Understanding context-dependency in plant–microbe symbiosis: The influence of abiotic and biotic contexts on host fitness and the rate of symbiont transmission

Niche Differentiation in the Dynamics of Host-Symbiont Interactions: Symbiont Prevalence as a Coexistence Problem

“…A previous greenhouse study of the A. hyemalis/E. amarillans symbiosis revealed effects of endophytes on host phenology, with Eϩ plants flowering earlier than EϪ plants (Davitt et al 2011). If other developmental processes are similarly shifted, temporal partitioning of the growing season could concentrate competition within host types.…”

“…Agrostis hyemalis produces a single seed per spikelet; spikelet counts therefore provide an upper bound on seed production (spikelets can be counted more reliably than the 0.5-mmlong seeds). Because endophyte symbiosis can advance flowering under some conditions (Davitt et al 2011), we assumed that, in a mixed population, the first flowering plant was likely to be Eϩ. The resulting seed scores allowed us to test this assumption (maternal plants with zero Eϩ seeds were assumed to be EϪ and excluded from the transmission rate data set).…”

“…Endophyte frequency in this population is high (96% Eϩ). Seeds were collected from approximately 100 individuals and a subset were heat treated in a 62ЊC water bath for 6 min, which kills the endophyte but not the plant (Davitt et al 2011). Heat treatment allowed us to decouple endophyte presence/absence from maternal plant genetic background, which are coinherited in naturally occurring lineages.…”

“…However, symbionts can have important effects on the physiology and ecology of their hosts and available evidence suggests that they can modify niche characteristics. For example, symbiosis with vertically transmitted microbes can alter host responses to abiotic stress (Malinowski and Belesky 2000;Davitt et al 2011;Saari and Faeth 2012), modify interactions with natural enemies (Haine 2008;Brownlie and Johnson 2009), and provide access to novel resources (Joy 2013) and means of nutrient acquisition (Stewart et al 2005;Kaiser et al 2010;Feldhaar 2011). We hypothesized that niche differences could offset a fitness difference between symbiotic and nonsymbiotic intraspecific competitors and promote their stable coexistence.…”

“…Epichloid fungal endophytes, primarily in the genera Neotyphodium and Epichloë, are estimated to occur in up to 30% of grass species (Leuchtmann 1992). In cases where grass and endophyte fitnesses are tightly coupled via exclusively vertical transmission, these symbionts are thought to function primarily as mutualists (e.g., Rudgers et al 2012), although interaction outcomes may depend upon environmental context (Davitt et al 2011;Miranda et al 2011). Epichloid endophytes produce alkaloids that can confer host resistance to various stresses, including herbivory (Schardl et al 2004).…”