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.)
II. Heredity and Variation
HEREDITY is concerned with the transmission of characters from parent to offspring. Animals of one species resemble each other, and this resemblance is due to heredity. Variations between individuals within any given species may be due to heredity or environment or both.
The body of the honey bee, like that of all the higher animals, is made up of many cells. Each cell contains a sort of cell within a cell, which is called a nucleus. Within the nucleus are the many genes, which are the hereditary determiners of form and function of all the parts and organs of the body.
Let us begin with the unfertilized egg. The egg is one cell having one nucleus and one set of genes. The genes are arranged in the nucleus in definite linear order on a series of string-like structures called chromosomes. The position of each gene on a chromosome is called its locus. A sperm cell also contains a nucleus with a single set of chromosomes and genes.
In fertilization the sperm enters the egg and its nucleus unites with the nucleus of the egg. Thus the fertilized egg nucleus has two sets of chromosomes and hereditary determiners.
In the development of an individual from the fertilized egg, the egg cell first divides into two cells and then the two daughter cells divide again. This process continues until the adult consists of millions of cells. Each time a cell divides each gene duplicates itself and all the chromosomes split longitudinally into two identical halves. These halves pass to opposite sides of the nucleus then divide into two. The cell then divides and each daughter cell gets one of these genetically identical nuclei. Each cell grows to the original size of its mother cell and the process of division is repeated. As growth proceeds there is an interaction between cells that determines the form and function of the body parts.
In the honey bee the queens and workers develop from fertilized eggs in this manner. Therefore all the cells, in their bodies contain two sets of chromosomes and genes and are said to be diploid. The drone, however, develops from an unfertilized egg, has only one set of chromosomes and genes in all the cells of its body and is said to be haploid.
During development reproductive organs are formed in which certain cells are set aside for the production of germ cells, sperms or eggs. In the formation of the egg a cell division occurs in which the chromosome behavior is different from that in the multiplication of body cells. As a result of this division the egg has only one member of each pair of chromosomes, or one set of chromosomes and genes as mentioned at the beginning of this discussion. This mechanism assures the maintenance of a constant number of chromosomes in the species.
In most organisms a similar reduction takes place in the formation of the sperm, but as the honey bee drone has only one set of chromosomes no reduction takes place.
In the formation of egg cells it is a matter of chance which member of a given pair of chromosomes reaches a given daughter cell. It receives an assortment of 16 chromosomes (one member of each pair) that might range from all those from the queen’s mother to all from her father or any mixture of the two. The genes on one chromosome usually pass as a unit and are said to be linked. Frequently, however, parts of a pair of chromosomes exchange places, so that an egg cell may receive chromosomes that are partly of maternal and partly of paternal origin. This exchange is called crossing over and, of course, increases the possibilities for variation tremendously by making many combinations of genes possible.
The separation of genes going into daughter cells followed by the creation of new combinations by fertilization is called segregation and recombination. This is the mechanism that brings about genetic variation between individuals in a family. Since in the honey bee all the sperms produced by a drone are identical, no variation between sisters (queens or workers) is introduced from the drone.
All the genes at a certain locus may be the same and have exactly the same effect in one individual as in another. Occasionally a gene may change so that it has a different effect. Such a change is called a mutation. Both the mutant and the parent gene are then referred to as alleles. The new allele multiplies in cell division and may become common in the bee population if it has a favorable action. There can be several alleles at a single locus.
Let us consider a simple case of alleles in the honey bee. It has been shown that the difference between a mutant white-eyed bee and the normal or wild-type black eyed bee is due to the action of alleles at a single locus on one of the chromosomes. It has further been shown that the normal or wild-type allele for the black eye is dominant to the mutant allele for white eye, which is then said to be recessive. A dominant gene masks the effect of its recessive allele.
In genetics it has become customary to denote genes by means of letters. The allele for black eyes may be designated by a capital W, since it is dominant, and the opposing recessive allele by w. One of these alleles is located at a particular locus on one of the chromosomes of all honey bees. Let us call this the W locus on chromosome number 1.
Since female honey bees have pairs of chromosomes, they can have any two of the alleles at this locus – WW, Ww, or ww. The WW and Ww genotypes (gene combinations) have black eyes, since W is dominant over w. The ww females have white eyes. Since drones have only one chromosome of each pair, they are either W (black eyes) or w (white eyes).
Inheritance of eye color in the female bee is thus duplicate and particulate. Each female receives either the W or w gene from its mother, and it also receives W or w from its drone father. Inheritance of eye color in the drone bee is particulate but not duplicate, for he receives either W or w from his mother since he does not have a father. The particulate nature of inheritance is illustrated by the gene W or w. In the drone if W is present the eye color is black, but it is white when the gene w is substituted for W.
If a queen bee has a W gene on both of her number 1 chromosomes, she is represented by the formula WW and is said to be homozygous for the W locus. Homozygous means likeness of genes. The drone can never be said to be homozygous, because he has only one chromosome of each pair and thus only one of the alleles, W or w.
Let us assume that a black-eyed virgin queen homozygous for W is mated to a white-eyed drone. The mating can thus be represented as follows
WW x w =
Since the queen is homozygous, WW, all her eggs will have a W gene. All sperms from the drone will have the gene w, and the female workers or queen offspring of such a mating will all be Ww, or heterozygous. Since W is dominant over w and all offspring get one W and one w, the eyes of these females will be black.
Let us suppose that from one of these Ww fertilized eggs a virgin queen is produced. She will be Ww, or heterozygous for the W locus. Now let us mate her to a white-eyed drone that is thus w. Since the queen is Ww, she will produce eggs that will have only one of these alleles. On an average 50 percent of the eggs of such a queen will receive a W gene and the other 50 percent will receive the w gene. The mating and the offspring will be as follows:
|Ww eggs either W or w|
|w sperm all w|
WW x w =
|50% Ww heterozygous black-eyed females 50% ww homozygous white-eyed females|
It can be seen that the heterozygous Ww black-eyed queen produces 50 percent of heterozygous black-eyed daughters and 50 percent of homozygous white-eyed daughters after mating to a drone that is whited-eyed and thus w. Now this heterozygous black-eyed queen can produce both black-eyed and white-eyed sons, since her eggs have either a W or a w. Since these eggs can develop without fertilization, the W eggs will develop into black-eyed drones while the w eggs will develop into white-eyed drones. This illustrates how it is possible for a drone to exhibit and transmit to his offspring something that was not expressed in his mother, yet all his inheritance is from his mother because he has no father.
INHERITANCE OF ABDOMINAL COLORATION IN TWO STRAINS OF HONEY BEES.
(Photos courtesy North Central States Bee Culture Laboratory, Madison, Wisconsin.)
In the discussion of the W locus we have considered only two alleles, one completely dominant over the other. It is not always true that one allele is completely dominant. The heterozygous condition (such as Ww) may have intermediate effects. Furthermore, there can be several alleles of one locus that interact in various ways. When one considers that there may be thousands of loci with interaction of genes at different loci as well as interaction of alleles, it becomes apparent that the possibilities for genetic variation are tremendous.
Bees vary in many factors, not all of which are so easy to evaluate as eye color. It has been shown that there are at least seven different loci for genes that affect the color of the abdomen. Thus many abdominal color patterns are possible, since seven pairs of genes can produce 2187 different genotypes. If the workers of a mixed population are arranged according to color from the darkest to the lightest abdomen, they will fall into a continuous series.* Such a variation is said to be continuous in contrast to one such as white eye color, which is said to be discrete. In most characters of economic importance, such as honey production, egg-laying ability, vigor or longevity, variation is continuous. Although such variation may be due to the interaction of a great many genes, it does not always imply a large number of inherited factors. It is possible that only a few genes are concerned and that much variation is due to environmental influences. These may be many and occur at various times from the beginning of queen rearing all the way through the life of the colony up to the time of harvesting the honey crop.
By a study of variation as such, we are never able to find out which part of the variation is certainly due to environment and which to heredity. The bee breeder must be ever alert and try to reduce the environmental variation to a minimum so that the variation he observes and upon which he bases his selection will be due to genetic factors. Queens in colonies to be compared as to performance should be reared under optimum conditions. (Queen rearing will be treated more fully in a later paper.) It is best to place the colonies in a single yard so that they will all be exposed to the same weather conditions and will have the same forage available. At the beginning of a performance test they should be equalized as to bees, pollen, and honey so that none start the test under a handicap. Colonies should be scattered and isolated as much as possible to prevent drifting and robbing.
The colonial nature of the honey bee makes the selection of breeding individuals particularly difficult. In contrast to the situation in most farm animals, the colony rather than the individual is the unit of performance upon which the selection of breeding individuals must be based. The colony performance depends upon the performance of a queen of one generation and all her worker daughters of the next generation. We cannot breed this unit. We can in effect breed the queen by using her sons, because, as we have seen they represent her germ cells. For breeding females we can only use virgin queens that are sisters to the workers of the colony.
It has frequently been stated that a beekeeper can improve his stock, even though he may know little about animal breeding, if he is willing to make the effort. Nevertheless, one cannot find conclusive evidence that much genetic improvement has been accomplished.
Among the principles of breed improvement employed by early plant and animal breeders were “like produces like or the likeness of some ancester,” “inbreeding produces prepotency or refinement,” and “breed the best to the best.” In following these principles why did the bee breeder fail to make the progress made by the early plant and animal breeders? Inadequate control of mating and the difficulty of recognizing genetic superiority of individuals were factors that hindered progress. There is yet another factor that adds complexity to bee breeding, which was not understood until very recently. This is the inheritance of sex determination, and it is a very important factor because it is associated with egg hatchability. The relationship of sex determination to bee breeding is the subject of the next article.
* Inheritance of abdominal coloration in two strains of honey bees is illustrated in figures 1, 2, 3. and 4.
Reprinted from AMERICAN BEE JOURNAL
No. 8. pagas 328-330, August 1951