In an excellent review of the Mutator system by Bennetzen, Springer, Cresse and Hendrickx (Crit. Rev. Plant Sci. 12(1/2):57-95, 1993), they discussed the evidence presented by Robertson and Stinard (Genetics 115:353-361, 1987) that the Mutator system induced deletions involving the yg2 locus. Bennetzen et al. concluded that, because of the absence of a proper control population, there was insufficient evidence presented in the Genetics paper to distinguish between deletions induced by the Mutator system and those that might occur spontaneously. They suggested that "...a non-Mutator line with a similar background, would have provided such evidence" (i.e., that the deletions were Mutator-induced).
The control data they desired were reported by Robertson (Mol. Gen. Genet. 200:9-13, 1985, see Materials and Methods section and Table 3). In this paper, the Mutator and the control stocks were described for the isolation plot involved in the production of the yg2-Mu isolates analyzed in the Genetics paper. "The control tests were carried out in the same manner [as those for the production of Mutator mutants] except that standard (non-Mu) lines (used for propagating the Mu lines used in the Mutator tests) served as the female parent." Thus, the requirements stipulated by Bennetzen et al. for a proper control were met. The pertinent data are as follows: 125 yg2 mutants out of a population of 779,338 from the Mutator population, zero yg2 mutants from a control population of 527,041. Eighteen of these yg2-Mu mutants have been shown to involve deletions (Genetics 115:353-361, 1987 and Genetics 136:1143-1149, 1994). Nine of these deletions were examined cytologically, and seven of them had the terminal knob found on the short arm of chromosome nine. Due to the failure of Robertson and Stinard to repeat a complete description of both the Mutator and the control populations involved in the production of the yg2-Mu mutants in the Genetics article or to include a citation of the Molecular and General Genetics paper as a source of this information, Bennetzen et al. were not aware of the control that had been involved in these tests.
Before submitting this note to the Newsletter, I sent the first draft of it to Jeff Bennetzen for his information and criticisms. Although he thought the controls utilized in the yg2 experiment provided a high likelihood ("90% certain") that the deletions were Mutator-induced, he suggested that "Mu-off" lines (i.e., lines that had once been active Mutator lines but in subsequent crosses had lost their ability to induce mutations) would serve as a better source for the control population. He felt that such lines would be more appropriate because the active Mutator stocks used in this test could have activated a breaker Ds-like activity of some other transposable element system carried by the Mutator stocks. The presence of other active transposable element systems in active Mutator stocks has been found by several workers (e.g., Patterson, et al., Genetics 127:205-220, 1991). There are, however, also difficulties with using "Mu off" lines as controls. Not all lines that first appear to be off are found to be so on further testing, and some such lines have been found to cycle back to an active state (Hardeman and Chandler, Dev. Genet. 10:460-472, 1989). Thus there is probably no way to be certain that any given deletion was actually induced by the Mutator system. It is not unreasonable to assume, however, that at least some of the 13 known deletions found in the Mutator lines were the result of the activity of this system. The observation that seven deletions were not terminal suggests that some were not induced by a Ds-like system.
Thirteen mutable yg2 mutants were found in this experiment and all had the small wildtype sectors typical of unstable Mutator-induced mutations. None had the larger wildtype sectors that are characteristic of many, but not all, unstable mutants produced by other transposable element systems.
Final proof as to the origin of any given deletion awaits molecular analyses of these deletion events. A first step in such a process would probably involve the cloning of the yg2 gene. The Mutator-induced mutable yg2 mutants are available from the Maize Genetics Cooperation or from me for anyone interested in pursuing this matter further.
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