Biology subjects

Orellana-Torrejon, C.

Publications and source records attributed to Orellana-Torrejon, C..

3 recordsLinked to original sources

bioRxiv2022

Multiple scenarios for sexual crosses in the fungal pathogen Zymoseptoria tritici on wheat residues: potential consequences for virulence gene transmission

Little is known about the impact of host immunity on sexual reproduction in fungal pathogens. In particular, it is unclear whether crossing requires both sexual partners to infect living plant tissues. We addressed this issue in a three-year experiment investigating different scenarios of Zymoseptoria tritici crosses on wheat according to the virulence ( vir) or avirulence ( avr) of the parents against a qualitative resistance gene. Co-inoculations ( vir x vir, avr x vir, avr x avr) and single inoculations were performed on a cultivar carrying the resistance gene (Cellule) and a susceptible cultivar (Apache), in the greenhouse. We assessed the intensity of asexual multiplication by scoring disease severity, and the intensity of sexual reproduction by counting the ascospores discharged from wheat residues. As expected, disease severity was more intense on Cellule for vir x vir co-inoculations than for avr x vir co-inoculations, with no disease for avr x avr. However, all types of co-inoculation yielded sexual offspring, whether or not the parental strains caused plant symptoms. Parenthood was confirmed by genotyping (SSR markers), and the occurrence of crosses between (co-)inoculated and exogenous strains (other strains from the experiment, or from far away) was determined. We found that symptomatic asexual infection was not required for a strain to participate in sexual reproduction, and that avirulent strains could be maintained asymptomatically "on" or "in" leaf tissues of plants carrying the corresponding resistant gene for long enough to reproduce sexually. In two of the three years, the intensity of sexual reproduction did not differ significantly between the three types of co-inoculation in Cellule, suggesting that crosses involving avirulent strains are not anecdotal. We discuss the possible mechanisms explaining the maintenance of avirulence in Z. tritici populations and supporting the potential efficacy of cultivar mixtures for limiting resistance gene breakdown. HighlightsO_LIAvirulent Zymoseptoria tritici strains can reproduce sexually in wheat plants carrying the corresponding resistant gene. C_LIO_LISymptomatic infection of plant tissues is not essential for a strain to reproduce sexually. C_LIO_LIAvirulent strains can be maintained asymptomatically "on" or "in" leaf tissues of plants carrying the corresponding resistant gene for long enough to reproduce sexually. C_LIO_LICrosses of virulent strains with virulent and avirulent strains in a plant host carrying the corresponding resistance gene can produce offspring with similar population sizes. C_LIO_LISeveral possible scenarios for sexual crosses can explain the maintenance of avirulence in Zymoseptoria tritici populations evolving in a wheat canopy, particular in cultivar mixtures. C_LI

microbiology↗

bioRxiv2022

Does the impact of cultivar mixtures on virulence dynamics in Zymoseptoria tritici populations persist after interseason sexual reproduction?

This study follows on from a previous study showing that binary mixtures of wheat cultivars affect the evolution of Zymoseptoria tritici populations within a field epidemic from the beginning (t1) to the end (t2) of a growing season. Here, we focused on the impact of interseason sexual reproduction on this evolution. We studied mixtures of susceptible and resistant cultivars (carrying Stb16q, a recently broken-down resistance gene) in proportions of 0.25, 0.5 and 0.75, and their pure stands. We determined the virulence status of 1440 ascospore-derived strains collected from residues of each cultivar by phenotyping on seedlings. Virulence frequencies were lower in mixtures than in pure stands of the resistant cultivar, as at t2, revealing that the impact of mixtures persisted until the beginning of the next epidemic (t3). The frequency of virulence was lower in the offspring population on the susceptible cultivar and, more surprisingly, the frequency of avirulence on the resistant cultivar increased after sexual reproduction. Our findings highlight two epidemiological processes in mixtures: selection within the local pathogen population between t1 and t2 driven by asexual cross-contamination between cultivars (previous study) and sexual crosses between avirulent and virulent strains between t2 and t3 driven by changes in the probabilities of physical encounters (this study). Mixtures therefore appear to be a promising strategy for the deployment of qualitative resistances, not only to limit the intensity of Septoria tritici blotch epidemics, but also to reduce the erosion of resistances by managing evolution of the pathogen population at a pluriannual scale.

ecology↗

bioRxiv2021

Annual dynamics of Zymoseptoria tritici populations in wheat cultivar mixtures: a compromise between the efficiency and durability of a recently broken-down resistance gene?

Cultivar mixtures slow polycyclic epidemics but may also affect the evolution of pathogen populations by diversifying the selection pressures exerted by their plant hosts at field scale. We compared the dynamics of natural populations of the fungal pathogen Zymoseptoria tritici in pure stands and in three binary mixtures of wheat cultivars (one susceptible cultivar and one cultivar carrying the recently broken-down Stb16q gene) over two annual field epidemics. We combined analyses of population size based on disease severity, and of population composition based on changes in the frequency of virulence against Stb16q in seedling assays with more than 3000 strains. Disease reductions were observed in mixtures late in the epidemic, at the whole canopy scale and on both cultivars, suggesting the existence of a reciprocal protective effect. The three cultivar proportions in the mixtures (0.25, 0.5 and 0.75) modulated the decrease in (i) the size of the pathogen population relative to the two pure stands, (ii) the size of the virulent subpopulation, and (iii) the frequency of virulence relative to the pure stand of the cultivar carrying Stb16q. Our findings suggest that optimal proportions may differ slightly between the three indicators considered. We argued potential trade-offs that should be taken into account when deploying a resistance gene in cultivar mixtures: between the dual objectives efficacy and durability, and between the size and frequency of the virulent subpopulation. Based on current knowledge, it remains unclear whether virulent subpopulation size or frequency has the largest influence on interepidemic virulence transmission.

microbiology↗