Comparison of the scoring methods for mutation frequency in self-pollinating disomic plants

Abstract

The merits and demerits of the various methods for scoring mutation frequency offered hitherto, namely, M₁-plant method, M₁-spike method, Gaul's M₂-plant (or seedling) method, modified M₂-plant method (M₂-plant method by randomly sampled M₂-plants), Yamaguchi's revised M₁-spike method (revised form of Gaul's indirect method) and Panicle-Branch method, Li and Rédei's standardized M₁-spike method, Ukai and Yamashita's expression of cell mutation rate and Walther's expression of ‘Factor of Effectiveness’, were comparatively discussed based on some theoretically derived formulae.

Based on our criteria for measuring the suitability of scoring methods, the M₁-plant, modified M₂-plant, Yamaguchi's Panicle-Branch and Walther's Factor-of-Effectiveness methods were considered inapplicable for our general use. Among the other methods, no one method seemed to be invariably the best, the utility of each scoring method being dependent on the aim and conditions of the particular experiment. Scoring methods regarded as least subject to error for some typical experimental conditions were presented. The necessity of recording the numbers and sizes of M₂-lines and the segregation ratios of the mutants regardless of the scoring method adopted in individual cases was made. These records should be useful not only for the transformation or standardization of mutation scores for our future needs, but also for refining our knowledge of the various factors causing distortions in the scores. It was also noted that the greatest care should be taken in all cases, so that each M₂-line might be derived from equally- or closely-ranked tillers. By this method, the errors stemming from irregularity in the numbers of germ-line cells contained in a spike primordium could be greatly reduced.