Topic 1 : How do pathogen/pest populations respond to deployment of host resistance?. Natural vs. human-managed systems Selective effects of qualitative resistance (R genes) How does evolution to virulence affect pathogens – is there a cost to virulence?
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% isolates virulent
% isolates virulent
Genotype A: 10 lesions/lesion/day
Genotype B: 9 lesions/lesion/day
For example:(Zhan et al, 2002, Local adaptation and effect of host genotype on the rate of pathogen evolution: an experimental test in a plant pathosystem. J. Evol. Biol. 15:637-647)
si = 1 - pit pj0
Fitness measured in field; aggressive-ness measured in greenhouse
Was aggressive-ness correlated with fitness?
Xi et al. , 2003, Mycol. Res. 107:1485-1492
Fig. 1. Frequency of pathotypes E97-2 and H97-2 of Rhynchosporium secalis co-inoculated over 4 cycles on two barley cultivars: (a) ‘Earl,’ susceptible to E97-2, and (b) ‘Harrington,’ susceptible to both.
E97-2 had complex virulence (to Earl, Harrington, and 5 more cvs).
H97-2 had simple virulence (to Harrington and 1 other cv, not Earl).
Bahri et al, 2009. Tracking costs of virulence in natural populations of the wheat pathogen, Puccinia striiformis f. sp. tritici. BMC Evol. Bio. 9:26.
So: if you remove a defeated R gene from commercial production, will the corresponding virulence in the pathogen population decline in frequency?
“A great advantage of the strategy is that it does not depend upon a supply of new resistance genes….Existing resistance genes can be recycled. For example, if a gene, or gene combination, is withdrawn because the frequency of the matching virulence increases in the pathogen population it can be re-introduced when thefrequency of the matching virulence allele(s) reduces.” (Pink & Puddephat)
Morris et al, 2009. Expanding the paradigms of plant pathogen life history and evolution of parasitic fitness beyond agricultural boundaries, PLoS Pathogens 5:1, www.plospathogens.org
So if avr genes affect fitness differentially, mutations in them should differentially affect fitness, and R genes corresponding to them should be differentially durable.
Tested in field with NILs, natural inoculum. Over 3 yrs, durability of Xa7 > Xa10.