Abstract
There is increasing evidence that honeybees (Apis mellifera L.) can adapt naturally to survive Varroa destructor, the primary cause of colony mortality world-wide. Most of the adaptive traits of naturally varroa-surviving honeybees concern varroa reproduction. Here we investigate whether factors in the honeybee metagenome also contribute to this survival. The quantitative and qualitative composition of the bacterial and viral metagenome fluctuated greatly during the active season, but with little overall difference between varroa-surviving and varroa-susceptible colonies. The main exceptions were Bartonella apis and sacbrood virus, particularly during early spring and autumn. Bombella apis was also strongly associated with early and late season, though equally for all colonies. All three affect colony protein management and metabolism. Lake Sinai virus was more abundant in varroa-surviving colonies during the summer. Lake Sinai virus and deformed wing virus also showed a tendency towards seasonal genetic change, but without any distinction between varroa-surviving and varroa-susceptible colonies. Whether the changes in these taxa contribute to survival or reflect demographic differences between the colonies (or both) remains unclear.
Highlights
There is increasing evidence that honeybees (Apis mellifera L.) can adapt naturally to survive Varroa destructor, the primary cause of colony mortality world-wide
We tracked the seasonal fluctuations in the quantitative and qualitative composition of the bacterial and viral metagenome in 14-day old adult bees from twelve colonies headed by 1-year old queens from the naturally varroa-surviving (MR) honeybee population on Gotland and from a local varroa-susceptible (MS) honeybee population
The bacterial community structure of the adult bees was determined through mass parallel 16S rDNA amplicon sequencing while the viral composition was determined through quantitative reverse transcription quantitative PCR (RT-qPCR) and RNA sequencing of the major bee viruses circulating in the Swedish honeybee p opulations[13,14]
Summary
There is increasing evidence that honeybees (Apis mellifera L.) can adapt naturally to survive Varroa destructor, the primary cause of colony mortality world-wide. Research has shown that these adaptive traits are linked to the bee population rather than the mite population[15]; that they can be inherited[16] and selected for[8,12] Some of these traits have been incorporated into honeybee breeding programmes[4,12] in order to reduce both the impact of varroa, and the beekeeping world’s dependency on chemical treatments[6]. The logical questions were : (1) if this was a singular occurrence or a consistent feature of the varroa-surviving population, and (2) if there were other components of the honeybee microbiome that were affected by, or contributing to, the natural varroa resilience phenotype developed by this isolated bee population on Gotland. The study focuses on the bacterial and viral metagenomes, with the analysis of the other microbial taxa (fungi, protists and other eukaryotic symbionts) deferred to future studies
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