Mechanism of instability of Rp1-J
--M.A. Sudupak and S.H. Hulbert
Rp1 is a complex rust resistance locus with multiple alleles. Several of the 14 alleles at Rp1 have been demonstrated to be meiotically unstable by Pryor (MNL 61:37) and Bennetzen et al. (Nature 332:369); susceptible progeny occur in testcrosses of Rp1 homozygotes. The origin of this instability has not yet been elucidated. Possible explanations include a recombinational origin or transposable elements. The identification of RFLP loci which flank the Rp1 locus, npi422 and npi285 proximally and bnl3.04 distally, allows possible mechanisms to be tested.
To test the role of recombination in this instability, an Rp1-J homozygote was constructed that was heterozygous for flanking RFLP loci. This was constructed by generating two parents that were recombinant between Rp1-J and either the distal or proximal flanking marker. The first parent carried Rp1-J with the npi285 allele exchanged for that of an Rp1-F R168 line. The second parent also carried Rp1-J but the bnl3.04 genotype was that of an Rp1-D R168 line. By crossing the two parents, an F1 was made that was homozygous at Rp1-J, but heterozygous at marker loci on both sides of the gene. A testcross population from this F1 was screened with the P. sorghi isolate KS1, which is avirulent on lines carrying Rp1-J. Five susceptible recombinants were observed in 9750 seedlings. All five individuals had nonparental combinations of flanking markers as would be expected if the progeny were derived from an unequal crossing-over event between tandemly duplicated repeats. This model is also supported by the fact that both nonparental marker combinations were recovered. Three of the recombinants had the npi285 allele of parent one and the bnl3.04 allele of parent two, while the other two recombinants had the opposite combination. This would be expected if mispairing between tandemly duplicated sequences occurred in both directions.
The frequency of susceptibles (about 1/2000) indicates that Rp1-J is moderately unstable when compared to most Rp1 alleles. This frequency is similar to that reported for Rp1-B or Rp1-C but less than that reported for Rp1-G. We are currently performing a similar experiment using Rp1-G homozygotes with heterozygous flanking markers. This should indicate whether instability at Rp1-G is also due to unequal crossing-over and determine if this is a general mechanism causing instability of resistance genes in the Rp1-Rp5 area.
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