Higher Variability in Fungi Compared to Bacteria in the Foraging Honey Bee

Abstract

Microbial communities in the honey bee gut have emerged as a model system to understand the effects of host-associated microbes on animals and plants. The specific distribution patterns of bacterial associates among honey bee gut regions remains a key finding within the field. The mid- and hindgut of foraging bees house a deterministic set of core species that affect host health. In contrast, the crop, or honey stomach, contains a more diverse set of bacteria that is highly variable in composition among individual bees. Whether this contrast between the two gut regions also applies to fungi, another major group of gut-associated microbes, remains unclear despite their potential influence on host health. In honey bees caught foraging at four sites across the San Francisco Peninsula, we found that fungi were much less distinct in species composition between the crop and the mid- and hindgut than bacteria. Unlike bacteria, fungi were highly variable in composition throughout the gut, and much of this variation was attributable to bee collection site. These patterns suggest that the fungi may be passengers rather than functionally significant gut symbionts. However, many of the fungi we found in the bees have been recognized as plant pathogens. Assuming that some fungi remain viable after passage through the gut, the distribution patterns we report here point to the potential importance of honey bees as vectors of fungal pathogens and suggest a more prominent role of honey bees in plant pathogen transmission than generally thought. Importance (Nontechnical explanation of why the work was undertaken)Along with bacteria, fungi make up a significant portion of animal- and plant-associated microbial communities. However, we have only begun to describe these fungi, much less examine their effects on most animals and plants. The honey bee, Apis mellifera, has emerged as a model system for studying host-associated microbes. Honey bees contain well-characterized bacteria specialized to inhabit different regions of the gut. Fungi also exist in the honey bee gut, but their composition and function remain largely undescribed. Here we show that, unlike bacteria, fungi vary substantially in species composition throughout the honey bee gut, contingent on where the bees are sampled. This observation suggests that fungi may be transient passengers and therefore unimportant as gut symbionts. However, our findings also indicate that honey bees could be major vectors of infectious plant diseases as many of the fungi we found in the honey bee gut are recognized as plant pathogens.