D. De Graaf 1, D. Laget 1, M. Brunain 1, L. De Smet 1, E. Danneels 1, B. Rotthier 1, P. Demaeght 1, R.F. Veerkamp 2, E.W. Brascamp 2

1 Ghent University, Ghent, BELGIUM, 2 Wageningen University & Research Animal Breeding And Genomics Centre, Wageningen, THE NETHERLANDS

Abstract:In 2012 we introduced the selection criterion ‘lack of vertical transmission’ in our honeybee breeding program. The trait was estimated by determining the virus load in a sample of 10 eggs. We hypothesized that queens capable of clearing a virus infection in the ovaria, resulting in the deposition of virus-free eggs, will inherit this trait to their progeny leading also to a better control of viral infection at colony level. Egg samples were analyzed for the presence of ABPV, BQCV, DWV and SBV by RT-PCR. After 6 years of breeding efforts, we found a remarkable decrease in the total number of virus-positive samples, which dropped in 2018 for the first time under 10% (7.02%) for DWV, whereas in the early years this was up to 44.12%. For the subgroup of queens that were the descents from virus-negative mother queens, the DWV infection was even lower (6.06%). With a data set of 694 records in hands, representing 2776 virus analyzes, we recently performed a statistical analysis in order to determine the heritability of this trait. One of the models used gave a statistical substantiated heritability h2 of 0.19 ± 0.18. The large estimation error can be expected because of the limited size of the dataset. The heritability value h2 confirms, however, that variation of this new trait can partially be explained by genetic effects. We have identified several pedigrees with 3 consecutive generations of queens with no vertical transmission of viruses and the trait seems to be inherited both from mother and father. Next bee season we will continue our efforts and further investigate what it means in terms of virus load of different tissues of the queen and the spread of the disease in the colony. The discovery that ‘lack of vertical transmission’ is inherited, opens new opportunities for breeding programs all over the world as it will strengthen the resilience of the honeybee colonies and allows to expand our focus in addition to the Varroa-mites, also to the viruses they transmit by vectoring.
  1. DWV and varroa-mite: mutualistic: undermines immunity and then provides transmission of virus
  2. Remedies:
    1. Mites:
      1. Acaridcides
      2. Bio technical methods
      3. Selection for varroa resistance
    2. Virus
      1. Indirect modalities
      2. RNAi attempt
  3. Cause of virus status
    1. Physiological In field
      1. High virus load > low expression level of RNAi machinery
      2. Low virus’s load > high expression level of RNAi machinery
    2. Genetic cause
      1. From 2015 on sanitary control of queens in Flemish breeding program
        1. Samples 10 eggs, taken from drone brood, frozen
        2. Lab analyses of RT PCR for
          1. ABPV Copley
          2. BQCV
          3. DWV
          4. SBV
        3. Participation
          1. Voluntary free
          2. Low number of queens involved in sanitary screening
          3. Virus status of the Queen was long undervalued
        4. Results
          1. Drop from 2015 onward worker versus drone brood - level
          2. only 6% eggs infected by DWV now today
        5. Discussion
          1. Carniolan queens only in the study
          2. Trait re-defined as ‘suppressed in ovovirus infection’ (SOV) rendering eggs free of viral infection
          3. It is heritable solely through the genotype of the Queen - moderate of .22 - .27
          4. It can be expressed against several viruses simultaneously or against each of the viruses individually
          5. low correlation with mite population growth >> so it contributes independently to the resilience of honey bees