THE GENETICS OF BEES
or How the Lord God Keeps Rolling the Dice
The Heredity That Builds the Honey Bee Colony
Viewed from the outside, the hive looks like a society with clearly assigned roles: one queen, thousands of workers, and drones that appear seasonally. Viewed through genetics, however, it is something far more extraordinary—a living system in which heredity acts not only upon the individual bee but upon the entire colony.
Genes determine the bee’s possibilities, but they do not predetermine every action it will take. Nutrition, temperature, age, the condition of the colony, pheromones, diseases, and the environment determine which hereditary possibilities will be expressed. This is why bee genetics cannot be understood only through the words “mother” and “father.” It must be regarded as the genetics of a society.
An Unusual Arithmetic of Heredity
The honey bee, Apis mellifera, has 16 chromosomes in its single, haploid set. Workers and queens possess two such sets—a total of 32 chromosomes. Drones normally have only one set of 16 chromosomes.
The reason is the unusual system known as haplodiploidy:
a diploid female develops from a fertilized egg;
a haploid drone develops from an unfertilized egg.
To some extent, the queen can control this by releasing or withholding sperm from her spermatheca while laying an egg. She normally lays a fertilized egg in a worker cell and an unfertilized egg in the wider drone cell. Thus, sex determination begins at the very moment the egg is laid.
This, however, is only the first level of explanation. Not every fertilized egg necessarily develops into a normal female.
The Gene That Determines Sex
In the honey bee, the csd gene—complementary sex determiner—plays a major role.
This gene exists in many different variants known as alleles. When a fertilized egg receives two different versions of the gene—one from the queen and one from the drone—it develops into a female. When the egg is unfertilized and possesses only one version, it develops into a normal haploid drone.
A problem occurs when a fertilized egg receives two identical versions of csd. A diploid drone then develops. Such larvae are usually recognized and removed by the workers during the early stages of development. Empty cells remain on the comb, and the brood acquires an irregular, “shotgun” appearance.
This is one reason why inbreeding is especially dangerous in honey bees. When the queen and the drones are closely related, the probability that they carry identical csd alleles increases. The consequences include:
more diploid drones;
loss of brood;
weakening of the colony;
inefficient use of food and nurse bees;
reduced viability of the population.
Genetic diversity in honey bees is therefore not merely a desirable quality. It is a condition for normal reproduction.
The Drone—A Living Haploid Message
The drone has no father. He develops from an unfertilized egg and receives his hereditary material only from his mother. This does not mean, however, that all the drones produced by one queen are identical.
During egg formation, the queen’s chromosomes are rearranged. Parts of the chromosomes she inherited from her mother and father are exchanged through recombination. Every unfertilized egg contains a different combination, so each drone represents a distinct genetic version of his mother.
The drone is haploid—he has no second copy of his genes that could conceal an unfavorable recessive variant. His hereditary characteristics are expressed more directly. This makes drones a powerful filter of natural selection: genetic defects often cannot remain hidden behind a normal second copy of the relevant gene.
Every successfully mating drone passes one complete paternal hereditary line to future workers. Drones are therefore not the “unnecessary males” of the hive. They represent half of the genetic future of the apiary.
Why the Queen Mates with Many Drones
The queen mates in flight with numerous drones. Their sperm is stored in the spermatheca and may be used over a long period. Because of multiple mating, the workers in one colony are not genetically identical.
They all have the same mother, but they may have different fathers. The hive therefore contains multiple paternal lines, known as patrilines.
Some groups of workers may respond more quickly to the odor of brood; others may shift more readily to nectar collection; some may be more sensitive to temperature changes; and others may display stronger hygienic behavior. The colony possesses a diverse range of responses instead of thousands of almost identical bees.
Experiments indicate that greater genetic diversity within a colony may reduce the risk that an infection will affect all bees equally severely. Benefits have been observed in disease resistance, colony growth, communication through the dance, and the division of labor. This does not mean that every genetically diverse colony is automatically healthy. It means that diversity provides a greater variety of biological responses when danger arises.
The hive resembles a team whose members share a common purpose but are not all made from the same mold.
Queen and Worker—Similar Genes, Different Destinies
The queen and the worker are diploid females. They may originate from similar fertilized eggs and possess the same basic set of genes. Yet the differences between the adult queen and the worker are enormous:
the queen has highly developed ovaries;
she lives considerably longer;
she lays an enormous number of eggs;
she has a different body structure and metabolism;
the worker develops organs and behaviors connected with brood care, comb building, guarding, and food collection.
How can a similar genome produce two such different female forms?
The answer lies in different nutrition and the subsequent regulation of genes. The larva destined to become a queen receives a special diet and abundant royal jelly. This affects metabolism, hormonal signals, organ growth, and the activity of large groups of genes.
It is often said that royal jelly “changes the genes.” It is more accurate to say that it helps change the way the genes are used. The DNA sequence itself is not normally rewritten. What changes is the activity of particular genes—which are activated, which are suppressed, when they function, and how strongly they function.
This process involves DNA methylation, changes in the proteins around which DNA is packaged, microRNAs, alternative RNA splicing, and complex nutritional and hormonal signals. Science does not support the simplified idea that a single component of royal jelly presses a “queen switch.” Caste development results from an interacting network of nutritional and regulatory mechanisms.
This phenomenon is known as phenotypic plasticity—a similar genetic plan can produce different bodily and behavioral forms depending on the conditions of development.
Genes Do Not Command Individually
In beekeeping practice, people often speak of a “gene for honey,” a “gene for gentleness,” or a “gene for low swarming tendency.” Such expressions are convenient, but they are not entirely accurate.
Most economically important traits are polygenic—they depend on numerous genes, each making a small or moderate contribution. Their expression is also strongly influenced by the environment.
Complex hereditary traits include:
honey production;
swarming tendency;
gentleness or defensive behavior;
hygienic behavior;
resistance to disease;
overwintering ability;
rate of spring development;
economical use of food reserves;
resistance to Varroa destructor;
orientation and foraging activity.
A colony may possess good genetic potential for high production but fail to express it during drought, poisoning, a shortage of forage, poor queen performance, or heavy parasite infestation. Conversely, an excellent nectar flow may temporarily conceal mediocre hereditary qualities.
A gene is not a sentence. It is a possibility that the environment may strengthen, restrict, or prevent from being expressed at all.
The Colony Is the Unit That Must Be Evaluated
The worker almost never reproduces, while the queen almost never collects food. The drone does not rear brood, but he carries genetic material between apiaries. No individual bee possesses all the functions of the colony.
For this reason, selection in the honey bee cannot assess only the individual. It must evaluate the colony as a whole:
how it maintains its brood;
how it responds to an interruption in forage flow;
how it overwinters;
how it defends itself;
how it removes diseased or damaged larvae;
how economically it uses its reserves;
how consistently it expresses its qualities over different years.
The queen provides only part of the answer. The other part comes from the drones, while the final result is expressed through thousands of workers with different fathers.
Selecting queens without controlling—or at least knowing—the drone background is therefore only half of selection.
Recombination—The Endless Shuffling
The honey bee genome is characterized by an exceptionally high rate of genetic recombination. During the formation of eggs, chromosomes exchange sections and create new combinations of hereditary variants. Modern estimates indicate an average rate of approximately 18–20 cM/Mbp, although it varies considerably among different regions of the genome.
This means that a queen does not simply produce copies of herself. She continuously creates new genetic combinations.
High recombination may support diversity and adaptability in the social colony. However, it also makes selection more difficult: an outstanding queen does not pass the same intact combination of qualities to all her daughters. Heredity is reshuffled in every generation.
For this reason, a good line should not be judged from a single exceptional queen but from a sufficiently large number of her daughter colonies.
Maternal and Paternal Lineage
Mitochondrial DNA is transmitted mainly from the mother to her offspring. It can therefore be used to trace the maternal line. However, it represents only a small part of the total inheritance.
Nuclear DNA is received from both the mother and the drone. It contains the overwhelming majority of the genes involved in determining bodily, physiological, and behavioral characteristics.
Two colonies may therefore have the same or a closely related maternal line but differ substantially because their queens mated with different drones. Conversely, bees that appear similar may have different genetic histories.
Determining a “breed” from abdominal coloration alone is unreliable. Color is influenced by numerous genes and by crossbreeding. Serious investigation uses a combination of pedigree, morphometric characteristics, behavior, productivity data, and molecular analyses.
The Local Bee and the Cost of Indiscriminate Crossbreeding
Bees adapt to the local climate, the duration of winter, the distribution of nectar flows, humidity, seasonal interruptions, and local parasites. This adaptation accumulates over generations.
The introduction of foreign genetics may sometimes bring useful qualities, but not every good bee is good everywhere. A line selected under a mild climate and a prolonged nectar flow may prove unsuitable where winters are cold, spring development begins early, and long periods without forage occur.
Free mating in drone congregation areas spreads genes over great distances. A beekeeper may carry out careful selection, yet the queens may mate with drones from numerous surrounding apiaries. This is a natural mechanism of genetic exchange, but it makes the preservation of a particular line more difficult.
Isolated mating stations and instrumental insemination provide greater control. If too narrow a range of parents is used, however, these methods may reduce genetic diversity and increase the risk of inbreeding.
Control without diversity is just as dangerous as diversity without direction.
The Genetics of Resistance to Varroosis
Resistance to Varroa destructor does not depend on a single magical gene. It includes numerous interacting characteristics:
recognition of infested brood;
uncapping and removal of affected pupae;
interruption of mite reproduction;
grooming—cleaning one’s own body and the bodies of other bees;
duration of sealed brood development;
physiological tolerance of viral infections;
specific features of brood development and odor.
A large genomic study involving more than 1,500 honey bee colonies showed that Varroa resistance has a complex, polygenic architecture. This means that future genomic selection will probably use information from many regions of the genome rather than from one single mutation.
The claim that “the gene against Varroa has been discovered” should therefore be treated cautiously. An important gene or marker may be found, but the colony’s actual resistance is almost always a system of characteristics.
How Genetic Wealth Is Lost
Genetic diversity may gradually decrease when:
too many apiaries use queens from the same breeder;
breeding is continuously based on a single “super queen”;
a small number of drone-producing colonies mate with many queens;
local lines are displaced by a narrow commercial stock;
severe losses destroy a large part of the population;
recovery after losses is based on only a few surviving colonies;
selection pursues one trait while neglecting all others.
A colony with record honey production may swarm excessively, overwinter poorly, or be susceptible to disease. Gentleness is valuable, but it should not be obtained at the cost of vitality. Varroa resistance is extremely important, but it must be accompanied by productivity, stable development, and normal behavior.
Good selection does not create a champion in one measurement. It seeks balance.
Practical Genetics for the Beekeeper
A beekeeper does not need a genetic laboratory in order to work in a genetically responsible manner.
Accurate records should be kept concerning queen origin, development, yield, swarming, overwintering, health, and behavior. Colonies should be compared under similar conditions, and conclusions should not be based on a single unusually good or bad year.
It is especially important:
Not to produce all queens continuously from one single line.
To maintain a sufficient number of high-quality drone-producing colonies.
To observe brood density and uniformity.
Not to replace all local genetics merely because of fashion.
To evaluate daughter colonies, not only the founding queen.
To seek stable qualities expressed over several seasons.
To combine controlled breeding with a sufficiently broad hereditary base.
Not to breed from colonies with unexplained spotty brood, poor vitality, or persistent health problems.
Selection begins not with grafting a larva, but with observation.
Genomic Selection—The Future That Has Already Begun
Modern methods make it possible to examine thousands of genetic markers simultaneously. These can be used to assess relationships, population mixing, and hereditary predispositions toward particular qualities.
But the genome does not replace the apiary. A genetic marker indicates probability, not a guaranteed result. The colony must prove its qualities under real conditions—with local forage, local climate, natural disease pressure, and normal beekeeping practice.
The strongest selection of the future will combine:
pedigree information;
genomic data;
field testing;
mating control;
preservation of diversity;
evaluation of the entire colony.
The laboratory can read DNA. Only the apiary can show how that DNA lives.
Conclusion: Heredity in Motion
The genetics of bees is not a motionless archive locked inside chromosomes. It flies with the drones toward congregation areas, is stored in the queen’s spermatheca, is reshuffled during egg formation, and is expressed through the work of thousands of workers.
A drone develops from an unfertilized egg. A worker or queen may develop from a fertilized egg. A difference in the csd gene may determine whether a normal female develops. Different nutrition can direct similar genomes toward two radically different lives. Multiple mating transforms the colony into a genetic mosaic capable of responding to a changing world.
The honey bee colony survives not because all its bees are identical, but because they are different in a way that allows them to act as one whole.
That is the great secret of honey bee genetics: heredity does not merely create bees. It creates a society.
