Clones with No Sons and Males with No Father
Let's get straight to the point - bee genetics are downright weird!
Most of us have heard of the importance of genes, how a mother and father each contribute their important element and so on. We accept, without question, that the mother contributes chromosomes from her egg and the father from his sperm. Much of the animal kingdom operates with these principles.
But that is not the case with honey bees.
But that is not the case with honey bees. They use an altogether different approach to the passing of chromosomes, with some strange implications. For example, 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
Confused? Read on...
Diploid and Haploid
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.
If you'd like to drill down a little more on this topic, check out this video from Jon Zawislak, at the University of Arkansas, Division of Agriculture.
The Impact on Genes
So what does all this mean in terms of the transfer of genes across generations? Let's consider, as a baseline, how humans compare.
Our genes are contained in chromosomes. We receive 46 chromosomes - 23 from our mother and 23 from our father. The transfer of genes through the generations means the characteristics of both sides of our family are evident in us and represented by factors such as eye color, height and so on. This is all because we result from an egg contributed by our mother and sperm contributed by our father.
With bees, the same basic principle exists - but only in female bees.
Female bees are created when a queen lays an egg that she then fertilizes. Though the numbers are different with bees (as compared to humans), each side of the family contributes the same number of chromosomes (16).
But it becomes a little stranger when we turn to the haploid - the drone. Male bees only include the chromosomes from the egg, since they have not been fertilized. Thus, drones have just 16 chromosomes, all from the queen.
Mixing vs. 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 Benefits of Genetic Diversity
With this fascinating background, let us turn to the issue of genetic diversity. One cannot underestimate the importance of diversity for bees. Where there is limited genetic diversity, the entire population is potentially vulnerable to the same diseases. If one of those hits the population then it can have a devastating effect.
By comparison, in a population with significant genetic diversity, a single disease may impact a percentage of the population, but others may be resistant. This means that genetic diversity reduces the chances of a catastrophic event wiping out the entire colony.
The Queen's Promiscuity
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.
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, resulting in either a worker or a drone. 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 genes.
How Beekeepers Can Hurt the Bee Population
As beekeepers we like to think we're helping the bee population and, therefore, the planet. However, an increasing number of beekeepers does not in itself mean the situation is improved. Without awareness, it may in fact be harmful.
As we have seen, genetic diversity is important for any species. And yet it's not uncommon for beekeepers - commercial or otherwise - to raise many queens from a single genetic line. This greatly constrains genetic diversity and, over time, leaves the population vulnerable to the wrong disease at the wrong time, with the potential to devastate the population.
For a deeper look at this topic here's a lecture by Debbie Delaney at the National Honey Show.