Comparative Transcriptome Investigation of Nosema ceranae Infecting Eastern Honey Bee Workers

Abstract

Apis cerana is the original host for Nosema ceranae, a widespread fungal parasite resulting in honey bee nosemosis, which leads to severe losses to the apiculture industry throughout the world. However, knowledge of N. ceranae infecting eastern honey bees is extremely limited. Currently, the mechanism underlying N. ceranae infection is still largely unknown. Based on our previously gained high-quality transcriptome datasets derived from N. ceranae spores (NcCK group), N. ceranae infecting Apis cerana cerana workers at seven days post inoculation (dpi) and 10 dpi (NcT1 and NcT2 groups), comparative transcriptomic investigation was conducted in this work, with a focus on virulence factor-associated differentially expressed genes (DEGs). Microscopic observation showed that the midguts of A. c. cerana workers were effectively infected after inoculation with clean spores of N. ceranae. In total, 1411, 604, and 38 DEGs were identified from NcCK vs. NcT1, NcCK vs. NcT2, and NcT1 vs. NcT2 comparison groups. Venn analysis showed that 10 upregulated genes and nine downregulated ones were shared by the aforementioned comparison groups. The GO category indicated that these DEGs were involved in a series of functional terms relevant to biological process, cellular component, and molecular function such as metabolic process, cell part, and catalytic activity. Additionally, KEGG pathway analysis suggested that the DEGs were engaged in an array of pathways of great importance such as metabolic pathway, glycolysis, and the biosynthesis of secondary metabolites. Furthermore, expression clustering analysis demonstrated that the majority of genes encoding virulence factors such as ricin B lectins and polar tube proteins displayed apparent upregulation, whereas a few virulence factor-associated genes such as hexokinase gene and 6-phosphofructokinase gene presented downregulation during the fungal infection. Finally, the expression trend of 14 DEGs was confirmed by RT-qPCR, validating the reliability of our transcriptome datasets. These results together demonstrated that an overall alteration of the transcriptome of N. ceranae occurred during the infection of A. c. cerana workers, and most of the virulence factor-related genes were induced to activation to promote the fungal invasion. Our findings not only lay a foundation for clarifying the molecular mechanism underlying N. ceranae infection of eastern honey bee workers and microsporidian–host interaction.

Transcriptome of Nosema ceranae and Upregulated Microsporidia Genes during Its Infection of Western Honey Bee (Apis mellifera)

“…Therefore, the upregulation of AAJ76_1600052943 and AAJ76_2000141845 might also play roles similar to those mentioned above to facilitate spore germination and mature spore development. The highly expressed SPW was consistent with our previous study on N. ceranae gene expression in A. mellifera [ 33 ] and the research on the N. ceranae gene expression in A. cerana, suggesting a significant role for SPW during N. ceranae infection [ 53 ].…”

“…The results indicated that a greater upregulation of N. ceranae’s common and stage-specific genes was found in the initial infectious stage, whereas more genes of N. ceranae were suppressed in the late stage of infection. This tendency was similar to the study of N. ceranae gene expression in Apis cerana, suggesting that the N. ceranae genes were expressed at an early infection stage, followed by the gene expression profile of N. ceranae switching from the merogony to sporogony stages [ 53 ]. Microsporidia lack mitochondria, and the loss of most metabolic pathways might involve an alteration of gene expression during the course of infection; therefore, host-generated ATP would be imported to support their metabolism [ 54 , 55 ].…”