In total, 5397 progenies were evaluated. Only one diploid variety could be used for this purpose ; its mutation frequency was 6.4%. A broad spectrum of varieties of different tetraploid species was studied. Some of them showed marked differences in their mutation frequency. The variety rubrovesticum of T. polonicum for instance showed an extremely low number of mutations (1.7%); the opposite situation was found in T. durum var. valenciae (10.0%). These differences could likewise be confirmed in a second treatment. The unexpected high mutation frequency of valenciae is in contrast to its strikingly high resistance to X-rays as far as its radiobiological response is concerned. The mutation frequency of the hexaploid varieties was extremely low. The values obtained are approximately adequate to the 40th part of the corresponding rates of the diploid and tetraploid material. Discussion It is well known, that the action of radiation can be influenced by specific physiological or physical peculiarities of the seeds such as water content, pH-value, nucleus volume, chromosome size etc. But the results described in this paper seem to be due to specific genetic differences of the varieties used. An increased radiosensitivity of a distinct strain of T. monococcum was interpreted to be due to the presence of an "X-ray susceptible factor" (SMITH l942). A corresponding concept was given by YAMASHITA (1956) likewise for T. monococcum. It is our opinion, that there is a close relation between the degree of radiosensitivity and the genetic composition of a variety. Not the number of genomes present but specific genes or gene combinations of the genomes could be responsible for the intensity of the reaction to irradiation, while the physiological criteria just mentioned have only a modifying action. It is very complicated to give a plausible explanation of the divergent mutation frequencies observed in our experiments. If we consider the voluminous literature concerning this problem, all relations conceivable between the level of ploidy and the mutation frequency can be found. A detailed discussion of the findings existing was given by GOTTSCHALK and IMAM (1965). One of the main problems for understanding these contradictory findings is obviously the clarification of the question, whether the amphidiploid character of the hexaploid wheats resulted in a reduction of the allelism of originally homologous genes. Therefore, a detailed comparison of the mutational behaviour of auto- and allo-polyploid species would be of great interest. |
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