A LETHAL MECHANISM in bees complicates breeding. To clarify this mechanism some preliminary explanations are necessary.

We have explained the location of the hereditary determiners, the genes, in linear order on chromosomes and how they are inherited in bees. When genes are paired as in the diploid queen, and one member has a different action than the other, each gene is called an allele of the other. Individuals in the population may carry still other alleles at the same locus on the chromosomes, and there may be a large series, each having a slightly different action. Fertilization may bring together various combinations of these alleles.

Genes have various effects. Some have detrimental effects. In some instances this effect is so great that the gene kills the individual inheriting it. Such a gene is called a lethal.

The lethals we are concerned with are members of a series of alleles which we have designated as a, b, c, d, et cetera. Females ( queens and workers ) are always heterozygous that is, they contain two of these alleles that are different, as, for example, a and b. A queen of this composition produces eggs one-half of which are a and one-half are b. Since the drones develop from unfertilized eggs, one-half of the sons are a and produce only a sperm, and the other half are b and produce only b sperm.

When an egg such as a is fertilized by a sperm carrying a different lethal allele such as b, a queen or worker having the composition a/b results. If it is fertilized by a sperm carrying a similar allele (a), this homozygous combination (a/a) causes the individual to die before maturity, usually in the egg stage. When a queen (a/b) is mated to a single drone carrying a different lethal allele such as c, then all the progeny resulting from fertilized eggs will have lethal alleles dissimilar (a/c, b/c) and will be viable; able to live to maturity. Efficiency in the brood nest will be high, and a populous colony will result. If, on the other hand, she is mated with a single drone having a similar lethal allele such as b, then one-half of her progeny resulting from fertilized eggs will be a/b and viable, and one-half will be homozygous (a/a) and will die. Because most of the dying eggs are not removed until hatching time, 3 days after they are laid, efficiency is low in such a brood nest, and a weak colony will result.

Failure of selection to eliminate the lethals indicates that a nonlethal gene does not exist at this locus. Inbreeding reduces the number of lethals in the population and increases the chances of similar lethals meeting to produce low viability. Outbreeding brings new lethals into the population and this increases the frequency of high viability.

A similar series of lethals in a related insect Bracon hebetor, better known to geneticists as Habrobracon, has been studied by P. W. Whiting, of the University of Pennsylvania. In that insect a definite association with sex has been established. Such an association has not been proved in the honey bee, where promotion of outbreeding may be justification enough for the existence of such a wasteful lethal mechanism.

As long as individual matings are made, the percentage of viable fertilized eggs will either be near 100 percent or near 50 percent. Of course, there may be a small percentage of deaths from other causes. If mated naturally, many queens will mate twice and often intermediate viabilities will result, depending on the composition of queen and drone and the proportion of types of sperm reaching the spermatheca. The same is true when several drones are used in artificial insemination.

By a series of individual matings something can be learned of the opposing lethal alleles in a given cross. If all the progenies are highly viable, then the opposing lethal alleles are different; if some of the progenies are poorly viable, however, then some of the opposing alleles are similar.

Lethal alleles can be identified most easily when we cross lines that contain only two alleles each. Crosses by individual matings between such lines will then fall into one of three classes : (1) All progenies of low viability, showing that the alleles are the same; (2) all progenies highly viable, showing the alleles to be different; and (3) one-half the progenies highly viable and one-half poorly viable, showing that the two lines have one allele in common. This procedure has been used to establish two-allele tester lines with definitely identified alleles for use in determining the alleles of any untested breeding stock.

Two-allele lines can be readily established in one of two ways : (1) By making individual matings and breeding from a low-viability progeny or (2) by mating unfertilized queens to their own sons. The second way is done by inducing virgin queens to lay by exposure to carbon dioxide, rearing drones from them, and mating these drones back to their mothers. As the virgin queen can contain only two lethal alleles, there are only two alleles in the line established.

Lethal alleles therefore are important in bee breeding. Matings that involve similar alleles cause low viability of the brood and lower colony population. This in turn reduces productivity of the colony. Selection for such qualities as honey production, which is profoundly influenced by colony population, is inefficient unless the lethal-allele conditions are comparable in all colonies. Because the lethals cannot be eliminated by selection, some form of controlled hybridization seems most promising.

THE EARLY BEE BREEDER raised virgin queens from his best colonies and thus controlled the female parent. He attempted to control male parentage by stimulating certain queens (colonies) to produce great numbers of drones. Thus he increased the chances that his selected queens would mate with these selected drones. Some mass selection thus has been practiced since early times. Progress was made in selecting for body color, type, and temperament, but we doubt whether much improvement was made in less easily measured characteristics, such as honey production and vigor. In fact, continuous selection for color, type, and temperament has resulted in lower vigor and honey yield, as exemplified by the golden bees developed in the United States. They looked beautiful but were inferior in productivity.

Seeing no real improvement through mass selection, the American bee breeder sought new stock from other beekeepers in this country or through the importation of races and strains from abroad. In mixing them with his own stock, he intentionally or unwittingly was hybridizing two races or strains. The superiority of the first few generations was inaccurately accredited to the new stock. Hybrids, of course, do not breed true, and it was impossible to maintain the superiority in later generations. As inbreeding progressed, low viability due to the mating of similar lethal alleles became more and more frequent.

The precepts of breed improvement successfully used by early plant and animal breeders included such ideas as like produces like or the likeness of some ancestor, inbreeding produces prepotency or refinement, and breed the best to the best. The development of all breeds of livestock has included some inbreeding to produce uniformity within the breeds.

If the beekeeper follows in the footsteps of the animal breeder and tries to fix characteristics by inbreeding or line breeding, he immediately runs into difficulties. These systems of breeding will almost invariably increase the proportion of low-viability matings by reducing the number of lethal alleles in the line. What the beekeeper gains in uniformity and fixation of desirable characteristics, therefore, might be more than nullified by increase in mortality.

In order to produce uniform colonies with high-viability brood.. one has to cross races and strains that are likely to contain different lethal alleles or specially selected lines of known lethal-allele composition.

Hybrid breeding seems to be the bee breeders' best solution to their special problems. Plants and animals have frequently been improved by crossing. Hybrid plants are generally taller than their parents, larger in size, more vigorous, longer lived, and more resistant to diseases. When it comes to heredity, animals behave as plants do. The effects of hybridizing chickens, mice, guinea pigs, and rabbits are the same as in plants. The superiority of hybrid corn is attested by the fact that 81 percent of all corn planted in the United States in 1951 was hybrid seed. Hybrid bees therefore appear to offer the surest and fastest method of producing superiority in production, egg viability, and performance.

Inbreeding, followed by crossing, has been the successful method employed by plant and animal breeders. Inbreeding is the mating of closely related individuals such as parent-offspring, brother-sister, or cousins. After several generations, each inbred line becomes constant and uniform within itself but distinctly different from other inbred lines. Inbred lines go through a purification process such that only those individuals that possess much of the best that was in the original stocks in the beginning can survive. Although these inbred lines themselves will be inferior, they have possibilities as parents. By crossing inbred lines, one can gather together again the best qualities that have been distributed to the several inbred lines and create a new variety. Size, vigor, fertility, and viability can be fully restored in the hybrid with the advantage of real improvement through the elimination of undesirable characters.

Crosses among certain inbred lines have shown a combination of desired characters that are definitely superior to those of the stocks from which the inbreds originated. This superiority could not have been reached as readily in the original stock by selection alone.

To produce hybrid bees, the breeder may cross different races, strains, or inbred lines of bees. Unless the races or strains are homozygous for the desired characters, the hybrids will be variable. Furthermore, the hybrids produced the following year or from other crosses of the same races or strains will differ from each other. The only sure method of having uniform hybrids is to cross strains or inbred lines that are homozygous for the desired characters.

To produce inbred lines, the bee breeder must know which matings to make to obtain the desired inbreeding with the least expense of time and labor. Because of the mating habits of bees, it is an economic necessity that all inbreeding matings be made by artificial insemination. The first chart shows the percentage of inbreeding in successive generations by several systems of inbreeding possible in bees. The percentage of inbreeding is the percentage of heterozygous loci in the original selected individuals that become homozygous by inbreeding. Inbreeding has no effect on genes already homozygous in the line so we are only concerned with those loci that are originally heterozygous. Since the bee breeder cannot know which genes were originally heterozygous and what effect each gene has, he can only measure the relative purity of the stocks by the percentage of inbreeding.