Sex and inbreeding (in bees).

By Peter Dearden 24/06/2014 1

Peter K Dearden

Tomorrow I am speaking at the National Bee Keepers Association conference in Whanganui and thought I might write a bit about what we have been doing to help me get things clear.

Much of my research work is on bees; trying to learn how they work, trying to find new ways to protect them and, occasionally doing research to help the beekeeping industry.

Beekeeping is a reasonably large business in New Zealand, making over $100 Million per annum in bee-related exports. More importantly, it is estimated that Bees bring $5.1 Billion each year to the New Zealand economy through pollination. Bees are a vital part of our primary production sector and we need to care about them.

Bees in New Zealand are undergoing, or have had, a kind of population crash. This is due to the incursion of the Varroa mite, a small mite that reduces the viability of the hive by attacking brood (young) and carrying viruses between bees. Without management, honeybee hives with Varroa will die. All those feral hives, which have lived for years in old trees and in town belts have died, or are dying. Managed beehive numbers are increasing, but sadly all those feral bees populations have gone.

This is a big issue for pollination, as feral bees probably played a significant role in many areas, particularly in clover pollination. This population crash also has implications for bee genetics. As the population of a species decreases, inbreeding is more likely, and inbreeding is particularly damaging to bees.

Bees are odd. They are weird animals in all kinds of ways, but the weirdness that is important here is to do with gender. Bees come in three forms; Queens and workers, both female, and Drones, male. Females come from fertilised eggs; they have two copies of each of their 16 chromosomes and are thus diploid. Males come from unfertilised eggs, they have one copy of their genome and are haploid. An early honeybee embryo needs to decide if it is male or female by working out if it is haploid or diploid. To do this, it counts. It counts the number of different versions it has of one single gene in the genome, a gene called ‘complementary sex determiner’ or csd. We call the variants of a gene ‘alleles’. If an organism is diploid, (it has two copies of its genome in each cell) it can have up to two versions of each gene in its genome – two alleles. If it is haploid it can only have one. So your honeybee embryo will count, through an as yet unknown process, the number of alleles of csd it has. If it finds two, it turns female, if it finds one it turns male. Simple right!

The problems occur if the bee embryo is diploid, but both copies of its csd gene are identical. If this is the case it can only count one allele, and it inappropriately turns male. Diploid males are killed by worker bees.

So honeybee health is linked to the number of alleles of csd present in the honeybee population. If you have lots of alleles, lots of variation, then the chances of any diploid bee getting two alleles that are the same is low. If genetic variation at the csd gene is low, then the chance of a diploid bee having two alleles the same is high. In this case, the workers kill the brood and the hive becomes much less productive.

In New Zealand we have had limited importation of bees. All honeybees have been imported into New Zealand, often in difficult circumstances. We also now have all our feral bees being killed by Varroa. As the population of bees decreases, so does genetic variation, so do the number of csd alleles in the population. Is it possible that our decreasing bee numbers is a threat to their unusual sex-determining system?

To address this question my lab, funded by AgMARDT  developed a genetic test for csd alleles, and for the last year we have been screening alleles from beekeepers around the country, funded by the fantastic Sustainable Farming Fund of MPI.

So what have we found? We have screened over 2000 bee samples from around the country and found, so far, 72 csd alleles. This is a relief. New Zealand has enough csd diversity without importing more bees, to deal with the Varroa challenge. Sometimes science gives you the answer!

Our work has also shown that there are some bee operations, often with selective breeding programmes, that need to worry about their number of csd alleles. We hope our work will enable us to help them maintain diversity. Our test is now commercially available (through the awesone Dnature ) which should allow beekeepers to monitor their own diversity if they are worried.

So we have learnt that we need to manage csd variation, but there is no crisis in New Zealand. Good news! I have also learnt how rewarding working with industry can be. I’d like to thank all those beekeepers that have helped us out. It is a privilege to work with such a well-informed and proactive group of people.

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