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 recombina nts can be observed at variable frequencies from different Rpl heteroz ygotes 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 typ e derivatives, to estimate their frequency, and to determine the mecha nism by which they arise. Testcross progeny were screened sequentially with two different rust isolates to assay each gene individually. Bot h susceptible noncrossover types and those with the combined resistanc e of each parent were identified in approximately equal frequencies. P atterns of recombination indicated that the noncrossover type derivati ves arose by gene conversion as opposed to mutation or intrachromosoma l crossing-over. While gene conversion events were observed frequently in both crosses (approx. 7 X 10(-4)), they were less frequent than cr ossover derivatives.