II.16 The frequency of chlorophyll mutations in relation to M1 sterility.
G. Kunzel and F. Scholz. Zentralinstitut fur Genetik und Kulturpflanzenforschung der DAW Gatersleben, DDR.
In mutation experiments it is interesting to know whether a selection for fertility or sterility, resp., in the M1 generation is effective as to the mutant frequency to be expected in M2, i.e., whether M1 spikes of less injured fertility give lower frequencies of mutants in M2 than rather sterile M1 spikes (from the same treatment). Studies in barley with X-rays only (GAUL 1958) as well as in Arabidopsis with X-rays, ethyl methanesulfonate and nitrosomethylurethane (MULLER 1967) have shown, however, that the frequencies of chlorophyll mutants in M2 tend to be constant over different fertility classes in M1.
In barley, comparable experiences with highly efficient chemical mutagens have apparently not been published. This contribution deals with results of studies with nitrosomethylurea, ethyl methanesulfonate and, for comparison, with X-rays. The M1 spikes of each treatment were harvested, using those of the first three tillers of a plant only, and divided into four fertility classes (see figure 1). In M2, the frequencies of chlorophyll seedling mutants were evaluated for these fertility classes. The M2 generations were grown in four replications under comparable conditions in the greenhouse.
Fig. 1 The frequency of chlorophyll seedling mutants in M2 in relation to the fertility of M1 spikes
The number of mutants evaluated in M2
A clear-cut difference between X-rays and the chemical mutagens was observed. In correspondence with the results obtained by GAUL (1958), X-rays were found to have induced about equal frequencies of chlorophyll mutants in each of the four M1 fertility classes. Ethyl methanesulfonate and nitrosomethylurea, on the other hand, yielded mutant frequencies increasing considerably with the degree of sterility of the corresponding M1 spikes. These results may be explained as the consequence of inhomogeneous mutagenic treatment, i.e., the embryos and their initial cells are reached by different doses of the chemical mutagen.
The divergency between barley and Arabidopsis (MULLER 1967) may be related at least in part to the differing size of their seeds. Several other factors may also influence the inhomogeneity. Different effects on the initial cells of the embryos probably arise, for instance, from differing rates of reaction and/or penetration of the chemicals. The faster rates of reaction and penetration of nitrosomethylurea as compared to ethyl methanesulfonate, indicated by studies in Arabidopsis (MULLER 1965) and barley (KUNZEL 1968), can explain the relative differences here observed between the two chemicals.
In any case, the results clearly indicate that a selection for fertility or sterility, resp., in M1 following treatment of barley seeds with chemical mutagens is highly effective. Decreasing fertility is accompanied by increasing mutation frequencies, and vice versa. In experiments for breeding purposes, as is well known, mutations causing sterility are undesired. After X-irradiation a selection for fertility may begin in M1 already, without lowering the probability of discovering desirable mutants in M2 and later generations. In experiments with chemical mutagens, on the contrary, a selection against sterility before M2 generation is not recommendable in general, since M1 spikes of good fertility will yield a considerably lower frequency of mutations.
References:
Gaul, H., 1958. Z. Pflanzenzucht. 40, 151-188.
Kunzel, G., 1968. Dissertation, Universitat Halle, pp. 125.
Muller, A. J., 1965. Mut. Research 2, 426-437.
Muller, A. J., 1967. In: H. Stubbe (Ed.), Induzierte Mutationen und ihre Nutzung, Erwin-Baur-Gedachtnisvorl. IV 1966, Abh. Dt. Akad. Wiss. Berl., Berlin, 89-97.
