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Race Specificity of QTL

Race Specificity of QTL. Genetic selection and adaptation of Cochliobolus heterostrophus to corn hosts with partial resistance. Kolmer, JA; Leonard, KJ Phytopathology. Vol. 76, no. 8, pp. 774-777. 1986. Recurrently selected for virulence of C. heterostrophus on one line- 316.

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Race Specificity of QTL

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  1. Race Specificity of QTL • Genetic selection and adaptation of Cochliobolus heterostrophus to corn hosts with partial resistance. Kolmer, JA; Leonard, KJ Phytopathology. Vol. 76, no. 8, pp. 774-777. 1986. • Recurrently selected for virulence of C. heterostrophus on one line- 316. P Balint-Kurti QDR lecture

  2. Saw line-specific as well as line-non-specific response. • What does this mean? P Balint-Kurti QDR lecture

  3. Do these dichotomies really exist? P Balint-Kurti QDR lecture

  4. Is quantitative resistance really durable? • This is certainly the received wisdom though no formal studies have been done (?) • Likely to be true • But some major genes Lr34 and Sr2 for instance are durable too. P Balint-Kurti QDR lecture

  5. Do these dichotomies really exist? P Balint-Kurti QDR lecture

  6. True, but major genes might also be developmentally and environmentally influenced. • At temperatures above 28°C, the HR and the restriction response associated with the tobacco N gene are inactive. P Balint-Kurti QDR lecture

  7. P Balint-Kurti QDR lecture

  8. Do these dichotomies really exist? P Balint-Kurti QDR lecture

  9. (almost) all available necrotrophic resistance is quantitative P Balint-Kurti QDR lecture

  10. Do these dichotomies really exist? P Balint-Kurti QDR lecture

  11. Quantitative resistance is, by definition, partial • But some (most?) R-genes do not confer complete resistance either. P Balint-Kurti QDR lecture

  12. P Balint-Kurti QDR lecture

  13. Lots of examples of this type of thing Randy Wisser P Balint-Kurti QDR lecture

  14. “Ghost” effects • Defeated R-genes still seem to be doing something! P Balint-Kurti QDR lecture

  15. CAVEAT • PERSONAL OPINION! P Balint-Kurti QDR lecture

  16. So are Quantitative resistance genes similar to major R-genes? • Maybe… • Most of the perceived differences are matters of degree • Rcg1, a quantitative gene for anthracnose stalk rot resistance was cloned and found to encode an NBS-LRR gene. • But Rcg1 has an extremely significant effect • What is the difference between a “major” gene and a strong QTL? P Balint-Kurti QDR lecture

  17. Perhaps R-genes that interact weakly with their corresponding Avr genes cause partial resistance? • There is some evidence for this. E.g. weak interations between L gene products in flax and corresponding Avr gene products is associated with weaker HR response • Presumably this happens often during evolution P Balint-Kurti QDR lecture

  18. QTL disproportionately colocalize with RGA in rice P Balint-Kurti QDR lecture

  19. Integrated disease-QTL map of rice • disease QTL • R-genes • RGAs (NBS-LRR) • Lesion mimic genes Randy Wisser Wisser et al., 2005 Genetics Wisser et al. 2005 Genetics 169: 2277 P Balint-Kurti QDR lecture

  20. Questions Are certain genes associated with the dQTL fraction of the genome? Do genes (or groups of genes) condition multiple disease resistance? Randy Wisser P Balint-Kurti QDR lecture

  21. genomic location differential expression defense response literature selection response Examining dQTL-gene associations QTL Many positional candidate genes Chromosome non-QTL fraction QTL fraction total gene dist. enriched in QTL fraction: Resistance (R) genes R-gene analogs (RGAs) Pathogen-induced genes Gene families (n=4/145) P Balint-Kurti QDR lecture Randy Wisser

  22. R-genes can’t be the whole story surely! • What other types of things might cause variations in quantitative resistance? • Structural features • Cell wall • Stomatal density, openness • Preformed defenses • Phytoalexins etc. • Components of the basal defence system? • PAMP receptors? • Effector targets? P Balint-Kurti QDR lecture

  23. Traits affecting leaf wetness may play a role P Balint-Kurti QDR lecture

  24. “race or cultivar-specific resistance mediated by single resistance (R) genes is thought to be of limited value in the field, because of the rapid evolution of new virulent races of the pathogens. On the other hand, nonhost and partial resistance appear more durable. However, the extent to which durable nonhost or partial resistance involves genetic components that are distinct from R genes remains unclear.” P Balint-Kurti QDR lecture

  25. Is quantitative resistance effective against multiple diseases • This area has not been looked at much. • If the resistance mechanism is somewhat general then you might expect it to be effective against a set of similar pathogens. • Mitchell-Olds, T., R. V. James, M. V. Palmer, and P. H. Williams, Genetics of Brassica rapa (syn. campestris). 2. Selection for multiple disease resistance to three fungal pathogens: Peronospora parasitica, Albugo candida, and Leptosphaeria maculans,Heredity, vol. 75 (1995), pp. 362-369 . P Balint-Kurti QDR lecture

  26. Positive correlations among multiple diseases Southern leaf blight Northern leaf blight Gray leaf spot R. J. Wisser P Balint-Kurti QDR lecture

  27. So what use are dQTL? • Knowledge of the location of disease resistance QTL can be used in “Marker Assisted Breeding” (MAS). • MAS uses markers to follow and incorporate into germplasm previously mapped genes for a beneficial phenotype P Balint-Kurti QDR lecture

  28. Jim Holland Catch-22 of MAS If phenotypic data are poor indicators of genotypes, you cannot adequately map QTLs to implement MAS. If phenotypic data are good, you do not need MAS. P Balint-Kurti QDR lecture

  29. Jim Holland The Catch-22 can be avoided if: • A small number of QTLs explain most of the genetic variation, in which case: • High heritability in the QTL mapping phase is optimal to identify QTL markers, then: • Markers can be implemented more easily/cheaply than phenotyping in future selection cycles. P Balint-Kurti QDR lecture

  30. In other words… • MAS makes sense: • If it is difficult/costly to select directly for resistance • Under what circumstances would this happen? • And QTL mapping is accurate. • And the QTL being selected for have reasonably large effects P Balint-Kurti QDR lecture

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