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