GENETIC STUDIES ON THE LOSS OF M u MUTATOR ACTIVITY IN MAIZE DONALD

Mutator activity of the Mu mutator system of maize can be lost by either outcrossing or inbreeding Mu stocks. The nature of these two kinds of Mu-loss phenomena was analyzed by testing the results of crossing Mu-loss stocks by active Mu lines. Outcross-Mu-loss stocks are capable of supporting Mu activity if crossed by an active mutator line. Inbred-Mu-loss stocks, however, inactivate the active Mu system contributed by a Mu line. Also, inbred-Mu-loss lines do not regain Mu activity after at least three generations of outcrossing to non-Mu stocks. These results suggest that, once the Mu system is inactivated by inbreeding, it remains inactivated for at least three generations of outcrossing. Further, once the system responsible for inactivation is established, it will, in turn, inactivate an active Mu system contributed by crossing with Mu plants. The outcrossMu-loss does not seem to involve such an inactivation system. These results are interpreted in the light of recent evidence that Mu inactivation results from the modification of Mu1 transposable elements involved in the Mu phenotype. HE Mutator (Mu) system of maize is responsible for a 50-fold, or higher, T increase in the spontaneous mutation rate (ROBERTSON 1978, 1983). Molecular analyses of Mu plants and Mu-induced mutants indicate that a transposable DNA element, Mul, is involved in the induction of mutants (STROMMER et al. 1984; BENNETZEN 1984; O’REILLY et al. 1985). This element was sequenced by BARKER et al. (1984) and was found to have the structure expected of a transposable element. Early genetic studies of the Mutator (Mu) system in maize indicated that mutator activity was lost occasionally when Mu plants were outcrossed (ROBERTSON 1978). A recent summary of the results from 563 outcrosses suggests that loss can be expected, on average, in about 10% of the outcross progeny (ROBERTSON 1985). Later studies (ROBERTSON 1983) demonstrated a second way in which mutator activity could be lost. This type of loss results from the inbreeding of Mu stocks. If two different Mu stocks, which previously had been propagated by outcrossing, are intercrossed, the mutator activity of the resulting F1 ( i e . , MU‘) will increase about 1.5-fold over that of the parents. If two Mu2 plants are crossed, the resulting Mu4 plants will show no further increase. If this Genetics 113: 765-773 July, 1986.