|Faeth, Stanley -|
|Hayes, Cinnamon -|
Submitted to: Microbial Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 8, 2010
Publication Date: October 1, 2010
Citation: Faeth, S.H., Hayes, C.J., Gardner, D.R. 2010. Asexual endophytes in a native grass: Tradeoffs in mortality, growth, reproduction, and alkaloid production. Microbial Ecology. 60:496-504. DOI: 10.1007/s00248-010-9643-4. Interpretive Summary: In many established agronomic grasses there is a positive benefit to the grass by hosting symbiotic microbes known as endophytes. Benefits include increased growth, reproduction, and resistance to herbivory from the production of toxic alkaloids by the endophytes. Little is known about such costs and benefits of endophyte infection in native grasses however. A four year experimental cost/benefit analysis was conducted using the native grass Achnatherum robustum (sleepygrass) in which herbivory and water availability were controlled and survival, growth, and reproduction were monitored with respect to endophyte infected and uninfected plants and endophyte infected plants with low and high alkaloid content. In the native sleepy grass, the endophyte infected grass with alkaloids had reduced growth and lower seed production and showed no advantage under full herbivory relative to low alkaloid grass or uninfected endophyte grass. Generally, the results are contrary to what has been established in most agronomic grasses and to the prediction that additional resources would offset the cost of alkaloid production. The results were also contrary to the predictions of the defensive mutualism hypothesis with respect to alkaloid content and herbivory. The endophyte infected grass with alkaloids did however have higher overwinter survival suggesting alkaloid may provide protection from underground herbivory. In native grasses there maybe a number of selective factors that maintain the presence of both uninfected and infected plants and infected plants with variable alkaloid levels.
Technical Abstract: Neotyphodium endophytes are asexual, seed-borne fungal symbionts that are thought to interact mutualistically with their grass hosts. Benefits include increased growth, reproduction, and resistance of herbivores via endophytic alkaloids. Although these benefits are well established in infected introduced, agronomic grasses, little is known about the cost and benefits of endophyte infection in native grass populations. These populations exist as mosaics of uninfected and infected plants, with the latter often comprised of plants that vary widely in alkaloid content. We tested the costs and benefits of endophyte infections with varying alkaloids in the native grass Achnatherum robustum (sleepygrass). We conducted a 4-year field experiment, where herbivory and water availability were controlled and survival, growth, and reproduction of three maternal plant genotypes [uninfected plants (E-), infected plants with high levels of ergot alkaloids (E+A+), and infected plants with no alkaloids (E+A-)] were monitored over three growing seasons. Generally, E+A+ plants had reduced growth over the three growing seasons and lower seed production than E- or E+A- plants, suggesting a cost of alkaloid production. The reduction in vegetative biomass in E+A+ plants was most pronounced under supplemented water, contrary to the prediction that additional resources would offset the cost of alkaloid production. Also, E+A+ plants showed no advantage in growth, seed production, or reproductive effort under full herbivory relative to E- or E+A- grasses, contrary to the predictions of the defensive mutualism hypothesis. However, E+A+ plants had higher overwintering survival than E+A- plants in early plant ontogeny, suggesting that alkaloids associated with infection may protect against below ground herbivory or harsh winter conditions. Our results suggest that the mosaic of E-, E+A+, and E+A- plants observed in nature may result from varying biotic and abiotic selective factors that maintain the presence of uninfected plants and infected plants that vary in alkaloid production.