Some of the questions we have been tackling

How common are mutualistic symbioses in natural populations? What ecological factors affect the frequency of symbiosis?

Endo myceliumVirtually all plants host endophytic fungi in above-ground tissues. However, we are just beginning to understand their diversity and roles in biological systems, and we know little about the factors that influence the distribution and abundance of these symbioses. I am currently focused on endophyte symbiosis in the grass family (Poaceae). Grasses dominate more than one-fifth of land area on earth and include ~10,000 species, many of which are economically important. Endophytic fungi occur commonly in grasses. For a very small fraction of grass species, endophytes have been shown to increase resistance to herbivores, pathogens, and drought, often via the provision of chemical compounds that are novel to the host. For most grass endophytes, however, their ecological consequences remain uninvestigated. I am surveying fungal endophyte distribution in grasses that span diverse host species and habitats to identify biotic (e.g., herbivory) and abiotic (e.g., drought) factors that may affect endophyte frequency in nature. Recent efforts have focused on grasses in the Rocky Mountains, particularly at the Rocky Mountain Biological Lab, where we are finding shifts in endophyte frequency along elevational gradients.

What are the ecological consequences of mutualism for populations?

The field of community ecology has long emphasized the importance of antagonistic interactions (competition, predation) in shaping the structure of communities.  Ecologists have lacked a body of work investigating mutualisms at the scale of whole communities – a gap that my work has aimed to fill.

Past work in collaboration with Keith Clay revealed that a fungal endophyte symbiotic with a dominant grass (Lolium arundinaceum) functions as a keystone species in grassland communities. In large-scale field experiments, the presence of this mutualist – which protects hosts from herbivory via secondary metabolites (alkaloids) that act as plant defenses – increased host dominance, but reduced plant diversity, arthropod diversity, herbivory by mammals and insects, rates of decomposition, and the progression of plant succession from grassland to forest. These results challenge the long-held paradigm that antagonistic species interactions are the most important forces shaping community structure. (publications)

I have also investigated whether microbial mutualists, such as endophytic fungi, can enhance the ability of their hosts to invade diverse plant communities. The biodiversity of a community can affect its functional properties, such as productivity or resistance to invasion. I developed a conceptual model that predicts plants with host-specific mutualists will have greater success invading diverse plant communities than plants without such mutualists. A long-term field experiment and a manipulative greenhouse experiment confirmed this model. Altogether, this work informs how microbial mutualists contribute to the diversity of natural ecosystems as well as to the success of exotic, invasive plants.

What are the ecological consequences of mutualism for ecosystems?

Collaborative research with Dr. Sarah Emery (former post-doc, not at the University of Louisville) is testing how fungal endophytes affect the success of a key ecosystem engineering plant, Ammophila breviligulata, in Great Lakes dune ecosystems.  Ammophila is the first species to colonize bare dunes during the process of primary plant succession.  While Great Lakes populations of Ammophila are uncommonly associated with fungal endophytes, restoration material is nearly 100% infected.  How will the presence of the endophyte shape dune successional dynamics, soil stabilization, and the carbon cycle?  And, how might these effects change with shifts in the climate?  An ongoing dune restoration experiment is manipulating the endophyte and water availability to test these questions.

What are the ecological consequences of mutualisms for populations?

Species vary fundamentally in abundance, spanning a range from invasive to rare. However, currently proposed mechanisms do not sufficiently account for this observed variation. Historically, abiotic factors (resource availability, disturbance) and antagonistic interactions (competition, predation) have been considered the major determinants of abundance.  Investigations of symbioses may help resolve the lack of empirical consistency in research on rarity and commonness.  Beneficial symbionts can increase host resistance to biotic and abiotic stress and may thereby promote ecological dominance, whereas pathogenic symbionts may reduce host abundance, particularly through negative plant-soil feedbacks. My ongoing work evaluates whether and how fungal endophyte symbioses explain the ecological dominance of plant species across several native, forest understory grass species.

We have expanded this effort to include soil microbes (both bacteria and fungi) in collaboration with post-doc Valérie Huguet.  Here, we are comparing soil microbial feedbacks to plants in experiments with pairs of rare and common grass species from the same genus.  Our field surveys show that rare and common plants can support fundamentally different bacterial communities.  Our next challenge is to understand the functional consequences of these differences for plant commonness and rarity.

What evolutionary processes maintain variation in mutualistic symbioses?

greenhouseVertically transmitted symbionts are inherited by host offspring and are associated with some of the most ecologically dominant species on earth, from humans to aphids and grasses. Fixation of vertically transmitted symbionts has led to major evolutionary innovations, such as mitochondria and chloroplasts. I use endophytes in plants to provide a window on the evolution of symbiont fixation. Rates of vertical transmission are key elements in mathematical models of symbiosis and can influence both the evolution of virulence and the stability of host-symbiont dynamics.  Despite this importance, vertical transmission rates have rarely been documented in mutualistic symbioses. Along with former MS student, Michelle Afkhami, we have shown that vertical transmission of endophytes is commonly imperfect (not all offspring become symbiotic), suggesting an important mechanism to maintain variation in symbiont frequency.

Current work in collaboration with Dr. Tom Miller and former undergraduate, Kelsey Yule, incorporates variation in vertical transmission rates into demographic models for grass populations.  We are particularly interested in the role of host life history in affecting symbiont dynamics.

Work with former undergraduate, Alex Gorischek, investigates whether uniparental transmission selects for symbionts that are reproductive manipulators, favoring host allocation to the transmitting sex.