THE GENETICS OF BEES, or How God Keeps Rolling the Dice

The Heredity That Builds the Bee Colony

Viewed from the outside, a beehive looks like a society with clearly assigned roles: one queen, thousands of workers, and seasonally present drones. Seen through the lens of genetics, however, it is something far more unusual — a living system in which heredity acts not only on the individual bee, but on the entire colony.

Genes determine the possibilities available to a bee, but they do not prewrite every action she will take. Nutrition, temperature, age, the condition of the colony, pheromones, diseases, and the environment determine which inherited possibilities will actually be expressed. That is why bee genetics cannot be understood solely through the words “mother” and “father.” It must be considered as the genetics of a society.

An Unusual Arithmetic of Heredity

The honey bee, Apis mellifera, has 16 chromosomes in a single, haploid set. Worker bees and queens possess two such sets — 32 chromosomes in total. Drones normally possess only one set of 16 chromosomes.

The reason lies in 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 as she lays each egg. In a worker cell she normally lays a fertilized egg, while in the larger drone cell she lays an unfertilized one. 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, a major role is played by the csd gene — complementary sex determiner.

This gene exists in many different forms 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 as a female. When the egg is unfertilized and carries only a single version, it develops into a normal haploid drone.

A problem arises when a fertilized egg receives two identical versions of csd. In that case, a diploid drone develops. Such larvae are usually recognized and removed by the worker bees at an early stage. Empty cells remain in the comb and the brood pattern becomes irregular and “shotgun-like.”

This is one of the reasons why inbreeding is particularly 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 bees is therefore not merely a desirable trait. It is a prerequisite for normal reproduction.

The Drone — A Living Haploid Message

A drone has no father. He develops from an unfertilized egg and receives his genetic material only from his mother. But this does not mean that all drones produced by one queen are genetically identical.

During the formation of eggs, the queen’s chromosomes are reshuffled. Segments of her maternal and paternal chromosomes are exchanged through recombination. Each unfertilized egg therefore contains a different genetic combination, which means that every drone represents a distinct genetic version of his mother.

The drone is haploid — there is no second copy of his genes to conceal an unfavorable recessive variant. His inherited traits are expressed more directly. This turns drones into a powerful filter of natural selection: genetic defects often cannot remain hidden behind a normal second copy of the corresponding gene.

Every drone that successfully mates contributes an entire paternal genetic line to future worker bees. 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 multiple drones. Their sperm is stored in the spermatheca and can be used over a long period of time. Because of multiple mating, the worker bees in a colony are not genetically identical.

They all have the same mother, but they may have different fathers. As a result, multiple paternal lines — or patrilines — arise within a single colony.

Some groups of workers may respond more quickly to the scent of brood, others may switch more readily to nectar foraging, others may be more sensitive to changes in temperature, and still others may display stronger hygienic behavior. The colony gains a diverse range of responses rather than thousands of nearly identical bees.

Experiments have shown that greater genetic diversity within a colony can reduce the risk that a single infection will affect all bees equally severely. Benefits have been observed in disease resistance, colony growth, dance communication, and division of labor. This does not mean that every genetically diverse colony is automatically healthy, but rather that diversity provides a wider range of biological responses when danger arises.

A beehive resembles a team whose members all share one common goal, but are not made from the same mold.

Queen and Worker — Similar Genes, Different Destinies

The queen and the worker are both diploid females. They may originate from similar fertilized eggs and possess the same basic set of genes. Yet the differences between an adult queen and an adult worker are enormous:

• the queen has highly developed ovaries;

• she lives significantly longer;

• she lays enormous numbers of eggs;

• she has a different body structure and metabolism;

• the worker develops organs and behaviors associated with brood care, comb building, guarding, and food collection.

How can a similar genome create two such different female forms?

The answer lies in differences in nutrition and in the subsequent regulation of gene activity. The larva destined to become a queen receives a special diet and abundant royal jelly. This affects metabolism, hormonal signaling, 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 participates in changing the way genes are used. The DNA sequence itself is normally not rewritten. What changes is the activity of particular genes — which are switched on, which are suppressed, when they act, and how strongly.

DNA methylation, changes in the proteins around which DNA is packaged, microRNAs, alternative RNA splicing, and complex nutritional and hormonal signals all participate in this process. Science does not support the simplified idea that a single component of royal jelly presses a “queen switch.” Caste development is the result of an interacting network of nutritional and regulatory mechanisms.

This phenomenon is known as phenotypic plasticity — a similar genetic blueprint can produce very different physical and behavioral forms depending on developmental conditions.

Genes Do Not Act One by One

In beekeeping practice, people often speak of a “gene for honey production,” a “gene for gentleness,” or a “gene against swarming.” Such expressions are convenient, but they are not entirely accurate.

Most important economically valuable traits are polygenic — they depend on many genes, each making a small or moderate contribution. Their expression is also strongly influenced by the environment.

Complex inherited traits include:

• honey productivity;

• tendency to swarm;

• gentleness or defensive behavior;

• hygienic behavior;

• disease resistance;

• overwintering ability;

• rate of spring development;

• economical use of food reserves;

• resistance to Varroa destructor;

• orientation and foraging activity.

A colony may possess excellent genetic potential for high yield but fail to express it during drought, poisoning, forage shortage, queen problems, or heavy parasite infestation. Conversely, exceptional forage conditions may temporarily mask mediocre inherited traits.

A gene is not a sentence. It is a possibility that the environment may strengthen, restrict, or prevent from being expressed altogether.

The Colony Is the Unit That Must Be Evaluated

The worker bee hardly reproduces, while the queen hardly ever gathers food. The drone does not raise brood, but he carries genetic material between apiaries. No individual bee possesses all the functions of the colony.

For this reason, selection in honey bees cannot evaluate only the individual. It must assess the colony as a whole:

• how it maintains brood;

• how it responds to interruptions in forage;

• 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 carries 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 understanding, the drone background is therefore only half a selection program.

Recombination — Continuous Genetic Shuffling

The honey bee genome is characterized by a very high rate of genetic recombination. During egg formation, chromosomes exchange segments and create new combinations of inherited variants. Modern estimates suggest an average recombination 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 generates new genetic combinations.

High recombination may support diversity and adaptability within the social colony. But it also makes selection more difficult: an outstanding queen does not transmit exactly the same intact combination of desirable traits to all of her daughters. The genetic inheritance is reshuffled in every generation.

For this reason, a good line should not be judged by a single exceptional queen, but by a sufficiently large number of daughter colonies.

Maternal and Paternal Lineage

Mitochondrial DNA is transmitted primarily from the queen to her offspring. It can therefore be used to trace the maternal line. But mitochondrial DNA represents only a small part of the total inheritance.

Nuclear DNA comes from both the queen and the drone. It contains the overwhelming majority of the genes involved in determining physical, physiological, and behavioral characteristics.

Two colonies may therefore have identical or closely related maternal lineages but display substantially different qualities because their queens mated with different drones. Conversely, bees that look similar may have very different genetic histories.

Determining a “breed” solely from the color of the abdomen is unreliable. Coloration is influenced by many genes and by crossbreeding. Serious evaluation uses a combination of pedigree, morphometric traits, behavior, productivity data, and molecular analyses.

The Local Bee and the Cost of Uncontrolled Crossbreeding

Bees adapt to local climate, the length of winter, the distribution of forage sources, humidity, seasonal forage gaps, and local parasites. This adaptation accumulates over generations.

Introducing foreign genetics can sometimes bring useful traits, but not every good bee is good everywhere. A line selected under mild climatic conditions and prolonged forage may prove unsuitable in an area with cold winters, early spring development, and long periods without forage.

Open mating in drone congregation areas spreads genes over considerable distances. A beekeeper may conduct careful selection, yet his queens may mate with drones from many 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. However, if too narrow a group of parents is used, 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 involves many interacting characteristics:

• recognition of infested brood;

• uncapping and removal of affected pupae;

• interruption of mite reproduction;

• grooming — cleaning of one’s own body and of other bees;

• duration of capped brood development;

• physiological tolerance to viral infections;

• characteristics of brood development and odor.

A large genomic study of 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.

Claims that “the gene against Varroa has been discovered” should therefore be treated with caution. An important gene or marker may indeed be identified, but real resistance at the colony level is almost always a system of traits.

How Genetic Wealth Is Lost

Genetic diversity may gradually decline when:

• too many apiaries use queens from the same breeder;

• breeding repeatedly relies on a single “super queen”;

• a small number of drone-producing colonies mate with large numbers of queens;

• local lines are displaced by narrowly based commercial stock;

• severe losses destroy a large part of the population;

• recovery after those losses depends on only a few surviving colonies;

• selection pursues a single trait while neglecting all others.

A colony with record honey production may swarm heavily, overwinter poorly, or be susceptible to disease. Gentleness is valuable, but it should not come at the cost of vitality. Varroa resistance is extremely important, but it must be combined with productivity, stable development, and normal behavior.

Good selection does not create a champion in only one category. It seeks balance.

Practical Genetics for the Beekeeper

A beekeeper does not need a genetic laboratory in order to work in a genetically responsible way.

Accurate records should be kept on queen origin, colony development, yield, swarming, overwintering, health, and behavior. Colonies should be compared under similar conditions, and conclusions should not be drawn from a single unusually good or unusually poor year.

It is especially important:

1. Not to produce all queens continuously from a single line.

2. To maintain a sufficient number of high-quality drone-producing colonies.

3. To monitor the density and uniformity of brood patterns.

4. Not to replace all local genetics simply because of fashion.

5. To evaluate daughter colonies, not only the founding queen.

6. To look for stable traits that are expressed over several seasons.

7. To combine controlled breeding with a sufficiently broad genetic base.

8. Not to breed from colonies showing unexplained patchy 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. They can be used to assess relatedness, population admixture, and inherited predispositions toward particular traits.

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 programs of the future will combine:

• pedigree information;

• genomic data;

• field trials;

• control of mating;

• preservation of genetic diversity;

• evaluation of the entire colony.

The laboratory can read DNA. Only the apiary can show how that DNA lives.

Conclusion: Heredity in Motion

Bee genetics is not a motionless archive locked inside chromosomes. It flies with 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 worker bees.

An unfertilized egg produces a drone. A fertilized egg may produce a worker or a queen. 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 turns the colony into a genetic mosaic capable of responding to a changing world.

The bee colony survives not because all bees are identical, but because they are different in ways that allow them to function as one whole.

That is the great secret of bee genetics: heredity does not simply create bees.

It creates a society.