Abstract
Sex-related differences in susceptibility to pathogens are a common phenomenon in animals. In the eusocial Hymenoptera the two female castes, workers and queens, are diploid and males are haploid. The haploid susceptibility hypothesis predicts that haploid males are more susceptible to pathogen infections compared to females. Here we test this hypothesis using adult male (drone) and female (worker) honey bees (Apis mellifera), inoculated with the gut endoparasite Nosema ceranae and/or black queen cell virus (BQCV). These pathogens were chosen due to previously reported synergistic interactions between Nosema apis and BQCV. Our data do not support synergistic interactions between N. ceranae and BQCV and also suggest that BQCV has limited effect on both drone and worker health, regardless of the infection level. However, the data clearly show that, despite lower levels of N. ceranae spores in drones than in workers, Nosema-infected drones had both a higher mortality and a lower body mass than non-infected drones, across all treatment groups, while the mortality and body mass of worker bees were largely unaffected by N. ceranae infection, suggesting that drones are more susceptible to this pathogen than workers. In conclusion, the data reveal considerable sex-specific differences in pathogen susceptibility in honey bees and highlight the importance of ultimate measures for determining susceptibility, such as mortality and body quality, rather than mere infection levels.
Highlights
Sex-specific phenotypic differences can influence susceptibility to various stressors encountered in the environment such as pathogens or toxins [1]
Host mortality and body mass Drones inoculated with N. ceranae spores, regardless of coinoculation with black queen cell virus (BQCV), had significantly greater mortality than those not inoculated with Nosema (Kaplan-Meier Log-Rank, both Ps,0.04), whereas worker mortality did not differ significantly among treatment groups (Kaplan-Meier Log-Rank, P = 0.89; Fig. 1)
12 to 16% of the workers died during the experiment, whereas drone mortality rates were much higher at 62.5%, 65% (BQCV), 87.5% (N. ceranae) and 92.5% (N. ceranae & BQCV)
Summary
Sex-specific phenotypic differences can influence susceptibility to various stressors encountered in the environment such as pathogens or toxins [1]. Drones are a critical element of both apicultural breeding and natural colony fitness selection [36] They exhibit substantial differences in behaviour and physiology compared to workers or queens [37] that may influence their susceptibility to pathogens. Drones have been the subject of numerous studies relating to their reproductive and genetic functions [38,39,40,41,42], relatively little is known about the impact and interactions between common honey bee pathogens in drones, and even less about differential disease susceptibility between the honey bee sexes. Nothing is known about sex-specific differences in susceptibility to BQCV between drone and worker bees The aim of these experiments was to assess the susceptibility of workers and drones to N. ceranae and BQCV, as well as their interspecific interaction, using common indices of honey bee health. The experiments were conducted using standardized laboratory hoarding cages, in order to maximize control over the infection process and the environmental conditions
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