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Principles of Honeybee Genetics
Adapted from a workshop presented by Tom Glenn at the EAS meeting,
Cornell University August 2002
The wonderful thing about genetics is that we find that all life on earth operates under more or less under the same rules. By understanding some basic principles, we can understand how a wide variety of organisms work, from bacteria to elephants. Now that genomes of various plants and animals are starting to be decoded, we are realizing just how closely related we all are. You've probably all heard that people share 99% of their genes with chimpanzees. But did you know that we also share 25% of our genes with bananas. Still, every species is obviously unique and honeybees have their share of odd genetic quirk. It's the quirk that make them what they are.
Probably the one factor that makes bees what they are is the fact that drones are born from unfertilized eggs. On the face of it, it doesn't seem that this should be such a big deal, but we'll see that it really is.
Chromosomes are the structures that contain the genes of an organism. Bees have about 15,000 genes. Most animals normally have two sets of chromosomes. One set comes from the mother and one from the father. They are called diploid. Di means two, and ploid stands for chromosome. In people, we have 46 chromosomes, we get 23 from our mother's egg and 23 from our fathers sperm. Bees have a different number of chromosomes. Females, workers and queens have 32, 16 are contributed by the queen's eggs and 16 come from the drones sperm. Since drones hatch from unfertilized eggs, they only have the 16 chromosomes that were in the egg. Drones are haploid because they only have one set of chromosomes.
You'll notice that the egg can only carry half of the queens 32 chromosomes so she can only pass on half of her genes to her offspring. Each egg contains a unique collection of her genes, so each egg is different. Drones on the other hand only have 16 chromosomes to begin with, so each sperm must contain all the genes of the drone. This means that each sperm from a drone is exactly identical, they are clones. This is different from most other animals, where each sperm is unique, just like each egg is.
The other odd thing about bees is their habit of multiple mating. The queen is quite promiscuous and mates with from 10-20 drones, usually in 1 or 2 mating flights over the course of a couple days. The sperm is stored for years in an organ called the spermatheca. Actually the sperm from one drone is more than enough to fill the spermatheca. So apparently the queen goes out of her way and at great risk to mate with so many drones, just to gather up extra genetic diversity for her colony. This extra diversity is thought to help in providing genetic resistance to diseases. Also since bees tend to specialize in performing certain tasks in the hive, by having a larger diversity of fathers, the colony may perform more efficiently.
The effect of this multiple mating is that the colony is composed of different subfamilies. Each subfamily has the same mother but different fathers. Remember that all the sperm from each drone is an identical clone. This means that the workers inherit 50% of the queens genes, but 100% of the drones genes. The workers that belong to the same subfamily are related by 75%, they are called supersisters.
In almost all other animals, parents and offspring or siblings are related by only 50%, except for identical twins. This may be the key factor why in bees and other social insects the workers have given up reproducing themselves, in favor of helping their queen raise more of their sisters. If a worker had her own offspring, it would only be related to herself by 50%. But by helping the queen raise her supersisters, they are related by 75%. There is a principle in biology that an organism will tend to do whatever most efficiently gets more of it's genes into the next generation. So the simple fact that drones hatch from unfertilized eggs, and therefore his sperm are all identical, goes a long way in explaining the social behavior of bees.
You can see that multiple mating makes for an extremely complex family. In this picture the different colors stand for different genes. I don't expect anyone to sort it all out, I just wanted to give an idea of the complexity involved. In the slides coming up, we'll simplify things by showing the queen mating with just one drone. This probably rarely happens in nature, but we can do it with artificial insemination.
Sex determination
We all know that females come from fertilized eggs and males from unfertilized eggs, right? Now I'm going to throw a monkey wrench in that and talk about sex alleles. Allele is just a word that means a version of a gene. For instance genes for blue eyes and brown eyes are alleles of the eye color gene.
It turns out there is a gene that controls the sex of bees, called a sex allele. There is a simple rule it operates by. If there are two different alleles present, the bee will develop into a female, either a worker or a queen. If there is only one allele present, the bee will develop into a drone. There are two ways that only one allele may be present. The egg may be unfertilized, so that it only contains one allele, in which case it develops into a normal drone. But there is another way for only one allele to be present. That is if an egg is fertilized, but both mother and father happen to contribute the same sex allele. This fertilized egg will also develop into a drone. But this drone will be abnormal because it is diploid, it contains two sets of chromosomes and cannot function as a normal drone. These diploid drones are always destroyed by the workers, who eat them as soon as they hatch.
The effect of this is that there are holes in the brood pattern when the larva is consumed, and the pattern is what is called shot brood. The worst effects are seen in inbreeding. A brother sister mating will produce only 50% viable brood.
It's thought that there are about 19 different sex alleles. There are probably more, but this is the most that has been calculated for a studied population. This is a very clear example of why it is so important to conserve genetic diversity. The more sex alleles we have in our bee population, the more solid the brood patterns will be, and there will be more bees in the hive to collect honey. An analogy would be a game of dice. Every time you role doubles you'd lose. If you had dice with 20 sides instead of 6 sides, it would be much less likely to role doubles. This is why it's a good idea to bring new blood into bees occasionally.
Hygienic behavior
Hygienic behavior is probably the most successful achievement in breeding bees. It's been very well studied and proven to be effective against chalkbrood, American foulbrood, and varroa.
Hygienic behavior is controlled by two recessive genes. One gene allows the worker to detect and uncap a cell that contains diseased brood. The other gene makes the workers remove the brood and discard it. It's possible for a hive to contain one of these genes and not the other, in which case the hive won't be hygienic. It's also possible for different individual bees to have one, but not the other gene.
For a recessive trait to be expressed, a worker needs to be homozygous for the gene. Homozygous means that it gets the same allele from the mother and father. Heterozygous means that the bee has one of the alleles and so is a carrier, but the trait is not expressed.
In this example we're starting with a queen homozygous for the hygienic traits and mating her to non hygienic drones. The offspring will not express the hygienic trait, but they will be heterozygous and so be carriers of the trait. It's important to remember that when you're dealing with recessive traits, it will not show up in the first or F1 generation. But if you have patience and continue with the program, you'll be successful in getting the trait into the following generations.
After a few generations of selecting and breeding from the colonies that express the trait, it can become fixed in the population. Then all the bees in that population will express the trait. We have reached this point with artificial insemination and closed populations.
Resistance to tracheal mites has recently been found to be a grooming behavior. The bees use their middle legs to groom the mites away from their tracheal opening. It's also been found that the trait is controlled by dominant genes. It hasn't been determined if there are more than one gene involved. In this example, we'll assume there's just one gene controlling it. Here we'll say that we're starting with a single drone that carries the resistant gene.
Dominant traits are easier to get established into your population because the first generation will express the trait. The trait will be expressed equally by bees that carry the gene for the trait on one or both of their chromosomes. You would prefer to breed only from queens homozygous for the trait. But there's no easy way to tell the homozygotes from the heterozygotes. For this reason it's actually more difficult to fix a dominant gene in the population, than it is for a recessive trait.
Suppressed mite reproduction SMR, (now renamed VSH, varroa sensitive hygiene) is the latest trait we have to work with. It holds great promise in solving the varroa problem, but it's too early to say exactly how and why it works. There is something about the bees that carry the trait that inhibits the varroa mites from reproducing normally. Some of the mites don't lay any eggs. Others lay eggs too late in the cycle to mature. And some will lay only a male egg. Still other mites get caught between the bee larva cocoon and the cell wall and die before they can lay any eggs. Whether all these are effects are one trait or several is still unknown. Also we don't know how many genes are involved. Fortunately, it's not necessary to know all these details to select for and utilize the trait.
Dr. Harbo and Dr. Harris have selectively bred bees for this trait to the point where few if any of the mites reproduce normally. By crossing these inbred bees to non SMR/VSH bees they found that the effects were intermediate between the two types. This indicates the trait is controlled by neither dominant nor recessive genes, but is what is called additive. This simply means that the more of these genes are present, the more the trait is expressed. Over time, as more drones begin to carry the trait We are trying to figure out just what the optimum level of
SMR /VSH is.
Mitochondrial DNA is used to track the maternal lineage of bees, or any other form of life. Mitochondria are little organelles that are found in every living cell. They are called the powerhouses of life because they are what make respiration possible. They release energy by burning sugar with oxygen. It's a very interesting little critter in that it is believed to have once been a free living bacteria. But a very long time ago it got incorporated into other living cells and has been there in a symbiotic relationship ever since.
The important thing to know about mitochondria is that they reproduce separately from the rest of the cell. When cells divide, the mitochondria divide at the same time. They contain a small amount of DNA , but this DNA remains separate from the nucleus. The mitochondria are present in the eggs when they form. But when the sperm unites with the nucleus at fertilization to create a new genetic combination, the mitochondria remain unchanged. So they get passed along from generation to generation through the eggs without their DNA ever being affected by males. They are passed on only through their mothers, and the DNA in them change only very slowly by occasional mutations. These changes do accumulate so scientists can tell the difference between the mitochondrial DNA of one type of bee from another. This is how African bees are distinguished from European bees.
Adapted from a workshop presented by Tom Glenn at the EAS meeting,Cornell University August 2002
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