Mutualistic microbes linked to broader niches but reduced diversification

Mutualistic interactions are widely hypothesized to broaden ecological niches and promote diversification, yet evidence for this relationship at macroevolutionary scales remains limited. Here, we investigate how fungal seed endophytes (Epichloë) influence niche breadth and diversification across over 100 species of the globally-distributed grass genus Poa. Combining global-scale climate, environmental, phylogenetic, and symbiosis data, we construct species distribution models for suites of symbiotic and non-symbiotic plant species, and use phylogenetic comparative methods to assess the influence of these symbionts on niche breadth, niche breadth evolution, and diversification. Endophyte-positive species have ~1.4 times broader primary niche axes (most often associated with temperature tolerances), ~2.5 times larger geographic ranges, and >30 times faster rates of niche breadth evolution than non-symbiotic species. Endophytic plants also have significantly drier climate centroids, consistent with endophyte-mediated drought tolerance, while simultaneously showing constrained niche centroid evolution. Despite pronounced niche expansion, hidden state speciation and extinction models reveal two modes of diversification: endophytes decrease diversification rates in young lineages, and have a neutral effect in older lineages. Together, our results indicate that mutualists can be associated with broader niches without necessarily increasing diversification, highlighting a potential decoupling between niche expansion and diversification. Mutualistic interactions are hypothesized to broaden ecological niches and promote diversification, yet evidence for this relationship remains limited. This study finds that, across Poa grasses, fungal endophytes are associated with broader niches and faster niche breadth evolution, but reduced diversification rates.