Variation in aggressiveness is detected among Puccinia triticina isolates of the same pathotype and clonal lineage in the adult plant stage

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

Evolution within a given virulence phenotype (pathotype) is driven by changes in aggressiveness: a case study of French wheat leaf rust populations

Plant pathogens are constantly evolving and adapting to their environment, including their host. Virulence alleles emerge, and then increase, and sometimes decrease in frequency within pathogen populations in response to the fluctuating selection pressures imposed by the deployment of resistance genes. In some cases, these strong selection pressures cannot fully explain the evolution observed in pathogen populations. A previous study on the French population of Puccinia triticina, the causal agent of wheat leaf rust, showed that two major pathotypes — groups of isolates with a particular combination of virulences — predominated but then declined over the 2005-2016 period. The relative dynamics and the domination of these two pathotypes — 166 317 0 and 106 314 0 —, relative to the other pathotypes present in the population at a low frequency although compatible, i.e. virulent on several varieties deployed, could not be explained solely by the frequency of Lr genes in the landscape. Within these two pathotypes, we identified two main genotypes that emerged in succession. We assessed three components of aggressiveness — infection efficiency, latency period and sporulation capacity — for 44 isolates representative of the four P. triticina pathotype-genotype combinations. We showed, for both pathotypes, that the more recent genotypes were more aggressive than the older ones. Our findings were highly consistent for the various components of aggressiveness for pathotype 166 317 0 grown on Michigan Amber — a ‘naive’ cultivar never grown in the landscape — or on Apache — a ‘neutral’ cultivar, which does not affect the pathotype frequency in the landscape and therefore was postulated to have no or minor selection effect on the population composition. For pathotype 106 314 0, the most recent genotype had a shorter latency period on several of the cultivars most frequently grown in the landscape, but not on ‘neutral’ and ‘naive’ cultivars. We conclude that the quantitative components of aggressiveness can be significant drivers of evolution in pathogen populations. A gain in aggressiveness stopped the decline in frequency of a pathotype, and subsequently allowed an increase in frequency of this pathotype in the pathogen population, providing evidence that adaptation to a changing varietal landscape not only affects virulence but can also lead to changes in aggressiveness

Etude du stock de semences de mauvaises herbes dans le sol : le problème de l’échantillonnage

L’étude quantitative du stock de semences de mauvaises herbes dans le sol pose un problème de nombre de prélèvements de sol à faire. L’objet de ce travail est d’apporter des éléments de réponse à la difficile question de l’échantillonnage. Des carottes de sol ont été prélevées selon un plan d’échantillonnage à 2 degrés (5 x 20 prélèvements par parcelle) pour 5 types de parcelles. Le traitement individuel des prélèvements de sol (tamisage, mise en germination en serre pendant 2 mois, dénombrement des plantules levées, reprise des semences dormantes par flottation) permet d’établir 3 types de lois de distribution spatiales des populations de semences (la loi dépend de l’effectif de la population mais pas de l’espèce considérée) : distribution de Poisson, distribution agrégée, distribution normale. Le calcul, fait séparément pour chaque espèce et dans chaque unité primaire de sondage, de la moyenne du nombre de semences et de son intervalle de confiance, permet de déterminer le nombre d’échantillons à prélever pour l’obtention d’une précision de 20 p. 100. A partir de ces résultats, il est possible d’établir une relation entre n, nombre minimum d’échantillons à prélever, et la moyenne de l’effectif de la population que l’on veut échantillonner. Ainsi on doit avoir n > 100 pour la plupart des espèces et pour une précision de 20 p. 100, ce qui est très difficile à réaliser en pratique. Pour la même précision, 50 100 for a majority of species to achieve a precision p = 0.05, which is very difficult to achieve in practice. For the same precision, 50 < n < 100 is correct if the distribution law is normal and if the species is abundant (more than 25 x 106 seeds per hectare)

Etude du stock de semences de mauvaises herbes dans le sol : le problème de l’échantillonnage

National audienceQuantitative study of the buried weed seed bank involves the problem of the number of cores to be taken, and this article seeks to solve this sampling problem. Soil cores were collected following a two-stage sampling (5 x 20 cores in each plot) for 5 types of plots. The cores were treated separately (sieving, germination in glasshouse for 2 months, seedling counts, recovery of dormant seeds by flotation), three types of spatial distribution laws could be established for seed populations, depending on the abundance of the species and not on the species itself ; Poisson distribution, aggregated distribution, normal distribution. The mean number of seeds per core and its confidence interval (calculated separately for each species in each subplot) was used to determine the number of cores to be taken to achieve a precision p = 0.05. From these results, a relation can be established between n, the minimum number of cores to be taken, and the mean number of the seed population to be sampled. In fact, n must be > 100 for a majority of species to achieve a precision p = 0.05, which is very difficult to achieve in practice. For the same precision, 50 100 pour la plupart des espèces et pour une précision de 20 p. 100, ce qui est très difficile à réaliser en pratique. Pour la même précision, 50 < n < 100 convient si la loi de distribution est normale et si l’espèce est abondante (plus de 25 x 106 semences par hectare)

Population structure and sampling of Puccinia recondita f.sp. tritici in France during 1993-1995.

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