GENETIC-CONTROL OF RESISTANCE TO BIPOLARIS-MAYDIS - ONE GENE OR 2 GENES

Bipolaris (Helminthosporium) maydis is the causal fungus of southern l eaf blight of maize, Resistance to this disease has been determined to be controlled by a single recessive gene, designated rhm. The dominan t allele, Rhm, confers susceptibility to the fungus, In our project to tag the rhm gene with transposable elements, Rhm/Rhm EIEI (El, elemen t) lines were crossed to an rhm tester, Screening for mutation to rhm was then conducted on the F-1, Rhm/rhm El, seedlings. Element insertio ns into Rhm are expected to be random events, and will mutate Rhm phen otypically to rhm (susceptible to resistant), In these tests mutation rates of Rhm to rhm were usually in the order of 10(-5). However, hybr id populations between two different Rhm El lines (Cy line and T line) yielded similar to 5% mutants, To account for the unexpectedly high m utation rate, a hypothesis is proposed that there are two linked reces sive genes controlling resistance, The two lines combined differed in genotypic content and the unexpected 5% mutants arose from crossovers between the two dominant alleles at the two linked loci in repulsion p hase (Rhm1 rhm2/rhm 1 Rhm2). It is also postulated that one of the two genes is a copy of the other created by duplication, The dominant sta tus at either locus makes a functional product and thus abolishes resi stance, The two-gene model is currently being tested, The significance and implications of this finding are discussed.