Environmental variables drive the latitudinal decline in diversity and functional guild distribution of seed endophytic fungi in the invasive plant Bidens frondosa.

Seed endophytic fungi are increasingly recognized as key determinants of plant fitness, yet the ecological processes governing their biogeographic patterns remain poorly understood. Here, we investigated the diversity, composition, and putative functional guilds of seed-associated fungal communities in the invasive plant Bidens frondosa along a latitudinal gradient from 28 to 41°N in China. Alpha diversity declined significantly with latitude, while beta diversity showed strong latitudinal structuring, with latitude explaining 45.7% of community variation. Community composition shifted from Sordariomycetes dominance at low latitudes to Saccharomycetes dominance at high latitudes, accompanied by a functional transition from saprotroph dominated to saprotroph-symbiotroph dominated guilds. Differential abundance analysis revealed that plant-pathogenic taxa (e.g., Fusarium) were enriched at low latitudes, whereas the cold-adapted yeast Mrakia dominated high-latitude populations. Distance-based redundancy analysis identified soil pH, cation exchange capacity, and soil organic carbon as the primary environmental filters structuring both taxonomic and functional community variation. In contrast, all climatic variables were excluded from the final models. These findings demonstrate that soil chemistry, rather than climate factors, acts as the dominant selective force associated with seed endophytic fungal communities along latitudinal gradients. This work provides mechanistic insights into the biogeographic organization of plant-microbe symbioses and highlights the potential role of seed-associated fungi in facilitating plant invasion across climatic gradients.