Group members involved: Mats Gyllenberg, Diana Preoteasa
Financial support: Finnish Academy of Sciences, ComBi
The term fungal endophyte refers to a fungus that lives inside a plant ("endo" - within, "phyte" - plant). When talking about grass endophytes, a fungus and a grass form a relationship that is mutually beneficial and enhances the reproductive success of each.
Endophytic fungi have been found in virtually all plant species studied to date. In addition to ideal research systems for testing ecological and evolutionary theory, fungal endophytes have economical value as biological control agents against pests and pathogens, or when ensuring the quality of animal feed. They are considered to be mutualists because they receive nutrition and protection from their hosts, while the hosts receive benefits through increased resistance to herbivores, pathogens and drought and flooding stress, and enhanced competitive abilities. However, all these benefits come with an associated cost which in poor resource conditions may outweigh the benefits, as the endophytes require energy from their hosts and the alkaloids they produce can occasionally poison the host plants.
Grasses, like most organisms, have a hierarchical spatial structure of several local populations which are connected by dispersal. Thus, persistence of fungal grass endophyte can most conveniently be modeled in the framework of metapopulation dynamics.
A metapopulation is a population of local entities which can be local populations or dispersers (seeds in our case). The local populations are structured by the densities of endophyte-free and endophyte-infected grass individuals and by the quality of the local habitat. Consequently, the metapopulation state is characterized by the distribution of the local states.
The question we address primarily is whether or not mutualism is a prerequisite for persistence of fungal endophytes in structured grass metapopulation. A fungus produces considerable mycelial biomass within the host and is considered to grow along the stem to developing flowerheads and seeds; therefore, the reproductive success of the host largely determines the distribution of the endophyte infected grasses. Thus, vertical transmission should align interests of partners, so that fungus-plant association should evolve towards mutualism. However, infection levels are relatively low within species and variation in infection frequencies among local populations can be very high. By modeling the grass-endophyte dynamics within the metapopulation dynamics framework we account for spatial variability and emphasize the importance of habitat diversity in relation to nature of endophyte-host association which can range from parasitic to mutualistic.