Further evidence on the timing of Mu activity
Evidence to date (see MGCNL 52:46-47, 1978) suggests that most, but not all, Mu induced mutations are probably meiotic in origin. The occurrence of occasional "clusters" of plants segregating for mutants of similar phenotype in outcross of Mu bearing plants as males suggested the existence of pre-meiotic mutants. Allelic tests of mutants from such putative "clusters" established that most were not allelic and hence were not derived from a premeiotic sector. However, 22 percent of the putative "clusters" did indeed possess allelic mutants and therefore probably were derived from a mutation producing a premeiotic sector.
If sectors are occurring, it should be possible to demonstrate them by planting ear maps of Mu plants outcrossed as females. Last year (MGCNL 52:47-49, 1978) I reported the results of three such ear maps. Two of them had putative "clusters" of mutants. This past summer mutants from seven of the eight possible clusters were tested to determine if the mutants in the putative sectors were allelic. Four pale yellow mutants found in one five plant sector of one ear all proved to be allelic to each other as did the mutants from a two pale-yellow and a white mutant sector of the second ear. Mutants from a putative two plant yellow-green sector proved to be non-allelic as did a yellow-green and luteus mutant in another, two plant putative sector. Sufficient data were not obtained on a possible three plant luteus sector to give a clear cut answer as to whether or not the mutants were allelic. Test plants from two putative two-plant pale green sectors did not appear to segregate for pale green seedlings. These may be temperature sensitive mutants which were not expressed in the warm greenhouse conditions this fall. This material will be retested under cooler growing conditions.
The ear map results confirm those from the allele tests of male derived mutants reported last year. Mu can induce premeiotic mutations which result in "clusters" of allelic mutants. Although the sample is small, it would appear that the mutations responsible for these sectors are extremely late since the sectors only seem to involve a few ovules. The largest sector demonstrated, to date, consists of 5 seeds. This is consistent with the tests for somatic sectoring in Mu bearing plants and seeds heterozygous for yg2, c, sh, bz, wx, a sh2, a2, and bt. The somatic sectoring rate for all of these loci was no higher in Mu plants than the non-Mu controls. If Mu affects the normal alleles of these loci, it does not do it early enough in ontogeny to produce observable somatic sectors.
Since it appears that the premeiotically induced mutations occur quite late it could be that some single mutations which are normally classified as meiotic in origin could in reality be due to a very late premeiotic mutation in which only one cell derived from the one in which the mutation occurred gave rise to an ovule. Thus it is impossible at this time to say what fraction of Mu induced mutations are meiotic or premeiotic.
Donald S. Robertson
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