The first spectrum of spore form and function reveals constrained evolution in mycorrhizal symbiosis
Arbuscular mycorrhizal (AM) fungi form one of the oldest and most widespread obligate mutualisms on Earth, yet they must survive independently while dispersing between hosts. Spores bridge this vulnerable host-free phase, and their morphology should therefore reflect the demands of persistence, dispersal, and establishment. However, the macroevolutionary trajectories of AM spore morphology remain poorly resolved, limiting our ability to determine whether spores diversified into multiple designs or remained constrained around a common architecture. Here, we construct the first quantitative morphospace of AM fungal spores and infer macroevolutionary patterns of trait evolution. We find that AM fungal spores have diversified mainly through scaling rather than redesign. The morphospace is dominated by size, with spore dimensions and wall volume coordinated through near-isometric scaling. Shape remains predominantly near-spherical across sizes, although the largest spores allocate proportionally less material to the wall, while ornamentation and coloration form a largely independent axis of surface variation. Most species occupy a narrow region of trait space, with distantly related lineages converging on similar trait combinations. We propose that adaptive filtering and construction economy jointly maintain this architecture. Near-spherical geometry may provide an efficient solution for packaging and protecting the reserves needed to persist between hosts while minimizing investment in wall material, whereas surface traits may mediate dispersal vectors. Functionally, this architecture suggests that AM fungal spores are shaped more by persistence through time than by dispersal through wind. The AM fungal spore morphospace thus links conserved spore design to the challenge of dispersal in an obligate mutualist.