Assessing Chemical Mechanisms Underlying the Effects of Sunflower Pollen on a Gut Pathogen in Bumble Bees

Abstract

Author contributions. LSA, REI and PCS conceived of and designed the study. PCS and IWF conducted chemical analysis of pollen and synthesized spermidines. AEF, RLM, PRA, LMC, PMD, and SL carried out bioassay experiments. AEF analyzed data and prepared figures. LSA wrote the manuscript with substantial contributions from PCS. All co-authors read and provided feedback on the manuscript.

Sunflower pollen reduces a gut pathogen in worker and queen but not male bumble bees

“…Although Crithidia grew in males relative to the initial inoculation dose, they exhibited remarkably low Crithidia counts compared to females regardless of diet, and there was no significant effect of diet on Crithidia counts. In workers and daughter queens, sunflower pollen dramatically reduced infection compared to wildflower pollen, consistent with previous findings (Giacomini et al ., 2018; Adler et al ., 2020).…”

“…The mechanism underlying the medicinal effect of sunflower pollen is currently unknown. Chemical extracts from sunflower pollen reduced the growth of bacteria and fungi (Fatrcová‐Šramková et al ., 2016) but increased Crithidia growth in vitro (Palmer‐Young & Thursfield, 2017) and did not reduce Crithidia to the level of pure sunflower pollen in bee assays (Adler et al ., 2020). A compound in heather nectar ( Calluna vulgaris ) reduced Crithidia infection by removing the flagellum and preventing attachment to the gut wall (Koch et al ., 2019); the spiky pollen coat of sunflower could mechanically inhibit attachment via similar mechanisms.…”

“…The low protein content is not likely responsible for reducing Crithidia since another similarly low‐protein pollen diet (buckwheat, Fagopyrum esculentum ) resulted in comparatively high Crithidia infections (Giacomini et al ., 2018). In addition, other chemical components of sunflower pollen, including several fatty acids and the secondary compounds rutin and triscoumaroyl spermidine derivatives, are not the mechanism by which sunflower pollen reduces Crithidia in the gut (Adler et al ., 2020). Currently, the mechanism underlying this medicinal effect is unknown.…”

“…We did not measure pollen consumption in this experiment. Previous work has confirmed that workers and newly emerged queens consume pollen (Woodard et al, 2019) and that workers consume similar quantities of wildflower pollen mix and sunflower pollen (Adler et al ., 2020). However, pollen consumption by male bumble bees has not been previously documented since male bees primarily forage for nectar during adulthood.…”

Linking bacterial diversity to floral identity in the bumble bee pollen basket

“…A positive correlation between Lactobacillus and Acinetobacter (and Sphingomonas) with Asteraceae illustrates an interesting bee-flowermicrobe interaction (bumble bee-Lactobacillus/Acinetobacter-Asteraceae) deserving of more research attention and indicative of an emergent and understudied driver of well-known bee-flower mutualisms. Despite some Asteraceae (e.g., dandelion; Taraxacum) being toxic to bumble bees when the sole source of food (Vanderplanck et al, 2020), many Asteraceae provide ample pollen and nectar that are nutritious and attractive to bumble bees (Hicks et al, 2016) and Asteraceae were present in 50% of all pollen samples in our study ( (Adler, Fowler, et al, 2020). In yet another study, Mockler et al (2018) showed that bumble bees having higher levels of Lactobacillus in their gut microbiome led to reduced infection rates of C. bombi.…”