https://pub.epsilon.slu.se/16331/1/thaduri_s_190912.pdf

Abstract
The ectoparasitic mite, Varroa destructor, together with its associated
viruses is the most significant threat to honeybee (Apis mellifera) health
world-wide. Since the introduction of varroa to the new host species, the
European honey bee (Apis mellifera), it has been responsible for the near
complete eradication of wild and feral honeybee populations in Europe and
North America. However, a unique honeybee population on the island of
Gotland, Sweden, has acquired resistance to the mite through a natural
selection
process. A recent study also showed that Gotland mite-resistant
population might
have adapted tolerance and resistance to virus infections. This suggests
that virus-host interactions may play a key role in the long-term survival
of this population. The aim of this thesis was to investigate the role of
the viral and bacterial microbiome in the enhanced survival of the mite
-resistant (MR) honeybees on Gotland, to compare the role of virus
tolerance and resistance in other naturally selected mite-resistant honeybee
populations, similar to the Gotland population, and to unravel individual level
virus-host interactions in honeybees. First, by using a combination of
high-throughput
sequencing and different bioinformatics tools we found Lake Sinai virus and
Apis rhabdovirus-1, including previously known honey bee viruses, in
Swedish honey bees. Further molecular studies showed that Gotland MR bees
have developed a colony-level resistance to these viruses, and
tolerance to Deformed
Wing Virus (DWV), the virus most commonly associated with mite infestation.
Secondly, differences in the bacterial microbiome between MR and
mite-susceptible
(MS) bees were studied using the 16S rDNA, but the results indicated little
differences between MR and MS bees throughout the season. Finally,
individual level susceptibility of MR and MS honey bees to oral virus
infection was tested for DWV virus and Acute bee paralysis virus (ABPV).
The results demonstrate that DWV and ABPV infection dynamics were nearly
identical in MR and MS bees, but that bees from the MR honeybee populations
had significantly lower mortality rates than bees from the MS population.In
conclusion, the results of this thesis present strong evidence that
naturally adapted mite-resistant honeybees have also adapted, through a
natural selection process, tolerance and resistance to virus infections at
both the colony and individual level. The bacterial microbiome did not
appear to play a role in the enhanced survival of Swedish mite-resistant
honeybees but more studies are required to investigate potential bacteria-virus
interactions on honeybee health. Future work should aim to identify key
genomic regions associated with virus resistance and tolerance that can be
incorporated into honeybee breeding programs to improve honeybee health.

Keywords:Apis mellifera, Varroa destructor, viral metagenomics, deformed
wing virus, acute bee paralysis virus, virus-host interactions Author’s
address: Srinivas Thaduri, SLU, Department of Ecology, P.O. Box ,7044, 750
07 Uppsala, Sweden. Email: [log in to unmask]

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