by William C. Roberts and Otto Mackensen
U.S.D.A., Agr. Res. Adm., Bureau of Entomology and Plant Quarantine*
(*In cooperation with the Wisconsin Agricultural Experiment Station and Louisiana State University.)
III. Sex Determination and Bee Breeding
EVERY beekeeper knows that the economically productive colony has a large population of worker bees. In order to provide this force the queen must lay a large number of fertile eggs, and they must hatch and be nurtured to develop into adults.
Many years ago Dzierzon discovered that male bees develop from unfertilized eggs. Drones have a mother but no father. This is known as parthenogenesis. It is not peculiar to bees, for many other animals have this method of reproduction. The female bees, workers and queens, develop from fertilized eggs. They have both father and mother. From each parent they receive 16 chromosomes and so have 16 pairs of chromosomes. The drone, who receives all his inheritance from his mother, has only 16 single chromosomes. Having paired chromosomes, the workers and queens are called diploid individuals. Since the chromosomes of drones are not paired, they are called haplold individuals.
Recent work indicates that the fundamental determiner of sex in bees is not the number of chromosomes or whether or not the egg is fertilized. Sex is determined by the action of certain genes at one locus on one pair of chromosomes. In the wasp Habrobracon, a relative of the honey bee, the females are diploid and the males are normally haploid, but under certain genetic conditions diploid males have been produced. No diploid males have been discovered in honey bees, but there is experimental evidence that sex determination in honey bees is similar to that in Habrobracon.
The position of a gene on a chromosome is called a locus. On one of the chromosomes there is a locus that is called the X, or sex-determining, locus. At this locus in honey bees there is a series of multiple allelic genes - instead of only two alleles, such as capital W and small w, which are alleles to each other, there are many alleles. We will call this the X locus and the alleles Xa, Xb, Xc, etc. To simplify matters we can drop the X and use only the letters a, b, c, etc., remembering always that these are the X alleles.
According to the theory of sex determination in bees, females develop from fertilized eggs that are heterozygous at the X locus. These eggs have two unlike alleles, such as ac, ad, bc, bd, etc. Any fertilized egg that happens to be homozygous at this locus would be a drone if it developed. However, it does not develop but dies instead. Thus all fertilized eggs that are homozygous - that is, having two sex alleles alike, such as aa, bb, cc, etc. - are lethal and do not hatch. Drones develop from unfertilized eggs and are haploid, having only one of these alleles - a or b or c or d, etc.
If a queen with sex alleles ab is mated to a drone having sex allele c, the fertilized eggs from this queen will be ac and be. Since these eggs are heterozygous, their hatchability is near 100 per cent. An ab queen mated to an a or b drone has fertilized eggs that are ab and aa. Since the aa eggs do not hatch, the viability of fertilized eggs from this queen will be only 50 per cent.
It can thus be seen that egg hatchability and the brood quality of a queen are determined by the sex alleles of the queen and the drone or drones that mate with this queen. To illustrate this point let us set up a breeding example. A beekeeper chooses a breeder queen from which he will raise a number of daughter queens. This queen has high-quality brood because she mated with a drone or drones whose sex alleles were different from hers. Since the queen is diploid and heterozygous for the sex-determining locus, we will assume that she is ab. Having good-quality brood, we know that she mated with a drone or drones that were not a or b. Let us assume that she mated with two drones, one c and one d. Fertilized eggs from this queen will be ac, ad, bc, and bd. Notice that all are heterozygous for the X locus, an indication of high hatchability and good-quality brood. Let us further assume that these queen daughters ac, ad, bc, and bd are allowed to mate with drones from another unrelated queen, which we will designate as yz, and that each queen mates with two drones - that is, drones y and z, y and y, or z and z. Their fertilized eggs will be either ay, cy, by, and dy or az, cz, bz, and dz. Consequently the brood quality will be good because the queens and drones have different sex alleles.
Let us assume that the next year the beekeeper raises queen daughters from one of these queens and drones from one of the other queens and allows them to mate, together. These are called matings of first cousins. The sex alleles of the daughters of ac x yz will be az, cz, ay, and cy in equal proportions, if the queen is ac and mated to one y and one z drone and received equal amounts of sperm from each.
Assume that the queen selected to produce the drones for mating with these virgins had sex alleles ad and that she was mated to z and y drones. Since drones develop from unfertilized eggs, all drones from an ad queen will be either a or d. If a large number of drones are produced, it is likely that half of them will be a and the other half d. If each virgin - az, cz, ay, or cy - mates with two drones, matings will be to drones a and d, or a and a, or d and d.
The queen az that mated with one a and one d drone will produce the following types of fertilized eggs with equal frequency: aa, ad, za, and zd. Since ad, za, and zd are heterozygous, they hatch, but the homozygous aa fertilized egg dies. Thus one fourth of the fertilized eggs produced by this queen fail to develop. Consequently she has low-quality brood and does not develop a colony with a large population of worker bees. This queen would have produced high-quality brood and a large colony if she had mated with two d drones instead of one a and one d, but only 50 per cent of her eggs would have hatched if she had mated with two a drones.
It is quite evident that the productivity of a colony or of a queen depends to a great extent on the sex alleles represented in the queen and in the drones that mated with the queen. Three sister queens can be identical genetically, and yet if each queen is mated to two drones of a group of brother drones, their egg hatchability can be 50, 75, or 100 per cent, depending upon the sex alleles in these drones. Other things being equal, if only 50 per cent of the fertilized eggs of a queen hatch, the colony is very unproductive. On the other hand, if the eggs are 100 per cent hatchable, very populous colonies will result. If these queens should mate only once, there would be only 50 per cent and 100 per cent hatchability of fertilized eggs from the several queens, with no 75 per cent hatchability matings.
What then can be expected of a breed-improvement program attempted by a queen breeder? Let us assume that from his own or selected stocks he chooses three mated (tested) queens as the source of all future breeding stock. He will try to improve his stock by breeding the best to the best. He will control matings by isolation so that they will be between progenies of these queens. It is assumed that each of these queens has different sex alleles. If each queen is mated to two drones and all these drones have sex alleles different from any of those in the three selected queens, there will be four different sex alleles in the progeny of each breeder, and a total of 12 alleles for all breeders. These sex alleles are given the following distribution:
U.S.D.A., Agr. Res. Adm., Bureau of Entomology and Plant Quarantine*
(*In cooperation with the Wisconsin Agricultural Experiment Station and Louisiana State University.)
III. Sex Determination and Bee Breeding
EVERY beekeeper knows that the economically productive colony has a large population of worker bees. In order to provide this force the queen must lay a large number of fertile eggs, and they must hatch and be nurtured to develop into adults.
Many years ago Dzierzon discovered that male bees develop from unfertilized eggs. Drones have a mother but no father. This is known as parthenogenesis. It is not peculiar to bees, for many other animals have this method of reproduction. The female bees, workers and queens, develop from fertilized eggs. They have both father and mother. From each parent they receive 16 chromosomes and so have 16 pairs of chromosomes. The drone, who receives all his inheritance from his mother, has only 16 single chromosomes. Having paired chromosomes, the workers and queens are called diploid individuals. Since the chromosomes of drones are not paired, they are called haplold individuals.
Recent work indicates that the fundamental determiner of sex in bees is not the number of chromosomes or whether or not the egg is fertilized. Sex is determined by the action of certain genes at one locus on one pair of chromosomes. In the wasp Habrobracon, a relative of the honey bee, the females are diploid and the males are normally haploid, but under certain genetic conditions diploid males have been produced. No diploid males have been discovered in honey bees, but there is experimental evidence that sex determination in honey bees is similar to that in Habrobracon.
The position of a gene on a chromosome is called a locus. On one of the chromosomes there is a locus that is called the X, or sex-determining, locus. At this locus in honey bees there is a series of multiple allelic genes - instead of only two alleles, such as capital W and small w, which are alleles to each other, there are many alleles. We will call this the X locus and the alleles Xa, Xb, Xc, etc. To simplify matters we can drop the X and use only the letters a, b, c, etc., remembering always that these are the X alleles.
According to the theory of sex determination in bees, females develop from fertilized eggs that are heterozygous at the X locus. These eggs have two unlike alleles, such as ac, ad, bc, bd, etc. Any fertilized egg that happens to be homozygous at this locus would be a drone if it developed. However, it does not develop but dies instead. Thus all fertilized eggs that are homozygous - that is, having two sex alleles alike, such as aa, bb, cc, etc. - are lethal and do not hatch. Drones develop from unfertilized eggs and are haploid, having only one of these alleles - a or b or c or d, etc.
If a queen with sex alleles ab is mated to a drone having sex allele c, the fertilized eggs from this queen will be ac and be. Since these eggs are heterozygous, their hatchability is near 100 per cent. An ab queen mated to an a or b drone has fertilized eggs that are ab and aa. Since the aa eggs do not hatch, the viability of fertilized eggs from this queen will be only 50 per cent.
It can thus be seen that egg hatchability and the brood quality of a queen are determined by the sex alleles of the queen and the drone or drones that mate with this queen. To illustrate this point let us set up a breeding example. A beekeeper chooses a breeder queen from which he will raise a number of daughter queens. This queen has high-quality brood because she mated with a drone or drones whose sex alleles were different from hers. Since the queen is diploid and heterozygous for the sex-determining locus, we will assume that she is ab. Having good-quality brood, we know that she mated with a drone or drones that were not a or b. Let us assume that she mated with two drones, one c and one d. Fertilized eggs from this queen will be ac, ad, bc, and bd. Notice that all are heterozygous for the X locus, an indication of high hatchability and good-quality brood. Let us further assume that these queen daughters ac, ad, bc, and bd are allowed to mate with drones from another unrelated queen, which we will designate as yz, and that each queen mates with two drones - that is, drones y and z, y and y, or z and z. Their fertilized eggs will be either ay, cy, by, and dy or az, cz, bz, and dz. Consequently the brood quality will be good because the queens and drones have different sex alleles.
Let us assume that the next year the beekeeper raises queen daughters from one of these queens and drones from one of the other queens and allows them to mate, together. These are called matings of first cousins. The sex alleles of the daughters of ac x yz will be az, cz, ay, and cy in equal proportions, if the queen is ac and mated to one y and one z drone and received equal amounts of sperm from each.
Assume that the queen selected to produce the drones for mating with these virgins had sex alleles ad and that she was mated to z and y drones. Since drones develop from unfertilized eggs, all drones from an ad queen will be either a or d. If a large number of drones are produced, it is likely that half of them will be a and the other half d. If each virgin - az, cz, ay, or cy - mates with two drones, matings will be to drones a and d, or a and a, or d and d.
The queen az that mated with one a and one d drone will produce the following types of fertilized eggs with equal frequency: aa, ad, za, and zd. Since ad, za, and zd are heterozygous, they hatch, but the homozygous aa fertilized egg dies. Thus one fourth of the fertilized eggs produced by this queen fail to develop. Consequently she has low-quality brood and does not develop a colony with a large population of worker bees. This queen would have produced high-quality brood and a large colony if she had mated with two d drones instead of one a and one d, but only 50 per cent of her eggs would have hatched if she had mated with two a drones.
It is quite evident that the productivity of a colony or of a queen depends to a great extent on the sex alleles represented in the queen and in the drones that mated with the queen. Three sister queens can be identical genetically, and yet if each queen is mated to two drones of a group of brother drones, their egg hatchability can be 50, 75, or 100 per cent, depending upon the sex alleles in these drones. Other things being equal, if only 50 per cent of the fertilized eggs of a queen hatch, the colony is very unproductive. On the other hand, if the eggs are 100 per cent hatchable, very populous colonies will result. If these queens should mate only once, there would be only 50 per cent and 100 per cent hatchability of fertilized eggs from the several queens, with no 75 per cent hatchability matings.
What then can be expected of a breed-improvement program attempted by a queen breeder? Let us assume that from his own or selected stocks he chooses three mated (tested) queens as the source of all future breeding stock. He will try to improve his stock by breeding the best to the best. He will control matings by isolation so that they will be between progenies of these queens. It is assumed that each of these queens has different sex alleles. If each queen is mated to two drones and all these drones have sex alleles different from any of those in the three selected queens, there will be four different sex alleles in the progeny of each breeder, and a total of 12 alleles for all breeders. These sex alleles are given the following distribution:
Queen No. | Queen | Drones Matedto Queens | Daughter Queens |