Mutualistic microbes drive niche expansion yet constrain diversification

Mutualistic interactions are widely hypothesized to expand ecological niches and promote diversification, yet evidence for this relationship at macroevolutionary scales remains limited. Here, we investigate how fungal seed endophytes (Epichloe) 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 constructed species distribution models for suites of symbiotic and non-symbiotic plant species, then used a range of phylogenetic comparative methods to assess the influence of these symbionts on niche breadth, niche breadth evolution, and diversification. Endophyte-positive species had [~]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 had significantly drier climate centroids, consistent with endophyte-mediated drought tolerance, while simultaneously showing constraints along specific niche dimensions. Despite pronounced niche expansion, hidden state speciation and extinction models revealed two modes of diversification: endophytes decrease diversification rates in young lineages, and have a neutral effect in older lineages. Together, our results demonstrate that mutualists can enhance ecological flexibility while simultaneously constraining macroevolutionary outcomes, revealing a decoupling of niche expansion and diversification, and highlighting the complex role of symbiosis in shaping host evolutionary trajectories.