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Climate change-related increases in thermal variability and rapid temperature shifts will affect organisms in multiple ways, including imposing physiological stress. Furthermore, the effects of temperature may alter the outcome of biotic interactions, such as those with pathogens and parasites. In the context of host–parasite interactions, the beneficial acclimation hypothesis posits that shifts away from acclimation or optimum performance temperatures will impose physiological stress on hosts and will affect their ability to resist parasite infection. We investigated the beneficial acclimation hypothesis in a bumble bee–trypanosome parasite system. Freshly emerged adult worker bumble bees, Bombus impatiens, were acclimated to 21, 26, or 29°C. They were subsequently experimentally exposed to the parasite, Crithidia bombi, and placed in a performance temperature that was the same as the acclimation temperature (constant) or one of the other temperatures (mismatched). Prevalence of parasite transmission was checked 4 and 6 days post-parasite exposure, and infection intensity in the gut was quantified at 8 days post-exposure. Parasite strain, host colony, and host size had significant effects on transmission prevalence and infection load. However, neither transmission nor infection intensity were significantly different between constant and mismatched thermal regimes. Furthermore, acclimation temperature, performance temperature, and the interaction of acclimation and performance temperatures had no significant effects on infection outcomes. These results, counter to predictions of the beneficial acclimation hypothesis, suggest that infection outcomes in this host–parasite system are robust to thermal variation within typically experienced ranges. This could be a consequence of adaptation to commonly experienced natural thermal regimes or a result of individual and colony level heterothermy in bumble bees. However, thermal variability may still have a detrimental effect on more sensitive stages or species, or when extreme climatic events push temperatures outside of the normally experienced range.

Abstract Comprehensive decisions on the management of commercially produced bees, depend largely on associated knowledge of genetic diversity. In this study, we present novel microsatellite markers to support the breeding, management, and conservation of the blue orchard bee, Osmia lignaria Say (Hymenoptera: Megachilidae). Native to North America, O. lignaria has been trapped from wildlands and propagated on-crop and used to pollinate certain fruit, nut, and berry crops. Harnessing the O. lignaria genome assembly, we identified 59,632 candidate microsatellite loci in silico, of which 22 were tested using molecular techniques. Of the 22 loci, 12 loci were in Hardy-Weinberg equilibrium (HWE), demonstrated no linkage disequilibrium (LD), and achieved low genotyping error in two Intermountain North American wild populations in Idaho and Utah, USA. We found no difference in population genetic diversity between the two populations, but there was evidence for low but significant population differentiation. Also, to determine if these markers amplify in other Osmia, we assessed 23 species across the clades apicata, bicornis, emarginata, and ribifloris. Nine loci amplified in three species/subspecies of apicata, 22 loci amplified in 11 species/subspecies of bicornis, 11 loci amplified in seven species/subspecies of emarginata, and 22 loci amplified in two species/subspecies of ribifloris. Further testing is necessary to determine the capacity of these microsatellite loci to characterize genetic diversity and structure under the assumption of HWE and LD for species beyond O. lignaria. These markers will inform the conservation and commercial use of trapped and managed O. lignaria and other Osmia species for both agricultural and nonagricultural systems.

Comprehensive decisions on the management of commercially produced bees, depend largely on associated knowledge of genetic diversity. In this study, we present novel microsatellite markers to support the breeding, management, and conservation of the blue orchard bee, Osmia lignaria Say (Hymenoptera: Megachilidae). Native to North America, O. lignaria has been trapped from wildlands and propagated on-crop and used to pollinate certain fruit, nut, and berry crops. Harnessing the O. lignaria genome assembly, we identified 59,632 candidate microsatellite loci in silico, of which 22 were tested using molecular techniques. Of the 22 loci, 12 loci were in Hardy-Weinberg equilibrium (HWE), demonstrated no linkage disequilibrium (LD), and achieved low genotyping error in two Intermountain North American wild populations in Idaho and Utah, USA. We found no difference in population genetic diversity between the two populations, but there was evidence for low but significant population differentiation. Also, to determine if these markers amplify in other Osmia, we assessed 23 species across the clades apicata, bicornis, emarginata, and ribifloris. Nine loci amplified in three species/ subspecies of apicata, 22 loci amplified in 11 species/subspecies of bicornis, 11 loci amplified in seven species/ subspecies of emarginata, and 22 loci amplified in two species/subspecies of ribifloris. Further testing is necessary to determine the capacity of these microsatellite loci to characterize genetic diversity and structure under the assumption of HWE and LD for species beyond O. lignaria. These markers will inform the conservation and commercial use of trapped and managed O. lignaria and other Osmia species for both agricultural and nonagricultural systems.

The overarching aim of my thesis is to investigate how increased thermal variability and temperature extremes that are associated with ongoing climate change could affect bumble bee health. In laboratory experiments with adult workers of the bumble bee Bombus impatiens, I investigate how these changes in the thermal environment influence immune investment and infection outcomes in the model host-parasite system of the bumble bee and its trypanosome parasite Crithidia bombi. The effect of increased thermal variability on health-related traits could manifest from different sources. The beneficial acclimation hypothesis states that an individual that is acclimated to a certain temperature will have better performance at that temperature relative to an unacclimated individual, because of acclimatized responses. Changes in environmental temperature, independent of direction, could result in situations where organisms are suddenly faced with a thermal environment that they have not acclimated to, potentially influencing their ability to perform physiological processes that could be involved in parasite defense. Additionally, exposure to thermal extremes outside of the organism’s optimum range through more frequent and longer heatwaves could affect immunity and infection outcomes by imposing thermal stress. Therefore, I have two independent but interlinked objectives: 1) to test the beneficial acclimation hypothesis in relation to the resistance of bumble bees to the parasite Crithidia bombi, and 2) to assess how a simulated natural heatwave interacts with bumble bee immunity and influences resistance to infection. I found no support for the beneficial acclimation hypothesis within the range of assayed temperatures, since neither acclimation temperature, the performance temperature under which infection took place, or the interaction between the two affected C. bombi infection. My experiments investigating the thermal stress hypothesis found that bees exposed to heatwave conditions had decreased survival and inducible anti-bacterial activity compared to control conditions. Additionally, I found that bees exposed to a heatwave and subsequently exposed to a live parasite showed increased infection loads. Finally, I found that bees under control thermal conditions showed a classical cost of immune activation (decreased survival), yet bees in heatwave conditions didn’t suffer the same consequences. This result is likely related to the underlying mechanism by which antimicrobial immunity is compromised in heatwave exposed bees. Although relative longevity is reduced in control thermal regime bees upon a non-pathogenic immune challenge, heatwave exposed bees will likely suffer more from elevated infections under natural conditions. Taken together, my work demonstrates that although infection outcomes in bumble bees may be robust to consequences of thermal variability within normal ranges, climate change may directly and indirectly increase threats to bumble bee pollinator populations due to climate extremes such as heatwaves.