Social Immunity in Aging Workers of Apis mellifera: Microbiota, Social Gene Expression and Disease Susceptibility in Honey Bee Colonies Overwinter.

Overwintering is a major contributor to honey bee colony loss worldwide and involves changes in environmental conditions, host physiology and behavior that render individuals more susceptible to disease. Aging, climate and social gene expression are all expected to play a major role in disease susceptibility throughout the winter months. Here we used a marked cohort of known-age worker honey bees to investigate the integrity of mouthpart and midgut microbiomes of overwintering workers at 19, 33, 50, and 70 days of age. We quantified gene expression associated with oxidative stress and social immunity from a mouthpart associated secretory (hypopharygeal) gland that interfaces with the extended social environment on many levels. Our results provide the first evidence that social immune expression is associated with the core gut microbiota in honey bees, and highlights the midgut as a vulnerable tissue overwinter in warm southern climates. Results suggest many microbiota centric hypotheses including cooperation, competition, and social hygienic host response to hive opportunists. We discovered a distinct physiological and microbiological trajectory of southern diutinus (long-lived) workers that differs drastically from the younger, short-lived workers in the colony. Antioxidant gene expression in aging bees was associated with decreased fungal load on the mouth, increases in core microbiota and increased longevity. The cold indoor overwintering environment was associated with a small and stable midgut microbiota structure and stable gene expression overwinter. In contrast, warm outdoor overwintering was associated with changes co-occurring throughout the alimentary tract microbiota that suggest both disease presence and resistance in older workers, but increased disease susceptibility for younger workers that emerged during the winter, including increases in Enterobacteriaceae, fungal load and other bacterial opportunists. Our results highlight social selection pressures that shaped the microbiome with evolution to a perennial life history.