Evolutionary Ecology of Grass-Endophyte Interactions
Because systemic endophytes predominantly inhabit caespitose perennial grasses and seedling recruitment may be relatively scarce, understanding how infection alters the population dynamics and evolutionary ecology of grass populations is challenging. Thus, genet and ramet survival, and size and seedling recruitment must be considered. In some cases, infected grasses increase in numbers and relative frequency while in others infection is neutral or negative with respect to relative frequencies, much like the case with individual host-endophyte interactions. In long-term studies of perennial ryegrass, infection can increase tiller production, change allometric relationships between growth and reproduction with grass ontogeny, and impact the dynamics of the grass population. Further complicating population dynamics of infected and uninfected grasses is the largely unknown dynamics of infected seeds in the seed bank. Nonetheless, this chapter suggests that changes in genotypic sorting in most grass populations based upon infection status is likely to be a relatively slow process that requires long-term experiments to observe. The observation that infection frequencies are highly variable among grass populations and among grass species suggests that there are many genetic, ecological, and geographical (e.g., metapopulation dynamics) factors that determine whether infected plants persist, increase or decline relative to uninfected counterparts. Like other species interactions, Thompson's (1994) geographic mosaic model of coevolution provides a conceptual basis for coevolution of grass-endophyte interactions. Local selection pressures acting on local populations shape endophyte-host grass interactions, and interacting populations may evolve antagonistically or mutualistically such that a broad characterization of the endophyte-grass interaction may prove difficult.
Keywords: asexual endophytes, coevolution, cost of reproduction, fungal hybridization, genotypic sorting, geographic mosaic theory, host reproduction, infection frequency, mutualism, population dynamics
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