TRADE-OFF BETWEEN VIRULENCE AND VERTICAL TRANSMISSION AND THE MAINTENANCE OF A VIRULENT PLANT PATHOGEN

The continuum hypothesis predicts that parasites should evolve reduced virulence if they have higher opportunity for vertical transmission. However, when there is a trade-off between Virulence and vertical tran smission, selection may favor horizontal transmission and higher virul ence. Atkinsonella hypoxylon is a fungal pathogen that reduces Danthon ia fitness by 50% or moro when it completely castrates hosts' chasmoga mous inflorescences, despite the high opportunity for vertical transmi ssion through cleistogamous seeds. Sporadically, infected hosts with p artially castrated inflorescences (which have higher fecundity than co mpletely castrated hosts) are observed in natural populations. Why are partially castrated plants rare if selection favors reduced virulence ! We investigated whether there was genetic diversity for virulence am ong A. hypoxylon genotypes and the relationship between virulence and vertical transmission. We found that the fungal genotype significantly affects the occurrence of partial castration in Danthonia compressa. The proportion of seedlings that were vertically infected by their mat ernal plant was lower for partially castrated than for completely cast rated plants. Our results demonstrate a trade-off between virulence an d vertical transmission, explaining the maintenance of more virulent, completely castrating Fungal genotypes in natural populations, and sug gest that vertical transmission in plants is more complex than what is considered in current models.