Honey Bee Sex: The Untold Story


Drone, Queen and Worker Bee Comparison The bee's universe is a tough monarchy, the proletariat has no say there. The queen honey bee decides the sex of her offspring, points a new research published in Behavioral Ecology, a finding that challenges the belief that queens are just 'egg laying machines' and that worker bees decide if the queen is going to deliver males or females.

Even though humans first started keeping honey bees at least 7000 years ago, it was not until the mid-1850s that people started to understand honey bee reproduction and genetics so that they could breed for desirable traits. Most bees in the hive are female, and the queen is responsible for all reproduction. Let’s take a look at some of the unique aspects of honey bee genetics and see what the recently sequenced genome can tell us about sociality and bee biology.

In a honey bee colony, the queen is responsible for all reproduction and does all of the egg-laying in the hive. Female workers do all of the work inside the hive and collect food outside of the hive. Male drones are fewer in number, and they contribute little to hive function.

Clones with No Sons and Males with No Father

Consider these facts about drones, the male bee.

•  They have no father
•  They have a grandfather
•  They can be fathers to daughters, granddaughters and grandsons
•  They can’t have sons

Diploid and Haploid

Honey Bee Chromosomes Honey bees (and the rest of Hymenoptera: bees, ants, and wasps) have a genetic system that is different than ours and most other insects. Haplodiploidy is a system where females have two copies of each chromosome, but males have just one copy. Haplodiploidy increases relatedness between siblings and is related to the social division of labor. Female honey bees have a full complement of chromosomes (one set from each parent) and are diploid. Males, however, are formed from unfertilized eggs and are haploid; they contain only one copy of each chromosome. Queens control whether or not they lay a fertilized egg because they are able to store sperm from previous matings. In this way, the queens manage the sex ratio in the hive.

The cause of all this weirdness is a sex-determination system called haplodiploidy. In high school biology we were taught that all eggs are fertilized and either female or male offspring result. Such is the case with humans, for example.

But in the curious world of the honey bee, a male is created from an unfertilized egg – just an egg laid by a queen, without being fertilized by sperm. The male bee is referred to as hapliod. The process of reproduction from unfertilized eggs is called parthenogenesis.

The more familiar situation, where the queen fertilizes an egg with sperm, results in a female bee. This means the egg is destined to be either a worker a queen bee. Female honey bees are referred to as diploid.

This figure shows only two chromosome sets for purposes of clarity. Diploid females have 16 pairs, or 32, chromosomes, and haploid males have only 16 single chromosomes.

Honey Bee Genome

Genomics is the study of DNA at the level of whole chromosomes, large clusters of genes, or the entire genome at once. A genome is the entire set of genetic material for an individual organism.

There is no special gene that controls whether a bee grows up to be a queen or a worker; their jobs in the hive are not determined by genetic makeup. For example, queen bees result from larvae that are fed large amounts of a substance called royal jelly, which leads to development resulting in a queen.

Furthermore, there are no specific genes predisposing the workers to the different tasks they do. Young adult bees perform nursing duties (feeding larvae, for example) inside the hive for about a week, after which there is a transition in behavior to performing foraging (collecting food) duties outside the hive. Other tasks that may be done for the colony include guarding the hive and removing dead bees from the hive.

There are a little over 10,000 genes in the honey bee genome, but this is likely to be a slight underestimate of the real total.

One use of a genome is to infer the history of the species. For example, an analysis showed that honey bees had at least two ancient migrations from Africa and into Europe. Another use of these SNPs is the study of African ‘killer’ bees, which were introduced to South America in 1954. These bees have now been hybridizing with bees in Central and South America.

The Queen's Mating Flight

Honey Bee Queen Nuptual Flight The queen’s behavior at mating time has evolved to encourage such diversity. During her mating flights, early in life, she will mate with many drones. Since these drones have various genetic traits, they offer a genetic diversity that will serve the queen and her offspring well.

Her mating flights, across a few days, will result in collecting sperm from 10-25 drones. She will store this sperm in her spermatheca for many years.

Back at home – perhaps in your beehive – she will start her life of laying eggs, around 2,000 per day. As she does so, she may or may not fertilize each egg, therefore deciding the gender. When she does fertilize an egg, she draws from a broad “stock” of sperm in her spermatheca. This means that all members of the colony have the same mother – the queen – but her female offspring have various fathers.

Her drone offspring will only have a mother, whereas if she fertilizes an egg to create a female she will use a sperm from one of many different potential fathers.

Mixing versus Cloning

A queen’s eggs contain 16 chromosomes. Thus, since she herself has 32 chromosomes (remember, as a female she has chromosomes from both her mother and father), she cannot “squeeze” all her 32 chromosomes into each egg. This means that eggs from a queen have a variety of combinations of chromosomes, from the queen and the sperm-donating drone.

Compare this with the drone. He has 16 chromosomes and his sperm can contain just that same number. And so, each sperm created by the drone is identical, since they all have the same 16 chromosomes. This makes for the creation of clones! Drones produce clones.

The Role of Royal Jelly

In looking at bee reproduction, the creation of queens vs. workers is interesting. Both, of course, are females and so have chromosomes from the mother (the egg-laying queen) and a father (through the sperm of a drone).

But what turns a fertilized egg result into a queen, as opposed to a worker? The answer lay in the feeding of royal jelly, a food fed to all larvae in their first three days.

In the majority of cases the feeding of royal jelly will cease after three days. This will result in either a worker or a drone, depending on whether the egg was fertilized. However, the colony will decide when it needs a new queen. When it does, a female larva will continue to be fed royal jelly, beyond the third day.

This changes the way in which genes are “marked” i.e. how they develop. Rather than promote the development of queen-relevant genes (as is often assumed), this takes place through the limitation of worker bee genes. It is the lack of these genes in a female larva that results in a queen.

Glossary

•  Diploid: paired complementary set of chromosomes

•  Haploid: unpaired, single set of chromosomes

•  Chromosome: long piece of DNA that contains many genes, genes are transported in these units

•  Gene: hereditary unit of DNA that codes for a specific protein

•  DNA: deoxyribonucleic acid. A double-stranded molecule that encodes proteins. Code is combination of A, T, C, and G.

•  Pheromone: a chemical produced by an organism of one species to communicate to a member of the same species

•  Resistance: a trait where an organism is less affected by something that should be harmful to it. For example, some insects are resistant to certain insecticides, meaning that those insecticides do not kill resistant insects.