If Mu induced mutations are occurring in mitotic divisions then there should be clusters of seeds carrying phenotypically similar mutants on the ear. In order to determine if such clusters were occurring, one ear of Mu plants with two ears was pollinated by a standard line while the second ear was self-pollinated (to make sure that the Mu parent was not segregating for a mutant). Six outcross ears with good seed set were selected and 150 seeds planted in an ear map pattern. Six rows of 25 seeds were taken off the ears in order and planted in order in the field.
Missing seeds on the ear were recorded and seeds that did not germinate were noted. The selfed progeny of the plants from these seeds were scored for mutations. Of the six tests made, 3 did not segregate for any mutants while three did. The ear maps are shown in Tables 1, 2, and 3.
There are indications that there might be mutant sectors in Tables 1 and 2. In Table 1 in row 5 there appears to be a sector of two plants (4 and 5) with both yellow-green and pale-yellow mutants. The white mutant in plant six is an offwhite, which may be an extreme expression of pale-yellow. If so, this would extend the sector over 3 seeds. A two-seed pale-green sector appears to be present in row 4 (positions 18 and 19). The luteus mutants in row 3, position 25 and row 5, position 21 although separated by six seeds may represent two members of a sector. Since only half the seeds in a mutant mitotic sector would be expected to carry the mutant, + seeds are not unexpected in sectors. In Table 2 there are also indications of sectors (e.g., luteus mutants in row 1, position 13 and row 2, position 15; pale-green mutants in row 3, positions 6 and 10; pale-yellow mutants in row 4, position 18 and row 6, position 19; pale-yellow mutants in row 6, positions 3, 4, 5, and 7; and yellow-green mutants in row 6, positions 19 and 22). Table 3 did not have enough mutants to provide any useful information.
Certainly there are strong indications of sectors. If there are indeed sectors occurring, the mutants making up a sector should be allelic. This prediction will be tested next summer.
Donald S. Robertson
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