EVIDENCE FOR THE INVOLVEMENT OF GENE CONVERSION IN MEIOTIC INSTABILITY OF THE RP1 RUST RESISTANCE GENES OF MAIZE

Rpl is a complex rust resistance locus of maize. Susceptible recombinants can be observed at variable frequencies from different Rpl heterozygotes and homozygotes by crossing-over and unequal crossing-over. In addition, susceptible progeny have been observed from one heterozygote that were not associated with crossing-over, as indicated by flanking marker analysis. We analyzed testcross progeny from this cross (Rpl-C/Rpl-J) and a related cross (Rpl-F/Rpl-J) to identify noncrossover type derivatives, to estimate their frequency, and to determine the mechanism by which they arise. Testcross progeny were screened sequentially with two different rust isolates to assay each gene individually. Both susceptible noncrossover types and those with the combined resistance of each parent were identified in approximately equal frequencies. Patterns of recombination indicated that the noncrossover type derivatives arose by gene conversion as opposed to mutation or intrachromosomal crossing-over. While gene conversion events were observed frequently in both crosses (approx. 7 X 10(-4)), they were less frequent than crossover derivatives.