1. The queen’s reproductive system

The queen’s reproductive system consists of two ovaries located in her abdomen, occupying almost its entire volume after fertilisation during the nuptial flight and mating with several males. Each ovary is composed of 150 to 180 ovarioles. These have the form of tubular filaments containing a succession of oocytes at different stages of development. Oocytes are the germ-line stem cells that will give rise to ovules, also known as female gametes or unfertilised eggs (male gametes are spermatozoa). The maturation of oocytes begins at the anterior end of the ovarioles, located in the pointed conical part of the ovaries near the junction between thorax and abdomen. Within the ovarioles are also nurse cells, called trophocytes, from which the oocytes draw nutrients during their development, allowing their differentiation into future ovules. At the posterior end of the ovaries, the ovarioles merge to form the oviducts (one on the right and one on the left). These two oviducts converge to form the median oviduct, which allows the ovule to progress towards the vaginal chamber. This vaginal chamber is the site where the unfertilised egg encounters the spermatozoon coming from the spermatheca. During the nuptial flight, the queen is fertilised by around fifteen drones, which transfer between 5 and 7 million spermatozoa. These are stored in the spermatheca, or seminal vesicle, a spherical structure with a capacity of about 1 ml. Surrounding the spermatheca is a very dense network of tracheae supplying the oxygen necessary for the survival of spermatozoa during their storage, which can last up to five years. Adjacent to the spermatheca is the Y-shaped gland, which maintains an alkaline pH of 9–9.5 and secretes nutrients for sperm metabolism. Just below this Y-gland, on the spermathecal duct, is a muscle acting as a valve and pump, regulating the release of a few spermatozoa destined to fertilise the ovule as it passes towards the vaginal valve.

Some figures: at the peak of the season, the queen lays up to 2,000 eggs per day, roughly equivalent to her own body weight. Over her 4–5 year lifespan, she may lay up to 500,000 eggs. During fertilisation in the nuptial flight, about fifteen drones provide 5–7 million spermatozoa, approximately 50,000 each. Under optimal conditions, the queen stores enough spermatozoa for around ten sperm to approach each ovule for fertilisation.

By comparison, in humans, the ovaries of a female foetus at the fifth month of pregnancy contain around 7 million oocytes. Most are eliminated, leaving only about 1 to 2 million at birth. After birth, no new oocytes are produced. At puberty, only around 300,000 oocytes remain, a number more than sufficient for the fertile period of life (1 ovule / 28 days × 30 years ≈ 450 ovules). In males, spermatogenesis—the formation of spermatozoa—takes place in the seminiferous tubules of the testes. It begins at puberty from stem cells called spermatogonia, which multiply by mitotic division and then by meiotic division, continuing sometimes into advanced age.

The scene is thus set…

2. But how does the queen manage to lay a fertilised, diploid egg or a haploid ovule (unfertilised)? (1)

It is recalled that wax combs contain so-called worker cells with a diameter of about 5.3 mm and so-called drone cells with a diameter of approximately 6.3 mm. Worker cells serve as brood nests for worker larvae and as storage sites for reserves (honey or pollen). Drone cells receive ovules (unfertilised eggs) from which drones emerge 24 days later. It has often been observed that during the laying period the queen carefully inspects the cells in which she is about to deposit an egg. She searches for impurities that would prompt her to choose another, cleaner cell. She also assesses the diameter of the cell. According to the work of Professor Martin Giurfa (2), bees are capable of counting up to five and organising numbers on a mental number line. They readily master relational concepts such as “smaller than” and “larger than”. Just before laying, the queen probes the cell with her forelegs and antennae and, using proprioceptive receptors known as mechanoreceptors (the fine hairs at the base of the legs and antennae), she measures not an absolute value in millimetres, but a comparison between the diameter of worker cells (“smaller than”) and that of drone cells (“larger than”).

• If the cell diameter appears “smaller”, she lays a (fertilised) egg by relaxing the valve muscle, allowing a few spermatozoa to pass from the spermatheca.
• If the cell diameter appears “larger”, she lays an ovule (unfertilised) by contracting the valve muscle, blocking the passage of spermatozoa.

Professor Joseph Hemmerlé reminds us “that workers initiate the construction of larger cells (6.3 mm in diameter instead of 5.3 mm) when drones need to be prepared for the fertilisation period. This is therefore an ‘architectural’ signal originating from the bee population, which in a way acts on the sex of future individuals. Nevertheless, things are somewhat more complex: the queen is not merely an executor. Indeed, she has the ‘freedom’ to choose the cells in which to lay (small or large) and thus to modulate the worker/drone ratio. Furthermore, there is ultimately a feedback loop controlled by workers, who are capable of influencing the final worker/drone ratio by not rearing (not feeding) drone ovules, or even by cannibalising them.”

3. Conclusion

Egg laying therefore results from complex mechanisms, modulated by a queen “skilled in mathematics” and whose reproductive system is a marvel of biological precision. The reproductive activity of this queen is also influenced by the worker population and by epigenetic factors (nectar and pollen intake, beekeeping season, etc.), confirming once again that the study of apiculture is truly fascinating.

4. Glossary

Meiosis: Meiosis is a process of double cell division occurring in germ-line cells to form gametes. This double division halves the genetic content of the resulting haploid nucleus.

Mitosis: Mitosis is the process by which a mother cell divides into two daughter cells. Nuclear division produces two genetically identical diploid nuclei.

Ovariole: Ovarioles are found in the ovaries of insects; they are tubular organs in which gametes form from germ-line stem cells located at the anterior end of the ovaries.

Oocyte: An oocyte is the primary female sex cell that will differentiate into an ovule after a more or less prolonged maturation.

Ovule: An ovule is the female sex cell or female gamete containing half of the genetic material, which after fertilisation by the male gamete (spermatozoon) enables sexual reproduction.

Trophocyte: A trophocyte is a cell differentiated into a nurse cell, supplying one or more other cells. Such cells are found, for example, in the ovarioles of meroistic insects and in the testes of various animals; Sertoli cells in seminiferous tubules are trophocytes.

Sources:

• (1) Introduction to honey bee genetics
• (2) Lecture materials by Prof. Martin Giurfa
• Guth: Bibliographic resources on Apis mellifera
• CARI – Biological overview of bee reproduction
• Bees can perform addition and subtraction
• L’Apiculture, une fascination, Vol. 2, VDRB Publishing, 2003