In vitro selection for methomyl resistance in CMS-T maize

Many plants resistant to methomyl (Lannate), an insecticide which selectively damages maize with the Texas (T) type of cytoplasmic male sterility (CMS-T), were obtained by in vitro selection and also without selection. The selection procedure used 0.6-0.7mM methomyl and callus from CMS-T versions of several field and sweet corn genotypes (W182BN, Wf9, P39, MDM1, SW1 and hybrids of SW1, IL766A1, IL766A2, and 442 with W182BN-N). Addition of 1 mM methomyl to the regeneration medium greatly reduced recovery of methomyl-sensitive escapes. Resistance was linked with reversion to male fertility and maternally inherited. Most progeny of resistant plants exhibited stable maternally inherited resistance for two generations in field tests. First-generation progeny of seven culture-derived plants segregated for resistance and sensitivity; this suggests that ears of these seven regenerants were cytoplasmically chimeral. Resistance to methomyl was associated with resistance to T toxin from Helminthosporium maydis race T and with changes in mitochondrial physiology. Prolonged culture (14-16 months versus 6-8 months) increased the frequency of resistance among both selected and non-selected regenerants. Little or no resistance was found among regenerants from certain genotypes. Selection with methomyl may be useful for production of improved sweet corn lines and as a source of mitochondrial mutants. This system is also convenient for studies of the effects of nuclear background and of culture and selection systems on the generation of cytoplasmic mutants.