Changes in Antioxidant Enzymes Activity and Metabolomic Profiles in the Guts of Honey Bee (Apis mellifera) Larvae Infected with Ascosphaera apis.

Mengshang Hou,Hongyi Nie,Bian Zhao,Yuanmei Qiu,Zhiguo Li,Songkun Su

doi:10.3390/insects11070419

Abstract

The fungus Ascosphaera apis, an obligate fungal pathogen of honey bee brood, causes chalkbrood disease in honey bee larvae worldwide. Biological characteristics of the fungal pathogen and the molecular interactions between A. apis and honey bees have been studied extensively. However, little is known about the effects of A. apis infection on antioxidant enzyme activities and metabolic profiles of the gut of honey bee larvae. In this study, sandwich enzyme-linked immunosorbent assay and LC-MS based untargeted metabolomic analysis were employed to determine the changes in the specific activities of antioxidant enzymes and the metabolomic profiles in gut tissues of A. apis-infected larvae (105 A. apis spores per larva) and controls. Results showed that specific activities of superoxide dismutase, catalase and glutathione S-transferase were significantly higher in the guts of the control larvae than in the guts of the A. apis-infected larvae. The metabolomic data revealed that levels of 28 and 52 metabolites were significantly higher and lower, respectively, in the guts of A. apis-infected larvae than in the guts of control larvae. The 5-oxo-ETE level in the infected larvae was two times higher than that in the control larvae. Elevated 5-oxo-ETE levels may act as a potential metabolic biomarker for chalkbrood disease diagnosis, suggesting that A. apis infection induced obvious oxidative stress in the honey bee larvae. The levels of metabolites such as taurine, docosahexaenoic acid, and L-carnitine involved in combating oxidative stress were significantly decreased in the gut of A. apis-infected larvae. Overall, our results suggest that A. apis infection may compromise the ability of infected larvae to cope with oxidative stress, providing new insight into changing patterns of physiological responses to A. apis infection in honey bee larvae by concurrent use of conventional biochemical assays and untargeted metabolomics.

Highlights

As one of the most important managed pollinators, the Western honey bee (Apis mellifera) is responsible for the pollination of crops worth $215 billion worldwide [1]
Most of the A. apis-infected larvae developed typical chalkbrood symptoms, and the percentage of 9 day-old pupae covered with white, cotton-like mycelium was 76% (n = 130)
Transcriptional profiling of honey bee larvae infected with A. apis revealed that the expression levels of vitellogenin and hexamerins are significantly down-regulated in A. apis-infected larvae [10], and the activities of both were positively correlated with antioxidant capacity in honey bees [11]

Summary

Introduction

As one of the most important managed pollinators, the Western honey bee (Apis mellifera) is responsible for the pollination of crops worth $215 billion worldwide [1]. The different threats to honey bee health caused by abiotic and biotic stresses have always been a major concern, and the combinations of parasites, pathogens, pesticides, and habitat loss could contribute to honey bee colony losses [1,2]. The fungus Ascosphaera apis, an obligate fungal pathogen of honey bee brood, causes chalkbrood disease in honey bee larvae [3]. Transcriptomic studies of A. apis indicated that fungal transcripts encoding chitinases may contribute to the penetration of the larval gut during host invasion by A. apis [6]. A. apis infection acts as a direct disease stressor causing chalkbrood in honey bees, and interacts with other biotic and abiotic stressors. Worker honey bees from chalkbrood-infected colonies exhibit significantly elevated deformed wing virus (DWV) viral load [7]. More severe symptoms can be found in A. apis-infected larvae, because the virulence of A. apis is likely to increase due to chilling stress [9]

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Conclusion