Abstract
Maternal immune experience acquired during pathogen exposure and passed on to progeny to enhance resistance to infection is called trans-generational immune priming (TgIP). In eusocial insects like honeybees, TgIP would result in a significant improvement of health at individual and colony level. Demonstrated in invertebrates other than honeybees, TgIP has not yet been fully elucidated in terms of intensity and molecular mechanisms underlying this response. Here, we immune-stimulated honeybee queens with Paenibacillus larvae (Pl), a spore-forming bacterium causing American Foulbrood, the most deadly bee brood disease worldwide. Subsequently, offspring of stimulated queens were exposed to spores of Pl and mortality rates were measured to evaluate maternal transfer of immunity. Our data substantiate the existence of TgIP effects in honeybees by direct evaluation of offspring resistance to bacterial infection. A further aspect of this study was to investigate a potential correlation between immune priming responses and prohaemocytes–haemocyte differentiation processes in larvae. The results point out that a priming effect triggers differentiation of prohaemocytes to haemocytes. However, the mechanisms underlying TgIP responses are still elusive and require future investigation.
Highlights
Recent studies involving different species of insects have expanded the knowledge of immunology in invertebrates, and increased the understanding of its limits [1,2,3,4,5,6,7]
Maternal immune experience acquired during pathogen exposure and passed on to progeny to enhance resistance to infection is called trans-generational immune priming (TgIP)
Our data substantiate the existence of TgIP effects in honeybees by direct evaluation of offspring resistance to bacterial infection
Summary
Recent studies involving different species of insects have expanded the knowledge of immunology in invertebrates, and increased the understanding of its limits [1,2,3,4,5,6,7]. We aimed at finding a spore concentration that produces a larval mortality in a range where any positive or negative effect owing to the priming of queens can be evaluated For this purpose, we exposed first instar honeybee larvae to different spore loads of Pl (exposure bioassays) to obtain the mortality rate due to Pl infection at day 12 of the experiment as a function of the spore inoculum per larva (see the electronic supplementary material determination of bacterial virulence and electronic supplementary material, figure S1 for spore doses and related mortality). Parallel to the exposure bioassays, larvae of queens challenged with ringer or Pl of the six colonies in 2012 were artificially reared in the laboratory under regular conditions (no exposure to spores) and were used on day 6 to perform haemocyte counts using a Burker-Turk haemocytometer. Non-parametric methods (Mann – Whitney U-test, Kruskal Wallis one-way ANOVA) were used for statistical analysis
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