Genetics of instability at the rust resistance (Rp) locus

There are at least 14 alleles known at the Rp locus (resistance against Puccinia sorghi) located at the tip of the short arm of chromosome 10 (R.K.S. Saxena and A.L. Hooker, PNAS 61:1300, 1968). Eight are unstable, giving rise to susceptible variants designated Rp' alleles (T. Pryor, MNL 61:37, 1987) in crosses of the following general type:

Rp-x/Rp-x x rp/rp ---> Rp-x/rp (Resistant) + Rp'-x/rp (Rare susceptible)

The frequency of the event appears to be allele specific and varies at least 100-fold between the extremes. However, the frequency with which a given allele produces susceptible variants may vary 5-10 times in different genetic backgrounds.
 
Allele Frequency range of instability
Rp-g 0.002 to 0.014
Rp-d 0.0001 to 0.0002

The Rp' alleles do not revert or alter to a form that confers resistance. No second-time resistant variants were observed in 11,716 progeny of Rp-g' or from 30,357 Rp-d' F1 test progeny, which were all susceptible.

No known transposable element system is associated with this event and the following crosses were designed to investigate the genetic nature of the event at the Rp locus that gives rise to this high frequency of susceptible Rp' variant alleles. All progeny were screened with the rust race R-1, which recognises the Rp-g and Rp-d alleles but is virulent (compatible) on the alleles Rp-c and Rp-m (Conversely the presence of the Rp-c and Rp-m alleles can be recognised with race R-2-1, which is virulent on Rp-d and Rp-g).

Crosses 1-3.

Unlike previous crosses with the Rp alleles homozygous, this cross demonstrates that the event producing the Rp' instabilities occurs in heterozygotes. In the above analysis the proposed genotypes have been assigned on the assumption that in this small population it was the Rp-g allele which gave rise to the Rp' variants. The observed frequency of Rp' variants is 28/3,982 or 0.007, which is not substantially different from 0.006 observed in the control population (Cross 1) designed to measure the stability of Rp-g in a related background during the same crossing season. This is considerably higher than that expected from Rp-d. However, differences in genetic background are probably too large to conclude whether these values are meaningful. Another consideration is whether our expectation in Cross 3 should have been 1/2 that of Cross 1 since there is only one Rp-g allele.

All of the susceptible seedlings in Cross 3 were green, yet from the estimate of recombination Rp to Oy in Cross 2 in a sample of size 28 one would expect 3.08 recombinants with the Oy marker (based on 11% recombination). This suggests that the event at Rp giving rise to the Rp' variants may inhibit crossing over in an adjacent region. By analogy to the rosy locus analysis in Drosophila (A.J. Hilliker and A. Chovnick, Gen. Res. 38:281, 1986) this might indicate a crossing over event rather than gene conversion, but certainly doesn't eliminate other mechanisms such as intrachromosomal events. Since the event occurs in homozygotes an unequal crossover would be necessary to generate a novel form. The observed numbers are small (at the borderline of significance) and will be increased. The cross will be repeated with the Oy marker linked to the Rp-g allele with the expectation that the Rp' variants should be oil yellow. Ideally a distal marker for a three point testcross is required but this is not yet possible for the Rp-g allele.

Tony Pryor


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