Interactions among Multiple Mutualists
In nature, most organisms interact simultaneously with multiple mutualistic species (e.g., plants with pollinators, microbial symbionts, and ants), however mutualism studies have traditionally focused on bipartite interactions between a single partner and host. Bipartite studies will underestimate the importance of mutualism for ecological and evolutionary dynamics if complementarity occurs among functionally different partners, and overestimate it if different partners are in conflict. In a recent synthesis paper (Afkhami et al. Ecology 2014), we merged two important, but disparate, research approaches (network and consumer-resource models) to define the multiple mutualist effects – MMEs – that occur when a focal species has multiple partner mutualists.
In current research, I have begun to integrate this conceptual framework with genomic approaches to provide new insights into the functional genomic basis of how organism balance and interact with multiple mutualists. I am using factorial experiments, genome-wide association mapping, and RNAseq with the tri-partite mutualism between Medicago truncatula, rhizobia, and arbuscular mycorrhizal fungi to (1) identify genes uniquely expressed in the presence of multiple mutualists and (2) to detect polymorphisms associated with plant fitness in response to multiple mutualists. This work is being done in collaboration with John Stinchcombe, Maren Friesen, and Yair Shachar-Hill. Our first paper looking at MME on genome-wide expression recently came out (Afkhami & Stinchcombe Molecular Ecology 2016), and we are now also pursing the interaction between these primary symbionts and the microbiome in Medicago (collaboration with grad student Colleen Friel). The lab is also working on coexpression network analyses in the context of multispecies mutualisms! (To see this great article on our Molecular Ecology paper click here).
Impact of Mutualisms on Species Distributions and the Niche
Understanding species distributions is a central goal of ecology and evolution,
and critical to predicting the consequences of climate change. Recent work has produced new theory on the roles of biotic interactions in shaping species’ distributions, but these effects are rarely investigated over large geographic scales. Unlike antagonistic species interactions which contract a species’ range, positive species interactions have the unique potential to ameliorate biotic and abiotic stressors, and may thus expand species niches, resulting in larger species ranges.
In a recent paper, I demonstrated that associating with a facultative mutualist can increase the range of a native species by thousands of square kilometers, or approximately 20% of the entire host range (Afkhami et al. Ecology Letters 2014). Combining (1) range-wide field observations from ~100 populations, (2) species distribution models, (3) 10 common garden experiments across the species range, and (4) greenhouse experiments, I showed that endophytic fungi can ameliorate drought stress and allow their grass host (Bromus laevipes) to grow in drier regions unoccupied by grasses that lack this partner. Range divergence between B. laevipes plants with and without endophytes was comparable to species-level divergence among congeners (other native Bromus), indicating that the mutualists’ effects were biologically significant. Collaborators on this project were Sharon Strauss and Pat Mcintyre. (To see this great article on our paper click here.)
Alterations to species niches and/or ranges caused by mutualisms have the potential to play an important role in the evolution and diversification of species. Mutualist-generated niche differentiation could pave the way for speciation, if selection diverges across niches, leading to reduced gene flow. Collaborators and I are examining how association with mutualistic nitrogen-fixing bacteria affects diversification rates across ~20,000 species of legumes, the third most speciose family of plants (Afkhami, Mahler et al., in review).
Conflict in Positive Interactions
Vertically transmitted symbionts (i.e., passed parent to offspring) associate with some of the most ecologically dominant species on Earth, and their fixation has
led to major evolutionary transitions (e.g., chloroplasts). Theory predicts that vertically-transmitted symbionts will be highly mutualistic because the fitness of the symbiont and host are coupled. However, partner conflict still exists in these intimate associations.
(1) Jenn Rudgers and I documented the first example of imperfect vertical transmission in the widespread symbiosis between plants and fungal endophytes (Afkhami & Rudgers American Naturalist 2008), which has since been identified as a common phenomenon and a central cause of variation in endophyte frequencies in nature (Rudgers, Afkhami et al. Ecology 2009).
(2) In a collaboration with Jenn Rudger’s lab, we documented subtle conflict due to the maternal inheritance of symbiosis, which biased host sex allocation toward maternal seed production at a cost to the host (Gorischek, Afkhami et al. American Naturalist 2013). To our knowledge this is the first example of a
symbiont altering sex allocation in plants.
(3) As part of an NCEAS working group, I have been collaborating on a novel
synthesis of the empirical and theoretical literature on cheating in mutualisms (Jones, Afkhami, et al. Ecology Letters 2015) and (Gould et al, Ecology Letters, In Review).
Microbial Mutualists as Community Players
Keystone species are important for understanding the maintenance of biodiversity and alternative stable states, as well as in guiding decisions on the conservation and restoration of ecosystems. While studies of keystone species have focused on highly visible macro-organisms, I have shown that microfungal endophytes can function as keystone mutualists. A 3-year field manipulation of endophyte in a native Californian grass showed that high endophyte abundance dramatically increased plant community diversity and altered community composition by suppressing a dominant invasive plant. The quantitative effects of endophytes on community diversity was on par with textbook examples of keystone species, such as starfish in the intertidal. Moreover, endophytes increased plant community diversity in 5 additional common gardens spanning >400km across the host grass range, demonstrating the keystone role of these fungi occurs on a range-wide scale (Afkhami & Strauss, Ecology, 2016). I have also found that endophyte effects on plant communities cascade up to higher trophic levels, such as predatory arthropods (in collaboration with Sharon Strauss and Megan Anderson).
Demography of Mutualisms across Species Ranges
Many studies of mutualisms quantify their short term fitness effects or the rewards exchanged among partners. To fully understand the consequences of mutualisms for long-term persistence of its participants, one must quantify fitness effects across the life cycle – because mutualists can have opposing effects at different life history stages. As a next step in investigating mutualism’s effect on species distributions, my lab will study demography across the species range. In 2009, I manipulated fungal endophyte abundance across five common gardens of plants from 11 Bromus laevipes populations. I have gathered 6 years of plant performance data (e.g., growth, reproduction, and survivorship) with which we will build integral projection models. To our knowledge, these gardens are the only set of replicated endophyte manipulations spanning a species’ native range, and have the potential to reveal complex, range-wide demographic effects of symbiosis.
Mutualistic Rewards = Fitness ?
We are interested in whether rewards traded among mutualist partners (in this case natural enemy defense) actually translate into meaningful fitness benefits for partners. Further, we are also interested in endophyte conferring protection to their hosts because it is another way in which they could impact communities, species ranges, and other ecological and evolutionary processes.
We know endophytes can deter natural enemies, reducing damage to their hosts for some species (e.g., Afkhami & Rudgers Environ. Entomology 2009), and I have recently found in quantitative surveys of damage and long term common gardens that endophytes in B. laevipes clearly deter enemies in their natural habitats. Further, genetic and chemical evidence showed that endophytes in B. laevipes produce a diversity of anti-herbivore alkaloids (Charlton, Craven, Afkhami et al. FEMS Microbiology Ecology 2014). After documenting that the reward was conferred, I have also set up and collected 3 years of data on a field experiment looking at whether endophyte’s ability to deter enemies causes (or is only correlated with) increases in plant fitness.