Collaborative Research: Ecological and Evolutionary Forces Reshaping Mutualism During Species Introductions

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Introduced species can wreak havoc on ecosystems by displacing natives and by altering environmental conditions. While much is known about the properties of invasive plants and animals, much less is known about the beneficial microbes (microscopic organisms) that accompany introduced species. Rhizobia are beneficial bacteria that inhabit leguminous plant root tissue where they convert nitrogen from the atmosphere to a plant-available form. Leguminous plants provide these microbes with sugars from photosynthesis. Legumes that disperse to habitats that lack these bacteria are typically unable to persist, however the bacteria can also spread through the soil or air. The aim of this research is to examine whether invasions change the nature of a mutualistic interaction. The investigators will study specialization and the evolution of cooperation in both natural and experimentally staged invasions for two plant species and their beneficial microbes. While this question has been addressed in aboveground systems, this is one of the first to utilize a belowground plant mutualism. This project will promote diversity in the sciences, provide research training and education in the biology of plant-microbe interactions, and inform efforts to control invasive plants. Diverse undergraduate students from Florida International University will participate in summer research at University of California at Riverside and Michigan State University. An educational module will also be developed on rhizobium-legume interactions in neighborhood soils, to be distributed to school children via MSU's Kellogg Biological Station Bioenergy Sustainability Program and the Fairchild Challenge, an environmental education program that reaches over 100,000 children a year in Miami.To understand whether mutualisms become less beneficial in the process of species invasions, this project will test the hypothesized trade-off between partner breadth and mutualism benefit across legume and rhizobium genotypes from the native and invasive range for two Medicago species. Medicago is the genus that includes alfalfa. Both species are native to Europe and invasive in the US. The researchers will 1) compare genetic diversity in native vs. invaded host range mutualists, using high marker density genotyping for hosts and whole-genome sequencing for symbionts, 2) assess divergence in mutualism properties between the native and invaded range to test hypotheses of reduced specialization, partner choice and mutualism dependence in invaded range mutualists using cross-inoculation trials, 3) test the impact of invasion on symbiont genome dynamics and mutualisms by simulated symbiont invasions in soil mesocosms under contrasting host density and diversity, and 4) identify the molecular underpinnings of mutualism specialization using a transcriptomic approach to nominate host-symbiont pathways. This research will shed light on the long-standing question of whether beneficial microbes face a trade-off between having a broad partner range and providing a high degree of benefit to a partner.