Most of the known Rp genes map to the Rp1 complex (Hulbert Ann. Rev. Phytopathol. 35:293-310, 1997) near the end of the short arm of chromosome 10. Different maize lines carry different numbers of rp1 genes in their rp1 haplotypes (generally between 4 and 20) but most do not confer resistance to any known rust biotypes. For example, the Rp1-D haplotype carries the Rp1-D gene and eight others with no detectable phenotypes (Collins et al., Plant Cell 11:1365-1376, 1999). Genetic recombination experiments have generated recombinant haplotypes with two or more Rp1 genes with characterized resistances. Some of these recombinant haplotypes confer resistance to a very broad spectrum of rust biotypes and have been incorporated into breeding programs (e.g. Hulbert and Drake, Hortsci. 35:145-146, 2000). The Rp5 and RpG loci map approximately two map units distal to the Rp1 locus. Recombinants with Rp5 and/or RpG combined with one or more Rp1 genes have also been constructed and allow multiple Rp genes to be manipulated as a single locus in breeding programs.
Resistance conferred by most Rp genes is dominant or incompletely dominant. Rp8 is unique in that only Rp8-A/Rp8-B heterozygotes confer resistance (Delaney et al. MPMI 11:242-245, 1998). Most maize lines carry the Rp8-B allele and some carry Rp8-C, which confers no known resistance in homozygotes or heterozygotes. To determine if Rp8 provides effective resistance against a specific rust population, an F1 hybrid between H95 and the Rp8-A line can be examined.
An extensive Rp differential series in the R168 genetic background was developed by Art Hooker and coworkers in the 1960s. A number of problems exist with this series in its current state (Hulbert et al., Plant Dis. 75:1130-1133, 1991). Due to their propagation for many years without routine testing with a set of characterized rust biotypes, different investigators stocks are missing the resistance genes they are thought to carry, or carry the wrong gene. The series is also redundant in the genes they carry. The Rp1-C, Rp1-L and Rp1-N lines all carry the same Rp1 haplotype as determined by gel blot analysis with an rp1 probe and by phenotype. The same is true for the Rp1-E, Rp1-I and Rp1-K lines, and the Rp1-A and Rp1-F lines. The Rp1-H and Rp1-J lines do not carry identical rp1 haplotypes but probably carry a functionally identical gene, since they confer resistance to the same rust biotypes. Six lines carrying Rp3 genes (designated Rp3-A to Rp3-F), originally identified from different sources (Wilkinson and Hooker, Phytopathology 58:605-608, 1968) also appear identical when tested with many rust isolates indicating they also carry a functionally identical gene. Some stocks of the Rp3-C line are resistant to a broader spectrum of rust isolates than the other Rp3 lines but we have found these stocks to be contaminated with an Rp1 gene. Another problem with the current series is that the R168 background makes the lines difficult to propagate in many environments.
We have recently constructed a new differential series in the H95 genetic background (Table 1). The H95 inbred line is susceptible to all known rust biotypes. The H95 Rp series was constructed by crossing lines carrying the resistance genes (often the R168 Rp lines) to the H95 line, and backcrossing progeny carrying the Rp genes to H95. Most lines have four or more backcrosses to H95 in their pedigrees, but some lines do not; therefore they should not be considered to be nearly-isogenic. After back crossing, lines carrying the Rp genes were self-fertilized and homozygous lines were selected by progeny testing with appropriate rust biotypes. The H95 series does not include each of the 14 original Rp1 genes (Rp1-A to Rp1-N), but includes a representative line for each different resistance specificity. For example, it does not include Rp1-F because this has the same rp1 haplotype and confers resistance to the same rust biotypes as the Rp1-A line. Similarly, the series includes only one of the six Rp3 genes because these are indistinguishable with our collection of rust biotypes. The series includes novel Rp1 genes and haplotypes that have been generated by recombination or spontaneous mutation and confer resistance to novel combinations of rust biotypes. Some of these carry different combinations of Rp1-area genes while others are thought to represent recombinant genes (Richter et al., Genetics 141:373-381, 1995). In cases where several recombinant rp1 genes or haplotypes were isolated that appeared phenotypically identical after challenging with multiple rust isolates, only one was included in the H95 series. The lines were designated for the specific haplotype they carry. For example, several different recombinant Rp1 haplotypes have been identified that carry both Rp1-J and Rp1-F, but we prefer to distribute only the Rp1-JF69 haplotype to prevent future confusion if phenotypic differences between these haplotypes are revealed by challenge with rust biotypes.
The new differential series does not include a line with the Rp6 gene. This is because we have not found a rust isolate that can be used to detect this gene. Those isolates used by Wilkinson and Hooker (Phytopathology 58:605-608, 1968) to identify the gene are no longer viable. It is possible the Rp6-R168 line may no longer carry the Rp6 gene, although we have tested seed samples from several different sources and all appear susceptible to our current collection of rust isolates. It is also possible that rust isolates carrying the avirulence gene corresponding to Rp6 have become very rare in North America.
A well-characterized differential series will be useful for determining the effectiveness of the Rp genes or gene combinations in different areas and for monitoring changes in P. sorghi populations. It will also be useful in characterizing specific rust biotypes that can then be used to estimate which Rp genes are carried in various maize lines and breeding material. The lines may be obtained from the Maize Genetics Cooperation Stock Center or by contacting the authors.
Table 1. Rp differential lines
in the H95 background.
|Rp1-FJC1||Rp1-F, Rp1-J, Rp1-C||10|
|Rp-GFJ||RpG, Rp1-F, Rp1-J||10|
|Rp-GDJ1||RpG, Rp1-D, Rp1-J||10|
|Rp-G5JCa||RpG, Rp5, Rp1-J, Rp1-C||10|
|H95||rp1, rp3, rp4, rp5, rp7, Rp8-B|
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