The Drone : Kingpin to the Colony.

Part One: Understanding some cell biology and genetics.

The drone is probably the least understood member of a honey bee colony. People persist in anthropomorphizing the drone and perpetuating myths of drones being greedy, lazy good for nothings that contribute little to the colony apart from mating with the queen, and how many of them get to do that? Not many, so what’s the point of them hanging about till the end of summer when they will get turfed out of the hive anyway? Some old beekeeping books even advocate sacrificing drones in the summer in order for the beekeeper to harvest more honey.

Caste Aside Myths.

Drones are mistakenly described in many books as being a caste. This is incorrect. The late E.O.Wilson (perhaps one of the most famous biologists of all time) tells us that there are 2 sexes, and 2 female castes in honey bees. Drones are the males. Workers and queens are females and comprise the two castes. The caste system in honey bees relates to age polyethism and the tasks that are performed inside the nest. The drones do not have a specific indoor role in this context. Professor Tom Seeley suggests that there are actually 4 female castes among the workers: cell cleaners, nurses, food storers and foragers. But we can stick with 2 castes for the females for now.

I’m sure we have all had a laugh at the drone’s expense at some point. I’m guilty of entertaining the local Women’s Institute and other groups on a winter’s evening with tales of the mating habits of honey bees and the drone’s dramatic post-coital demise. The drone’s parentage comes into question on beekeeping quiz nights, and we learn that he has a mother but no father though he does have a grandfather.

How can it be so? Well, it is quite complicated to explain but I want to start here because it can be more than a little confusing for some beekeepers, and those who have not studied basic biology. It is probably true to say that only well-trained biologists and geneticists thoroughly understand the complications of cell division and the genetics of honey bees. I am neither and I don’t claim to understand more than the basics involved with honey bee reproduction. I have gone into more detail than some might care for, but the reason is to support beekeepers who need to understand a little more for the UK beekeeping exams and module 7, Selection and Breeding of Honey Bees.

I plan to cover the biology, physiology and behaviour of drones in subsequent blogs.

It’s all About Cell Division.

Every organism is made up of cells which are the key components and essential to its structure and function. Different cells make up every part of our bodies but they are organised in more or less the same way inside with a nucleus, mitochondria, organelles and other parts surrounded by a cell membrane. Cells replicate during growth and repair of damaged tissue. You have probably noticed how quickly a cut on your body heals and this is because skin repair involves cell replication.

Human body somatic cells have 23 pairs of chromosomes in each cell giving a total of 46 chromosomes. Chromosomes are found in the nucleus of every cell and they are threadlike structures that carry genetic information in the form of a very long deoxyribonucleic acid (DNA) molecule and associated proteins. DNA is formed in a double helix shape and provides the information for the structure of a cell’s protein.

This is really important when it comes to repairing body tissue. When you cut your finger, it will heal quickly if you are healthy and very soon new skin will cover the damaged area. This is because somatic cells divide by a process called mitosis to produce 2 identical cells, in this case finger cells. The DNA in the cell has the genetic recipes (genes) for all types of cells in your body, but when you are cut only the genes needed to repair the skin on your finger are activated to repair skin on your finger.  This is reassuring because it would be no good at all if the body made liver cells instead to repair the cut finger.  Each new cell contains 23 pairs of chromosomes and all cells produced during mitosis are identical.

It helps to visualise the cell division process so borrow a school biology book from an offspring, or watch the Amoeba Sister’s fun educational videos here. https://www.bing.com/videos/search?q=the+amoeba+sisters+youtube&qpvt=the+amoeba+sisters+youtube&FORM=VDRE They cover all sorts of basic biology concepts that are also useful in beekeeping. I bought a second- hand sixth edition copy of Biology by Reece and Campbell which is published by Benjamin Cummings and I find this very useful.

Before going further, it is useful to know a couple of terms like ploidy which describes the number of sets of chromosomes in a cell. We humans are diploid because we have 2 sets of chromosomes. Di means twice, two, or double. Somatic cells are denoted by 2 n so human cells have 23 x 2 =46 chromosomes.

Polyploid describes the genetics of an organism that has more than 2 copies of each chromosome. Haploid describes an organism that contains just one copy of each chromosome. It is a word that is important when it comes to understanding drone genetics. Haploid comes from the Greek word haplous meaning single.

Human sex cells or gametes are termed haploid because they contain just one copy of each of our 23 chromosomes. Gametes are either a male sperm or a female egg/ovum. Gametes are produced differently from diploid cells by a more complicated process called meiosis. It is different from mitosis because each gamete producing cell (called a gametocyte) has 23 chromosomes, and the information on the two chromosomes of each pair gets recombined in a process called recombination. This is much like shuffling and dealing cards and means that every egg or sperm produced by a diploid creature has a different subset of its parent’s genetic material. In other words, each sex cell (sperm or egg) carries different genetic information.

The fusion of a sperm and an ovum, in a diploid species like ourselves and most other plants and animals, results in an offspring that possesses a unique sample of genes from both mother and father. Identical twins arise when the fertilized cell splits in two parts early in development.

Some animals and plants can reproduce asexually by a process called parthenogenesis the name of which is derived from Greek and means virgin birth. Parthenogenesis is an adaptive strategy that allows some organisms to reproduce when sexual reproduction is not possible due to environmental conditions. It allows some animals to pass on genes without having a mate. The Komodo dragon is an example and if a female gets washed up on an uninhabited island, she can create a new population through parthenogenesis. Animals reproducing in this way can save time and energy and also produce more offspring but there are disadvantages such as lack of genetic diversity and reduced hybrid vigour leaving offspring more vulnerable to disease.

Honey bees belong to a group of insects known to science as the Hymenoptera. It includes the wasps, ants, bees, and a number of similar insects. The name is compounded from two Greek words. “Hymen” means membrane and “ptera” means wings.

The oldest known bee fossil was found in Myanmar by an American entomologist, Dr George Poiner from Oregon State University. It was carbon dated and found to be 100 million years old, which is 40 million years older than previous fossils found preserved in amber. Very recently, work at York University, by Professor Amro Zayed and colleagues, settles the long debate about the origins of honey bees (all species in the genus Apis). It is now clear that this group arose in the Far East.

The Hymenoptera, like some other insects and animals, are able to reproduce in part by parthenogenesis (by means of unfertilized eggs). In 1835, the Polish beekeeper Johann Dzierzon discovered that this is the means of the production of drones in honey bees, and in 1845 he published a paper on it, but his discovery was not accepted till 1906, the year of his death.

In honey bee colonies, when a female (either a queen or a worker) is produced, the queen lays an egg in a queen or worker cell which she first measures with her front legs. If the cell size is the correct size for a worker, then she releases sperm from her spermatheca as the egg passes through the median oviduct. If the cell is larger, then the queen doesn’t release the sperm. So, the egg is not fertilised and develops as a drone.

Honey bees have a haplo-diploid system of sex determination. The females (queen and workers) are diploid. So, a queen’s somatic (body) cells have 16 pairs of chromosomes which means that her cells have 16 x 2 =32 chromosomes. The drone is haploid, so his cells have 16 chromosomes. Every sperm produced by a drone contains exactly the same genetic material because he doesn’t have pairs of chromosomes that can exchange genes during meiosis. This means that a drone cannot produce sperm with distinct combinations of genes. Note the contrast in egg production whereby every egg produced by the queen contains a unique sample of half her genes.  A drone’s gametes undergo something called incomplete meiosis which produces 4 identical sperm cells from each sperm-source cell (“spermatocyte”).

At some point in the millions of years of their history, honey bees evolved a means to overcome the problem that every sperm produced by a drone is genetically identical: the queen mates with many drones. This is called polyandry. The queen stores living sperm from between 1-59 drones. The average is 14.6 (1).

Unlike male honey bees, male humans are diploid and so have 2 sex chromosomes. A large one called the X chromosome and a small one called the Y chromosome. Female humans have 2 of the large X chromosomes. So, in humans, sex is determined by these two chromosomes.

In honey bees, sex is determined by something called a sex locus. It is a single gene on one chromosome, and it occurs in different forms called alleles within a population of honey bees. It used to be believed that there are only about 12 of these sex alleles, but recently geneticists have discovered many more, and modelling by Lechner et al (2) suggests that there may exist as many as 116-145 sex alleles among populations of Apis mellifera. The queen has 2 sex alleles but the drone, being haploid, has only one.

Having so many sex alleles in a honey bee population reduces the risk of producing infertile diplod drones which is what happens if a queen mates with a drone who shares one of her two sex alleles. A diploid drone will start to develop but its larva gives off an odour, or some other cue that is abnormal, and so they are usually cannibalised by worker bees in the nest. Inbreeding of a queen (i.e, when a queen mates with brothers), when might be suspected if one sees many empty cells in a brood frame, giving it a spotty appearance.

References:

(1)& (2) Koeniger G, Koeniger N, Ellis J, Connor C, (2014) Mating Biology of Honey Bees, Wicwas Press, Kalamazoo, Michigan.

Acknowledgement:

Thank you to Professor Tom Seeley for correcting gaffes in my draft copy.

Follow Up.

 The next part will cover some interesting features of drone development and appearance.