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Fertilised egg or unfertilised egg

The queen’s primary function is to lay eggs, enabling the emergence of all individuals that make up the population of a bee colony. The queen’s many other functions are not addressed in this brief article.

1. The queen’s reproductive system

The queen’s reproductive system consists of two ovaries located in her abdomen, where they occupy almost the 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 a tubular filament containing a succession of oocytes at different stages of development. Oocytes are the germ-line stem cells that will give rise to ovules, also called female gametes or unfertilised eggs (the male gametes are called 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 the thorax and the abdomen. Within the ovarioles there are also nurse cells called trophocytes, from which the oocytes draw nutrients during their development, enabling 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. This canal allows the ovule to progress toward the vaginal chamber. This vaginal chamber is the site where the unfertilised egg meets the spermatozoon originating from the spermatheca. During the nuptial flight, the queen is fertilised by around fifteen drones, which transmit between 5 and 7 million spermatozoa. These are stored in the spermatheca, or seminal vesicle, a kind of sphere with a capacity of about 1 ml. Surrounding this spermatheca is a very dense network of tracheae that supply the oxygen necessary for the survival of the spermatozoa during the five years of their storage. Attached to the spermatheca is the Y-shaped gland, which maintains an alkaline pH between 9 and 9.5 and secretes the nutrients required for sperm metabolism. Just below this Y-shaped gland, on the spermathecal duct, there is a muscle that serves as a valve and pump, regulating the flow of a few spermatozoa intended to fertilise the ovule as it passes toward the vaginal valve.

 

 

Fig. 1: dorsal view of the diagram of the queen’s sexual organs.

  

Fig. 2: lateral view of the diagram of the queen’s sexual organs.

  
Fig. 3: schematic view of the spermatheca, the spermathecal duct, and the muscular sphincter that allows or blocks the passage of spermatozoa toward the ovule (credit: L’apiculture, une fascination).  

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 years of life, the queen may lay up to 500,000 eggs. During fertilisation in the nuptial flight, about fifteen drones contribute 5–7 million spermatozoa, or around 50,000 each. If everything proceeds normally, the queen stores enough spermatozoa for about 10 spermatozoa to approach each ovule for fertilisation.

By comparison, in the human species, at the fifth month of pregnancy, the ovaries of a female fetus contain around 7 million oocytes. Most of these are eliminated, and at birth only about 1 to 2 million remain. After birth, no new oocytes are produced. At puberty, only around 300,000 oocytes are retained, a quantity more than sufficient for the fertile period of life (1 ovule every 28 days × 30 years = ~450 ovules). In males, spermatogenesis, or the formation of spermatozoa, takes place within the seminiferous tubules of the testes. It begins at puberty, from stem cells called spermatogonia, which multiply by simple cell division and then by meiotic division, sometimes continuing to a very 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)

We recall that wax combs contain so-called “worker” cells with a diameter of about 5.3 mm and so-called “male” cells with a diameter of around 6.3 mm. Worker cells serve as the nest for worker brood and as storage sites for reserves (honey or pollen). Male cells receive ovules (unfertilised eggs) from which drones emerge 24 days later. We have all observed that, during the laying period, the queen meticulously inspects the cells in which she is about to deposit an egg. She looks for impurities that would cause her to choose another, cleaner cell for laying. She also inspects the diameter of the cell. According to the work of Prof. Martin Giurfa (2), bees are capable of counting up to five and organising these numbers along a mental number line. They easily master relational concepts such as “smaller than” and “larger than.” Just before laying, the queen probes the cell with her forelegs and antennae and, thanks to proprioceptive receptors called mechanoreceptors (the famous 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 male cells (“larger than”).

  • If the diameter of the cell appears “smaller than,” she lays an egg (fertilised) by relaxing the valve muscle that allows the passage of a few spermatozoa from the spermatheca.
  • If the diameter of the cell appears “larger than,” she lays an ovule (unfertilised) by contracting the valve muscle, which blocks the passage of spermatozoa.

Prof. Joseph Hemmerlé reminds us that “workers initiate the construction of larger cells (6.3 mm in diameter instead of 5.3) when it is necessary to prepare males for the fertilisation period. This is therefore an ‘architectural’ signal originating from the bee population that, in a way, acts on the sex of future individuals. Nevertheless, things are somewhat more complex than that: the queen is not merely an executor. Indeed, the queen has the ‘leisure’ to choose laying cells (small or large) and thus modulate the worker/male ratio. And that is not all: there is ultimately a feedback loop, controlled by the workers, who are able to influence the final worker/male ratio by not rearing (not feeding) male ovules, or even by cannibalising them.”

 

Fig. 4: Mechanoreceptor hairs record angular variations of the joints.  

Fig. 5: Laying of an egg.  

 

3. In conclusion

Egg laying therefore results from complex mechanisms, modulated by a “mathematically gifted” queen whose reproductive system is a marvel of biological precision. However, 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 that the study of apiculture is truly fascinating.

 

4. Glossary

Meiosis: Meiosis is a process of double cell division that occurs in germ-line cells to form gametes. This double division halves the genetic material 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 are formed from germ-line stem cells located at the anterior end of the ovaries.

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

Ovule: The ovule is the female sex cell or female gamete, which contains half of the genetic material and, after fertilisation by the male gamete or spermatozoon, enables sexual reproduction.

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

 

Sources:

See also:

Author
www.ApiSion.ch, Claude Pfefferlé
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