Interim Report: Sustainable Farming Fund Grant: Honeybee
genetics for sustainability and pollination security.
Associate Professor Peter K. Dearden, University of Otago.
peter.dearden@otago.ac.nz
Executive Summary.
The genetic diversity of the sex-locus in bees (csd) is critical to sustainability of
bee populations. We have surveyed csd alleles across New Zealand looking at
diversity.
Key finding and outcomes are:
1) New Zealand has considerable diversity in csd, and will be able to maintain
sustainable bee populations by managing the diversity we already have.
2) In specific beekeeping operations, csd diversity is very low. This needs to be
managed at the operation level.
3) Purchasing selectively-bred closed-population bees from breeders is a very
effective way to spread improved genetics through a population. This comes at
the cost of reduced csd diversity. This needs to be managed, we suggest by the
bee breeders rather than their customers
4) csd testing and management will be a useful tool, especially for bee breeding.
This technology is now available commercially.
We are now in a position to begin feeding back the information on csd alleles to
the beekeeping industry, and would value your feedback on how best to do that.
Background
This project aims to support both beekeeping and pollination-requiring
industries in the aftermath of the (near) nation-wide spread of Varroa mite. The
project has two key goals: the first to determine if bee populations in NZ are
genetically sustainable; the second to determine if pollination traits can be
selected in bees to improve the quality & value of pollination.
This interim report focuses only on the objective in this project to monitor
genetic diversity at the sex locus in New Zealand bees.
In the past few years the Varroa mite has been spreading through NZ. This pest
destroys beehives that are not carefully managed with miticides. This invasion
has had a profound effect on bees. It has effectively turned them from a wild
species that can live without human intervention, to a domestic one, requiring
our input to survive. Varroa mite has had an important impact on beekeepers,
increasing their costs. But its more devastating impact has been on wild or feral
bee populations, which have collapsed and almost completely vanished.
This loss of our feral bee population (considered to have outnumbered managed
bees by some considerable extent) will inevitably have significantly reduced the
genetic diversity of NZ bees. Compounding this dilemma is the increasing trend
for beekeepers to purchase livestock from a limited number of bee breeders.
Both these factors will reduce the overall genetic diversity of NZ bees, reducing
the effective population size of the NZ bee population and their overall resilience
to new diseases and man-made threats. Bringing in overseas genetics as a
potential solution is not a viable option for beekeepers because of the very real
risk of the importation of new pests and viruses (with even bee semen able to
contain viruses).
Small population size increases the risk of inbreeding within any species;
however, because of a quirk of honeybee genetics, inbreeding in honeybees is
particularly serious. Honeybee sex is determined at a single locus called csd. If
a bee is heterozygous at csd it is female, if it is hemizygous, or homozygous, it is
male. Honeybees control sex by laying un-fertilized eggs, which are hemizygous
and male, and fertilised eggs that are female. If the population is inbred, the
proportion of bees homozygous at csd increases, making diploid males, which
are killed by the hive. Therefore as inbreeding increases, survival decreases.
Because of this unusually strong effect of inbreeding it is important that we
survey the NZ bee population for their sex alleles to determine the seriousness /
extent of any inbreeding problem, and to help plan breeding strategies that
introduce greater diversity for the future sustainability of our bee industry and
pollination capacity.
Methods.
We have collected drone samples from around New Zealand (see figure 1) and
assayed their csd alleles. We used a technique developed in the lab that requires
the use of Drone bees to accurately define different alleles. Each new allele we
have discovered has been checked by sequencing, and entered into our csd allele
database, and tracked in our other samples. We have also compared the csd
sequences we have found in NZ bees, with those from the rest of the world.
During the course of the work we have also developed a new method of
identifying csd alleles using high throughput sequencing. This technology allows
us to examine the csd alleles within workers, allowing us to identify not only the
two csd alleles in the queen in each hive, but also the csd alleles of the drones she
has mated with. We have used this technique for a specific side project, and will
be publishing the method in the near future.
Results.
We have identified 71 alleles of csd in bees nationwide (screeing over 3000
drones. Figure 1 shows our sampling locations and number of samples collected.
We are still extending this survey, particularly using our new high-throughput
Figure 1: Sampling location and depth of Drone survey
sequencing technique, to fill in areas of low coverage, and to identify alleles more
efficiently.
Our population modelling indicates that 71 alleles is more than enough to keep
the NZ population sustainable. While we have (weak) evidence that this is a
decrease in the number of alleles since the arrival of Varroa mite, this number of
alleles is sustainable.
Two issues are, however, more worrying. The first is that in particular
beekeeping operations, especially those carrying out their own queen breeding,
csd diversity is very low. Those beekeepers that keep relatively closed
populations, and breed within their stock, tend to have low csd diversity, and risk
low brood viability. Csd diversity within these populations must be managed to
ensure the health of these populations.
The second problem is that we do not know the rate at which we are loosing csd
alleles. Given no importation of bees, and the rate of evolution of this gene being
fast, but fast enough to generate lots of alleles on a human time-scale, we are
likely losing alleles. The rate of this loss is unknown.
csd diversity in managed breeding.
As part of this study, we examined, using our new high-throughput technique,
the csd alleles of a large commercial beekeeping operation, which 5 years ago,
began to import queens from a single closed-population queen breeder.
We identified, in this survey, 45 alleles (most previously found in other parts of
the country) in this one, large operation. This is more than enough diversity to
ensure the future health of this population. Worryingly, however, the most
common alleles, by far, were those from the closed population they were
importing bees from. As this analysis is only a snapshot of diversity, we cannot
be sure how this situation has arisen. It is compelling to think, however, that
because of the importation of bees, the population of this operation is changing,
becoming similar to the closed population and reducing the diversity of csd
alleles.
The benefits of selective breeding in bees are clear, with genetic gains improving
production. These benefits come at the cost of genetic diversity, particularly at
the csd gene. This must be managed for these breeding programme to be
sustainable long-term, and for the beekeepers using those stocks to maintain
productivity. We are delighted that one closed population breeder is developing
plans to manage csd allele diversity in their populations.
Outcomes and implications.
Our work has a number of key outcomes and implications for the beekeeping
industry.
1) csd allele diversity across New Zealand is high. We do not need to import bees
to maintain or increase the number of csd alleles. Management of the alleles we
have will provide all the diversity we need to have a genetically sustainable
population in New Zealand. We do not, however, know if this number if alleles is
decreasing, and if so, at what rate?
2) csd allele diversity in particular beekeeping operations is low, and needs to be
managed. We aim to feed-back information to beekeepers about their alleles and
what might be done to improve their allele diversity. In many cases we may be
able to provide information as to where to get bee stocks with new alleles from.
We need some advice as to how best achieve this.
3) We have shown that purchasing breeders from closed-population, selective
breeding operation is a very effective way to spread the improved genetics from
these breeders into a population. This comes, however, at the cost of a reduction
in diversity in csd alleles. This needs to be managed, and we suggest it is best
managed by the bee breeders themselves.
4) csd information and testing will be a very useful tool for managing selective
breeding. A commercial DNA testing laboratory, DNature diagnostics limited
(Gisborne), has trialled and is offering the test on a commercial basis.
Associate Professor Peter K. Dearden
Director of Genetics Otago
6 December 2013
peter.dearden@otago.ac.nz
03 479 7832