Fashionably late partners have more fruitful encounters: Impact of the timing of co-infection and pathogenicity on sexual reproduction in Zymoseptoria tritici.

The wheat pathogen Zymoseptoria tritici is a relevant fungal model organism for investigations of the epidemiological determinants of sexual reproduction. The objective of this experimental study was to determine which intrinsic factors, including parental fitness and timing conditions of infection, affect the numbers of ascospores produced. We first performed 28 crosses on adult wheat plants in semi-controlled conditions, with 10 isolates characterized for their fitness traits. We validated the efficiency of the crossing method, opening up new perspectives for epidemiological studies. We found that the ability to reproduce sexually was determined, at least partly, by the parental genotypes. We also found that the number of ascospores released was correlated with the mean size of the sporulating lesions of the parental isolates on the one hand, and the absolute difference in the latent periods of these isolates on the other. No functional trade-off between the two modes of reproduction in Z. tritici was revealed: there was no adaptive compromise between pathogenicity (asexual multiplication on leaves) and transmission (intensity of sexual reproduction on wheat debris). Moreover, a few days' difference in the latent periods of the two parental isolates, such that one progressed more rapidly in the host tissue than the other, seemed to be slightly beneficial to ascosporogenesis. This may be because the first parental isolate breaks down host defenses, thereby facilitating infection for the other parental isolate. However, a larger difference (a few weeks), generated by leaving two to three weeks between the inoculations of the plant with the parental isolates, was clearly detrimental to ascosporogenesis. In this case, the host tissues were likely colonized by the first isolate, leaving less host resources available for the second, consistent with a competition effect during the asexual stage.