Pathogen evolution and risk assessment Flashcards
Describe how gene-for-gene coevolution is thought to operate in natural ecosystems and compare this with how it operates in agricultural ecosystems.
Begins with a mutation for resistance in the host. In the presence of disease, resistant hosts produce more offspring and thus the resistance gene increases in frequency. As resistance increases, the ability of the pathogen to reproduce is reduced. If the pathogen population has a mutation from avirulence to virulence detected by this resistance gene, there will be selection for the virulence allele. Alleles for resistance and susceptibility may coexist in natural populations for a long time.
We expect htat coevolution will occur most quickly in places where the amount of disease is high and in places where host and pathogen have coexisted for along time
Coevolution in agricultural ecosystems is guided by man,evolution is not reciprocal because there is not a natural feedback loop. evolution of the pathogen is directed by humans
What is a “boom-and-bust” cycle and why is it so common in agroecosystems?
There is a phase lag oscillation in frequencies of major resistance alleles in the host population and corresponding virulence alleles in the pathogen population.
the cycles are a by-product of plant breeding programs and modern agricultural practices.
What are the five factors that determine population genetic structure and affect the evoulutionary potential of pathogen populations? Explain how each factor is likely to affect pathogen evolution.
Mutation: source of new alleles
Mating/reproduction system: new combinations of alleles if outcrossing, hold together old combinations of alleles if inbreeding or asexual
Gene/Genotype flow: Unites geographically separate populations into one genetic population
Population size: if small, genetic drift occurs, caused by bottlenecks and founder effects
Selection: Imposed by resistant varieties or fungicides will increase frequencies of selected genes or genotypes
What is “durable” disease resistance?
remains effective during prolonged and widespread use in an environment favorable to the disease
How does diversity affect the durability of disease resistance?
The more diverse the pathogen population is, the easier it will be for the pathogen to overcome resistance. The more diverse the host population is, the harder it will be for the pathogen to overcome resistance.
What are five strategies that can be used to achieve durable disease resistance in agroecosystems?
- Search for single genes with a large effect that are durable
- Discard major genes and focus on quantitative resistance
- Make pyramids of major R-genes or QR genes with large effect
- Rotate R-genes through time and space
- Use multilines or cultivar mixtures carrying different R-genes
Describe five strategies that can be used to deploy resistance genes in agricultural ecosystems.
- Plug, plant, and pray, where single R-genes are deployed one at a time matching boom-and-bust cycles;
- Rotation of R-genes in time or space;
- R-gene pyramids, where all R-genes are placed together in a plant;
- Regional deployment of different R-genes;
- Cultivar mixtures and multilines with different R-genes
What is a resistance gene pyramid? Under what conditions would you expect that a pyramid of resistance genes would lead to durable disease resistance? Under what conditions would you expect that a pyramid of resistance genes would not be durable? Explain your answers.
Pyramids are the combination of more than one R-gene into the same cultivar. Multiple mutations to virulence in the same pathogen are less likely.
Automatic recycling of R-genes becomes possible. Pyramids with 3-4 novel R-genes impose very unlikely criteria for pathogen evolution. However, pyramids are most likely to work for pathogens with no sexual stage and with low genotype flow potential. Microorganisms with sexual reproduction may quickly overcome resistance pyramids. Moreover, pyramidization with conventional breeding takes a long time, but MAS can help. Lastly, selection pressure for gene pyramids promotes ‘super pathogens’, resistant to all R-genes used
Explain four mechanisms through which cultivar mixtures and multilines can decrease disease.
lower density of compatible host genotypes
induced resistance due to inoculation of susceptible plants by avirulent pathotypes
Blockage of spore movement by the taller, sticky varieties
changes in the microclimate of the canopy making a less conducive environment or disease development
How is a multiline different from a cultivar mixture? What are the advantages and disadvantages of each approach?
Multilines are mixture of different host lines that are genetically almost identical but carry different resistance genes.
Advantages are, that components can be changed as the pathogen population evolves and that the genetic uniformity of the background genotype gives a uniform product that is desired by food processors
Disadavantage is that synthesis of a multiline takes too long using traditional breeding practices
Cultivar mixtures can be implemented without much breeding effort because they are based upon cultivars that have already been developed
Both have the advantage of considerable flexibility
What is a resistance gene cassette?
The r-gene cassette represents the highest available technology. The underlying premise is that plants already evolved r-gene clusters as a natural evolutionary response to rapidly evolving pathogens. The next step is for humans to control the process of making r-gene clusters, imitating nature’s strategy. The r cassette would have several permutations composed of different cocktails of r-genes custom-engineered for different environments and pathogen populations. The r-gene cassette would be inserted into an existing r-gene cluster at the final stage of plant breeding, after all the other quantitative traits have been fixed by the breeder.
Describe five non-genetic strategies that can be used to minimize the risk of pathogen evolution in agroecosystems.
- Limit potential for long distance dispersal (movement of infected material, soil, equipment, etc)
- Eliminate ‘living bridges’ of susceptible host materials
- Remove alternate hosts or crop stubble;
- Use regular crop rotations;
- Avoid cultivation of extremely susceptible varieties, which allow pathogen populations to explode in size
