Abstract
External immune defense, such as antimicrobial secretions, is not generally viewed as part of the immune system. Nevertheless, it constitutes a first barrier to pathogens and manipulates microbial environments. Hygienic measures ranging from the protection of oneself or conspecifics, the nesting site, to stored food may be more efficient with secreted antimicrobials. However, the relationship between external immune defense and internal immunity, including potential life-history trade-offs, is not well understood. As hymenopteran venom often contains antimicrobial peptides it could serve as an external immune defense. Assuming that antimicrobial venom is costly its production might be traded-off against the internal immune defense. Here we compared the antimicrobial activity of venom and hemolymph in fourteen different bumblebee species according to their life-history strategies and characteristics, i.e. overwintered queens, workers and young queens and cuckoo queens. We found no direct relation between antimicrobial activity of hemolymph and venom. Across all species, hemolymph mainly showed lysozyme-like activity (LLA) whereas venom mainly showed antimicrobial peptide (AMP) activity. While LLA activity in the hemolymph was similar among species and life-history strategies and characteristics, both factors significantly differed in venom AMP activity. Independent of body size or fat body content, young queens showed the highest venom AMP activity, followed by workers, overwintered queens and cuckoo queens. Venom as a potential external immune trait seems not directly linked to internal immunity in bumblebees. However, the investment in external defense depends on the species and the life-history strategies and characteristics of an individual, such as social status or condition.
Highlights
Organisms are constantly exposed to parasites and opportunistic microbial pathogens
Female Groups Overall, the three different types of queens and workers significantly affected the proportion of individuals showing internal or external lysozyme-like activity (LLA) (LME: X2 = 13.304, df = 3, p < 0.01) (Figure 1A), but not antimicrobial peptide (AMP) activity (LME: X2 = 2.760, df = 3, p = 0.43) (Figure 1B)
Species Overall, we found that species significantly varied in the proportion of individuals expressing LLA (LME: X2 = 31.740, df = 13, p < 0.01) (Figures 2A,B) and AMP activity (LME: X2 = 59.715, df = 13, p < 0.001) (Figures 2C,D)
Summary
Organisms are constantly exposed to parasites and opportunistic microbial pathogens. Due to this constant microbial selection pressure, organisms have evolved numerous defense mechanisms, most of which are part of their immune system. External Versus Internal Bumblebee Immunity interaction of humoral and cellular immune reactions exists to minimize the threat (Bulet et al, 1999; Lemaitre and Hoffmann, 2007) These immune responses can be constitutively expressed and/or induced upon recognition of a pathogen or of stress induced by pathogens (Schmid-Hempel, 2011). Antimicrobial peptides (AMP) are induced and expressed in many different tissues (SchmidHempel, 2011) Both AMPs and lysozyme can be found in the fat body, in hemocytes (Cotter et al, 2004), in the midgut, in salivary glands (Hamilton et al, 2011), within reproductive organs (Samakovlis et al, 1991; Lung et al, 2001; Otti et al, 2009), and even on the cuticle (Ashida and Brey, 1995)
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