The drivers of major transitions in mutualistic dependence

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Cooperation among species (mutualism) is widespread in nature. Such partnerships between species can be of very different kinds. Mutualisms vary from associations between species that can also live without each other (facultative) to tight obligate partnerships binding species' fates together. When do these different types of mutualisms evolve or get lost? What makes a species obligately dependent on another one? Why can some associations peacefully disassemble while, in other cases, divorce is a dead end? This project aims to understand when and how mutualistic dependence evolves. Addressing this major scientific challenge requires analysing mutualistic associations at different scales. I will do so using three main approaches. My first approach will involve mapping different kinds of animal/plant mutualisms presence and absence across the plant tree of life in order to understand when they evolve or break down. To do this, I will use three major animal/plant mutualisms: pollination, seed dispersal and plant defence against herbivores or pathogens. These three ecologically important types of animal/plant mutualisms converge in that the key function can be dependent or independent on an animal mutualism. I will map these strategies on a large evolutionary plant trees (phylogenies). This will allow me to test several hypotheses regarding the factors promoting mutualistic dependence or its breakdown.My next goal will be to determine what changes in the genetic makeup are responsible for shifts in mutualistic dependence. So far, our knowledge on this area is restricted to mutualisms involving microbes living inside a host. My previous worked has shown that facultative mutualisms are lost easily while obligate mutualisms are evolutionary conserved. What defines the reversibility of these mutualistic dependencies? To address these questions, I will focus on a type of symbiotic mutualisms where plants host ants in return for extra nutrients (and sometimes defence against herbivores). This group of ~105 plant species from the coffee family is ideal system to answer this question since they have various levels of dependence. Using DNA sequencing, I will focus on three main questions: (1) Does the loss of functional genes drive mutualism dependence in these symbioses between ants and plants, as it does in partnerships between microbes and hosts? (2) How do different levels of mutualistic dependence affect the pace of gene evolution? And (3) Can we identify the genomic changes associated with trait evolution in the transition from facultative to obligate dependence?Finally, I will test the hypothesis that obligate dependence on ants in this group is the result of the loss of pathogen defence function (implying that this function is performed by the ants), which arises from my preliminary data. To do so, I will perform experiments in the field where I remove ants from obligate and facultative plants, and examine plant heath across all these treatments, notably by using a state-of-the-art method to compare microbes communities in these different cases. This will, in turn, reveal the impact of mutualistic dependence on microbial communities.At the end of this project, I hope to have identified the conditions fostering the evolution of mutualistic dependence, the mechanisms by which obligate dependence is mediated and how it impacts the evolution of the genetic makeup in an animal/plant mutualism. Since mutualistic dependence affects many ecological and evolutionary processes, understanding how it evolves will enhance our understanding of species evolution. Moreover, because highly dependent, obligate mutualists are deemed to be more vulnerable to extinction since their fate is bound to their mutualistic partner, this research has the potential to inform conservation.