Evolutionary theory suggests that obligate hereditary symbionts should evolve towards mutualism or else become extinct. Studies of hereditary Epichloë fungi, which occur in aboveground tissues of poöid grasses, indicate some benefits of these endophytes for the host grasses. However, infection frequencies of grass populations hosting these endophytes vary from 0 to 100%, and numerous mechanisms have been proposed to explain these observations. We developed a stage-structured, continuous-time, population dynamics model to examine host-grass population infection frequencies based on effects of (1) imperfect endophyte transmission at various host life stages and (2) endophyte costs or benefits for host population vital rates. The model structure captures the general relationships and processes necessary to model any grass-Epichloë host-endophyte association, provided that parameter estimates are obtained. Here, the model was parameterized using empirical data from Schedonorus arundinaceus (tall fescue)–Epichloë coenophiala.
The model results show that infection frequencies are highly dependent on endophyte benefits to host fitness (tiller survival and fecundity) and transmission losses at most host life-history stages except the seed bank. In the absence of endophyte benefits, equilibrium infection frequencies approach 0% due to transmission failure. However, a host performance advantage of ~35% due to the endophyte results in a population infection frequency of ~60%, even with transmission failure. Thus, small changes in the cost-benefit ratio of hosting the endophyte can substantially alter the equilibrium population infection frequency. Given that tiller survival and fecundity are not well studied for many of the long-lived perennial grass–Epichloë sp. associations, future studies should attempt to determine the environmental conditions under which the endophyte poses costs or benefits for these fitness components.