Nutrigenomics in honey bees: digital gene expression analysis of pollen's nutritive effects on healthy and varroa-parasitized bees

Abstract

Background: Malnutrition is a major factor affecting animal health, resistance to disease and survival. In honey bees (Apis mellifera), pollen, which is the main dietary source of proteins, amino acids and lipids, is essential to adult bee physiological development while reducing their susceptibility to parasites and pathogens. However, the molecular mechanisms underlying pollen's nutritive impact on honey bee health remained to be determined. For that purpose, we investigated the influence of pollen nutrients on the transcriptome of worker bees parasitized by the mite Varroa destructor, known for suppressing immunity and decreasing lifespan. The 4 experimental groups (control bees without a pollen diet, control bees fed with pollen, varroa-parasitized bees without a pollen diet and varroa-parasitized bees fed with pollen) were analyzed by performing a digital gene expression (DGE) analysis on bee abdomens.

Results: Around 36, 000 unique tags were generated per DGE-tag library, which matched about 8, 000 genes (60% of the genes in the honey bee genome). Comparing the transcriptome of bees fed with pollen and sugar and bees restricted to a sugar diet, we found that pollen activates nutrient-sensing and metabolic pathways. In addition, those nutrients had a positive influence on genes affecting longevity and the production of some antimicrobial peptides. However, varroa parasitism caused the development of viral populations and a decrease in metabolism, specifically by inhibiting protein metabolism essential to bee health. This harmful effect was not reversed by pollen intake.

Conclusions: The DGE-tag profiling methods used in this study proved to be a powerful means for analyzing transcriptome variation related to nutrient intake in honey bees. Ultimately, with such an approach, applying genomics tools to nutrition research, nutrigenomics promises to offer a better understanding of how nutrition influences body homeostasis and may help reduce the susceptibility of bees to (less virulent) pathogens.

Figure 1

Marker genes affected by pollen feeding or varroa parasitism. Expression values of the 4 genes chosen among the set of genes differentially transcribed between the different treatments. RT-PCR data normalized to β-actin are shown. Means ± SE are shown for 8 pools of 3 bees per treatment. Different letters indicate significant differences detected by Mann-Whitney U tests.

Figure 2

Functional (Gene Ontology) analysis of genes affected by pollen feeding (A, B) or varroa parasitism (C, D). The lists of genes regulated by pollen feeding or varroa parasitism were analyzed for statistical enrichment of associated GO terms (P < 0.05), relative to the representation of these terms for all genes analyzed with DGE. Molecular function and biological process that were enriched in upregulated (A, C) and downregulated gene sets (B, D) are shown.

Figure 3

mTOR signaling pathway affected by pollen feeding or varroa parasitism. Genes from the Drosophila mTOR signaling pathway that were differentially transcribed after pollen feeding or varroa parasitism. The pathway was modified from KEGG: Kyoto Encyclopedia of Genes and Genomes http://www.kegg.com/. Grey indicates the presence of genes in the bee genome. Squares and circles indicate genes and enriched functions that were up- and downregulated between two treatments, respectively.

Figure 4

Immune and lifespan genes affected by pollen feeding or varroa parasitism. A) Genes from our gene sets that are involved in lifespan in Drosophila and honey bees (i.e. Vg). B) Immune genes found in honey bees (the list of genes originated from [42]). Colors scale from the heatmap (green to red) indicates log2 transcription ratios. For each gene, accession number and annotation are indicated.