Mutualism abandonment in land plants and the origin of novel adaptations

ended

Evolutionary outcomes are constrained by lineage history. Episodes of drastic shifts in selection, as in cases of radical environmental change, often lead to the loss of traits that were once essential. While the loss of such traits may narrow niche breadth, it also creates opportunities for adaptive innovations. Whether trait losses are compensated by novel adaptations, and the extent to which similar outcomes evolved independently in unrelated lineages is largely unknown. In this project, I will investigate this problem by capitalizing on the recurrent loss of mutualism in land plants. Mutualism with soil microorganisms is ancestral in land plants, and provides plants with facilitated access to nutrients. Instances of secondary loss of mutualism are often linked to novel resource acquisition strategies, such as carnivory or parasitism. Some lineages, however, including vascular and non-vascular plants, have not undergone drastic niche shifts, and yet have largely diversified across the ecological space over time after losing mutualism. In this project, I will use comparative genomics and experimental approaches to test the hypothesis that mutualism abandonment was associated with adaptive genetic changes that compensated for the loss of mutualism. To determine whether the loss of mutualism operated as a source of selection on functions formerly facilitated by the mutualistic association, I will identify gene family expansions and genomic signatures of adaptive evolution that preceded and followed mutualism abandonment. I will then experimentally test this hypothesis in a comparative framework using closely related mutualists and non-mutualists of the liverwort genus Marchantia in conditions expected to differentially affect fitness according to mutualism status. This work will determine how novel adaptations evolve following the loss of a widely conserved trait, and reveal the extent to which similar outcomes originate in lineages with widely different histories.