POTENTIAL VERSUS ACTUAL CONTRIBUTION OF VERTICAL TRANSMISSION TO PATHOGEN FITNESS

Theory predicts that virulent parasites cannot be maintained at high p revalence if they are only vertically transmitted. However, parasites with high rates of vertical transmission that cause severe reduction i n host fitness have been reported. Atkinsonella hypoxylon is a fungal pathogen capable of both vertical and horizontal transmission that dra stically reduces its host's fitness. In contrast with theoretical pred ictions, field and laboratory observations suggested that the primary mechanism of transmission was vertical. Using randomly amplified polym orphic DNA markers, we investigated the effective contribution of vert ical and horizontal transmission to the genetic structure of three nat ural populations of A. hypoxylon. We found high genotypic diversity an d low linkage disequilibrium, indicating that most established genotyp es are derived from horizontally transmitted, sexual spores. The low c ontribution of vertical transmission to the parasite's fitness despite its high potential might be due to lower establishment of cleistogamo us seeds (through which vertical transmission occurs) or lower vigour of vertically transmitted fungal genotypes. Low establishment of verti cally infected hosts might explain the persistence of virulent parasit es with high apparent vertical transmission. Our results suggest that caution must be taken when using the potential for vertical transmissi on to make predictions about the evolution of parasite virulence.