So far all reported tests for the presence of mutator have involved the crossing of Mu stocks as males. To test for the presence of Mu in a Mu line 50 outcross plants are grown and self-pollinated and the progeny of the selfed ears scored for the presence of seedling mutants. In tests made so far almost all outcross progeny of Mu plants carry Mu and transmit it to all of their outcross progeny. Only rare exceptions to this non-Mendelian transmission have been observed. Out of 130 outcross plants scored for the presence of Mu only 16 (12.3%) gave negative tests, suggesting the loss of Mu. Some of these have proven to be the result of false negative tests due to sampling error (see MGCNL 51:36, 1977). The true loss rate of Mu is probably less than 10%.
With the development of Mu lines that consistently produce two ears, it has been possible to test for the presence of induced mutation in outcross progeny when Mu plants are crossed as females. The results of such tests are shown in Table 1.
Thirty percent of the crosses (3 out of 10) did not segregate for any mutations (have lost Mu?) compared to about 10% of the crosses where Mu plants are used as males. However, the female sample is much too small for any firm estimates of loss rate at this time.
If the families which did not show any mutations are eliminated the resulting mutation rate is close to that observed for other Mu crosses. Thus it appears that Mu induced mutations occur in the female inflorescence as well as the male.
Donald S. Robertson
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