The effect of imperfect transmission on the frequency of mutualistic seed-borne endophytes in natural populations of grasses

We develop a simple mathematical model to explain the lower than expected levels of infection of wild perennial ryegrass populations in France by endophytic Neotyphodium fungi (formerly named Acremonium). Indeed, seed-borne Neotyphodium endophytes are considered as mutualistic symbionts, because they increase survival, growth and flowering rates of their hosts, and should therefore be present at very high frequencies in all host populations. However, recent surveys have shown that 70% of wild populations of perennial ryegrass harbour such endophytes in France. Moreover, most infected populations exhibit a low level of infection. Our simple model, taking into account the life-cycIes of the host and the fungus, shows that these patterns can be satisfactorily explained if the vertical transmission of the fungus is imperfect. Such imperfect transmission, thou,gh never measured in natural populations, is likely because of the reported mortality of the endophyte in stored seeds. This process, analogous to the mutation-selection balance of classical population genetics theory, may explain the observed patterns even better when we consider random fluctuations of selection coefficients over time and genetic drift.