Plants with the Mu mutator system have a mutation rate 30-50 times higher than non-Mu plants. The standard test for Mu activity is made by outcrossing Mu plants to a standard (non-Mu) line and selfing the outcross progeny. The frequency of selfed ears that segregate for seedling mutants is used to calculate the mutation rate.
It is possible that Mu activity may not be uniformly expressed at all times in all tissues of a plant. If this were so, samples of pollens taken from different days from the same plant may result in outcross progeny with different mutation rates. To test this, MU bearing plants were bagged as soon as the central spike began to shed pollen. An outcross was made the next day and each consecutive day as long as plants kept shedding pollen. The mutation rates in three of these outcross series are given in Table 1. Although there is some variation in mutation rates in pollen samples taken on different days it is no greater than that expected from sampling error.
Table 1. Mutation rates in three serial outcrosses of plants sampled
on 5-7 consecutive days of pollen shedding.
| Plant No. | Day | Total Plants Selfed | Total Different Mutants | % Different Mutants | Heterogeneity c2 Calculated Over Days (and P) |
| 5062-3 | 1 | 77 | 4 | 5.2 | |
| 2 | 63 | 5 | 7.9 | ||
| 3 | 52 | 8 | 15.4 | ||
| 4 | 47 | 8 | 17.0 | ||
| 5 | 60 | 7 | 11.7 | ||
| 6 | 71 | 8 | 11.3 | ||
| 7 | 57 | 4 | 7.0 | 7.1615 (P=.30-.50) | |
| 5061-9 | 1 | 49 | 5 | 10.2 | |
| 2 | 51 | 6 | 11.8 | ||
| 3 | 40 | 4 | 10.0 | ||
| 4 | 44 | 5 | 11.4 | ||
| 5 | 55 | 5 | 9.1 | ||
| 6 | 52 | 6 | 11.5 | 0.3017 (P = > .99) | |
| 5062-4 | 1 | 51 | 4 | 7.8 | |
| 2 | 53 | 2 | 3.8 | ||
| 3 | 46 | 4 | 8.7 | ||
| 4 | 64 | 5 | 7.8 | ||
| 5 | 52 | 3 | 5.8 | 1.3105 (P=.80-.90) |
In Table 2, the mutation rates for each cross for each day were combined to determine if a consistent pattern of Mu activity relative to days after the initiation of pollen shedding is present. No significant difference is seen between crosses within days nor is any significant difference in the combined mutation rates indicated between days.
Table 2. Mutation rates combined by days for three
serial outcrosses.
| Day | Plant No. | Total Plants Selfed | Total DifferentMutants | % Different Mutants | Heterogeneity c2 for Each Day (and P) |
| 1 | 5062-3 | 77 | 4 | 5.2 | |
| 5061-9 | 49 | 5 | 10.2 | ||
| 5062-4 | 51 | 4 | 7.8 | ||
| Total | 177 | 13 | 7.3 | 1.1303 (P=.50-.70) | |
| 2 | 5062-3 | 63 | 5 | 7.9 | |
| 5061-9 | 51 | 6 | 11.8 | ||
| 5062-4 | 53 | 2 | 3.8 | ||
| Total | 167 | 13 | 7.8 | 2.3153 (P=.30-.50) | |
| 3 | 5062-3 | 52 | 8 | 15.4 | |
| 5061-9 | 40 | 4 | 10.0 | ||
| 5062-1 | 46 | 4 | 8.7 | ||
| Total | 138 | 16 | 11.6 | 1.2051 (P=.50-.70) | |
| 4 | 5062-3 | 47 | 8 | 17.0 | |
| 5061-9 | 44 | 5 | 11.4 | ||
| 5062-1 | 64 | 5 | 7.8 | ||
| Total | 155 | 18 | 11.6 | 2.2426 (P=.30-.50) | |
| 5 | 5062-3 | 60 | 7 | 11.7 | |
| 5061-9 | 55 | 5 | 9.1 | ||
| 5062-1 | 52 | 3 | 5.8 | ||
| Total | 167 | 15 | 9.0 | 1.1863 (P=.30-.50) |
Heterogeneity c2 for totals of each day 3.1119 (P = .50 - .70)
The results of these tests are consistent with the view that Mu activity is uniformly expressed in all sporophytic tissue of the tassel over the 5-7 day interval of pollen shedding tested.
Donald S. Robertson
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