title: Microbiome Assembly and Function in the Solitary Mason Bee, <i>Osmia lignaria</i> (Megachilidae)
Creators: name: Crowley, Bailey
Publisher: name: Utah State University
Year: 2024

description: Animal-microbe interactions can influence host biology, ecology, and evolution. The assembly and function of microbes found within animal hosts oftentimes depends on which species are involved. Advances in sequencing technologies have permitted the exploration of host-microbe interactions in a variety of animals, including bees. Early research aimed at understanding the microbiomes of social bees, such as honey bees and bumble bees, found that microbes prevent the spoilage of stored pollen, breakdown indigestible nutrients into smaller molecules available for uptake by the host, and also protect the host from pathogens. When environmental stressors, such as increased temperatures, disrupt the microbiome, the host can be negatively affected through direct harm to their symbionts or reductions in symbiont-provided benefits. Only about 10% of bee species worldwide are social however, so the advantages of bee-microbe interactions described above may not apply to most bees species. There is less research describing the microbiomes of solitary bees, including how bacterial and fungal communities change across their bee development and in response to warming temperatures.
In the first study of my thesis, I characterized the microbiome of the solitary mason bee, Osmia lignaria, across its development. Specifically, I used amplicon sequencing to determine which bacterial and fungal species are present in provisions (larval food), larvae, and the guts of adult bees before and after overwintering. I found bacterial and fungal diversity did not change across bee samples. However, the composition of bacterial and fungal communities was significantly different between larvae and adult bees before the onset of winter. Larvae seem to acquire their microbiome from provisions, as many of the bacterial and fungi found in pollen was also detected in larvae. Notably, Arsenophonus was the most abundant bacterial genus, and had high sequence similarity to a vertically transmitted species that results in the death of male offspring. The causative agent of chalkbrood, Ascosphaera, was also found in provisions and larvae. Most other bacteria and fungi present were plant pathogens and those commonly found in soil. This study suggests that solitary bees harbor microbes with diverse functions that are acquired from the environment or are maternally transmitted.
For my second project, I conducted an experimental study to determine how temperature affects the microbiome composition of provisions and the resulting effects on solitary bee development and health. Specifically, we measured the body weight and fat content of male O. lignaria reared on sterile or microbe-rich provisions within incubators emulating past, current, and projected temperatures for the Great Basin region (USA). As anticipated, the time it took larvae to develop shortened with increasing temperatures. We detected a positive relationship between temperature and the mean relative abundance of Arsenophonus, a putative male-killing symbiont. While our sterilization method removed the reproductive parasite from pollen in the sterile treatment, there was no difference in larval survivorship between bees reared on sterile and microbe-rich provisions. Contrary to past research, larvae reared on microbe-rich provisions had a lower biomass and total fat content than those reared on sterile provisions, indicating that an intact provision microbiota may not always be beneficial to larval health. Temperature increased the difference in the mean weight and fat content between larvae consuming sterile and microbe-rich provisions, with the warmest microclimate having the greatest effect. We conclude that microbes found in the provision of solitary bees may have played a role in decreasing bee body size over the past several decades of warming.