Aggressiveness, the quantitative component of pathogenicity, and its role in the adaptation of plant pathogens are still insufficiently investigated. Using mainly examples of biotrophic and necrotrophic fungal pathogens of cereals and Phytophthora infestans on potato, the empirical knowledge on the nature of aggressiveness components and their evolution in response to host and environment is reviewed. Means of measuring aggressiveness components are considered, as well as the sources of environmental variance in these traits. The adaptive potential of aggressiveness components is evaluated by reviewing evidence for their heritability, as well as for constraints on their evolution, including differential interactions between host and pathogen genotypes and trade-offs between components of pathogenicity. Adaptations of pathogen aggressiveness components to host and environment are analysed, showing that: (i) selection for aggressiveness in pathogen populations can be mediated by climatic parameters; (ii) global population changes or remarkable population structures may be explained by variation in aggressiveness; and (iii) selection for quantitative traits can influence pathogen evolution in agricultural pathosystems and can result in differential adaptation to host cultivars, sometimes leading to erosion of quantitative resistance. Possible links with concepts in evolutionary ecology are suggested.
Experimental evidence on the capacity of pathogen populations to quantitatively adapt to their hosts and on the life traits that are involved is lacking at this time. In this article, we identified a situation in which a leaf rust pathotype (P1) was found at a high frequency on a widely grown cultivar (Soissons) and we tested the hypothesis that P1 was more aggressive on Soissons than other virulent pathotypes (P2 and P3). Several components of the pathogen life cycle were measured on adult wheat plants in two different experiments under greenhouse conditions: latent period, spore production per lesion and per unit of sporulating tissue, uredinium size, and lesion life span. Regardless of the component, pathotype P1 was repeatedly found to be more aggressive than at least one of the other two pathotypes, with differences of 5 to 51%. Breaking down spore production per lesion into uredinium size and spore production per square millimeter of sporulating tissue showed that the three pathotypes presented different aggressiveness profiles, suggesting different development constraints for the pathogen, either for its growth capacity into host tissues or its ability to exploit the host resources for spore production. Although leaf rust pathotypes present a clonal structure, quantitative differences were found for aggressiveness traits within a pathotype.
Crop pathogens are notorious for their rapid adaptation to their host. We still know little about the evolution of their life cycles and whether there might be trade-offs between fitness components, limiting the evolutionary potential of these pathogens. In this study, we explored a trade-off between spore production capacity and latent period in Puccinia triticina, a fungal pathogen causing leaf rust on wheat. Using a simple multivariate (manova) technique, we showed that the covariance between the two traits is under shared control of host and pathogen, with contributions from host genotype (57%), pathogen genotype (18.4%) and genotype × genotype interactions (12.5%). We also found variation in sign and strength of genetic correlations for the pathogen, when measured on different host varieties. Our results suggest that these important pathogen life-history traits do not freely respond to directional selection and that precise evolutionary trajectories are contingent on the genetic identity of the interacting host and pathogen.
Puccinia triticina reproduces asexually in France and thus individual genotype is the unit of selection. A strong link has been observed between genotype identities (as assessed by microsatellite markers) and pathotypes (pools of individuals with the same combination of qualitative virulence factors). Here, we tested whether differences in quantitative traits of aggressiveness could be detected within those clonal lineages by comparing isolates of identical pathotype and microsatellite profile. Pairs of isolates belonging to different pathotypes were compared for their latent period, lesion size and spore production capacity on adult plants under greenhouse conditions, with a high number of replicates. Isolates of the same pathotype showed remarkably similar values for the measured traits, except in three situations: differences were obtained within two pathotypes for latent period and within one pathotype for sporulation capacity.One of these differences was tested again and confirmed. This indicates that the average aggressiveness level of a leaf rust pathotype may increase without any change in its virulence factors or microsatellite profile.
<p><strong>Background: </strong><em>Pepper huastecto yellow vein virus</em> (PHYVV) is one of the main viruses affecting pepper (<em>Capsicum</em> spp.) plants in Mexico.</p><p><strong>Question: </strong>Why there are no pepper resistant cultivars to PHYVV currently? Could it be due for the lack of new pepper resistant sources and knowledge about the heritability of the resistant trait?</p><p><strong>Study species: </strong><em>Capsicum annuum, Pepper huasteco yellow vein virus </em>and<em> Bemisia tabaci</em>.</p><p><strong>Study site: </strong>Culiacan<strong> </strong>Sinaloa, Mexico; January 2013 to August 2014.</p><p><strong>Methods: </strong>Two assays were performed in 2013 and 2014 with three resistant wild lines of <em>Capsicum annuum</em> in the S2 and S3 generation under greenhouse conditions to analyze the resistance to the <em>Pepper huasteco yellow vein virus</em> (PHYVV) and its heritability. Plants were inoculated with PHYVV through <em>Bemisia tabaci</em> G. and by grafting.</p><p><strong>Results: </strong>Line UAS12 showed a significantly higher proportion of resistant plants, longer incubation time, and less amount of viral DNA, followed by lines UAS13, UAS10 and the Maverick cultivar under both inoculation methods in both assays. Distribution of symptoms revealed a bimodal tendency in both assays. The novel gene "<em>CchGLP</em>" which confer resistance to PHYVV in pepper plants, was identified in the three lines evaluated on this study. Heritability of line UAS12 was of 0.35 and 0.26 in the insects and grafting inoculations, and of 0.58 and 0.10 in the first and second assays, respectively. Lines UAS13 and UAS10 showed close to zero heritability in the first and second assays with both inoculation methods.</p><strong>Conclusions: </strong>Line UAS12 is the most promising genetic resource for its high resistance and for showing heritability for the resistance trait. The intermediate resistance of lines UAS13 and UAS10 could be also useful for breeding programs. At least two genes are involved in the resistance trait to PHYVV. Part of the resistance shown in these lines may be due to the presence of the "<em>CchGLP</em>" gene. Line UAS12 count with variability for the resistant trait and can, therefore, be used to improve resistance and the other two lines possibly are stable as they did not show heritability.
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