Fungal hyphae regulate bacterial diversity and plasmid-mediated functional novelty during range expansion.

The amount of bacterial diversity present on many surfaces is enormous; however, how these levels of diversity persist in the face of the purifying processes that occur as bacterial communities expand across space (referred to here as range expansion) remains enigmatic. We shed light on this apparent paradox by providing mechanistic evidence for a strong role of fungal hyphae-mediated dispersal on regulating bacterial diversity during range expansion. Using pairs of fluorescently labeled bacterial strains and a hyphae-forming fungal strain that expand together across a nutrient-amended surface, we show that a hyphal network increases the spatial intermixing and extent of range expansion of the bacterial strains. This is true regardless of the type of interaction (competition or resource cross-feeding) imposed between the bacterial strains. We further show that the underlying cause is that flagellar motility drives bacterial dispersal along the hyphal network, which counteracts the purifying effects of ecological drift at the expansion frontier. We finally demonstrate that hyphae-mediated spatial intermixing increases the conjugation-mediated spread of plasmid-encoded antibiotic resistance. In conclusion, fungal hyphae are important regulators of bacterial diversity and promote plasmid-mediated functional novelty during range expansion in an interaction-independent manner. [Display omitted] • Fungal hyphae promote and maintain bacterial diversity during range expansion • Fungal hyphae increase bacterial dispersal and counteract ecological drift • This increases the spatial intermixing of different bacterial populations • The increased spatial intermixing promotes plasmid-mediated functional novelty Ruan et al. demonstrate that fungal hyphae can promote and maintain bacterial diversity during range expansion. Fungal hyphae increase bacterial dispersal, which counteracts the effects of ecological drift at the expansion frontier. This increases the spatial intermixing of different populations and promotes plasmid-mediated functional novelty. [ABSTRACT FROM AUTHOR]