Imperfect clones without sons, males without fathers but with a maternal grandfather, twins through their fathers, sisters through their mothers, daughters as full or half-sisters …

Let us get straight to the point: honey bee genetics is truly unusual! With bees, one must set aside what is known from human reproduction, where each parent contributes half of the chromosomes—the mother via the egg, the father via the sperm. This is not the case in bees!

Consider, for example, the case of drones, the male members of the colony:

• They have a maternal grandfather (but no paternal grandfather) and no father at all.
• They can have daughters, granddaughters, and grandsons, but they cannot have sons.

Diploid and haploid

The cause of all these peculiarities lies in a system of sex determination (known as “haplodiploidy”) that is fundamentally different from that of humans. We know that all eggs (for example hen’s eggs—exactly the same in humans) must be fertilised to produce a chick or a human child, male or female.

In the curious world of bees, however, a male is produced from an egg laid by the queen without fertilisation by sperm. This reproductive process from unfertilised eggs is called parthenogenesis. The male bee (the drone) is said to be haploid because it carries only a single set of chromosomes.

The more familiar situation, in which the queen fertilises an egg with sperm, results in a female. This female egg is destined to become either a worker or a queen. Female bees are described as diploid because, unlike males, their chromosomes occur in pairs.

In the beginning was the egg …

In humans, both mother and father each pass on half of their genetic material, that is, half of their chromosomes (a process known as chromosomal reduction). We can therefore say that we are, in a sense, only half-children of our parents, since each contributes only half of their 46 chromosomes: 23 from the mother via the egg and 23 from the father via the sperm. This inheritance explains our resemblance to both parents.

In bees, the same principle applies, but only for females. Female bees result from fertilised eggs. As in humans, the combination of the queen’s 16 chromosomes and the drone’s 16 chromosomes gives rise to a female bee. Unlike in humans, however, a fertilised egg in bees always produces a female, never a male.

On the male side, matters become more complex … As seen, males (drones) possess only one copy of each chromosome and are therefore haploid: they have only 16 chromosomes, all derived from their mother, the queen. Female bees, being diploid, possess 32 chromosomes derived from both their mother and their father.

In summary :

16 chromosomes from the queen + 16 chromosomes from drone sperm = female (worker or queen) with 32 chromosomes.

16 chromosomes from the queen + 0 chromosomes from drone sperm = male with only 16 chromosomes.

 

Mixing versus cloning

Females are thus the result of a mixture of chromosomes originating from drone sperm and from the queen’s eggs. The number of possible chromosomal combinations is therefore very large.

Males, on the other hand, result from identical sets of 16 chromosomes. Drones are therefore nothing more than imperfect clones of their mother: they inherit her entire genetic makeup, but only in the form of 16 chromosomes. Consequently, the queen’s eggs ensure genetic variability, while the ten to twenty spermatozoa stored in the spermatheca provide a certain degree of genetic stability.

The benefits of genetic diversity in bees

Genetic diversity is fundamental for bees. Where genetic diversity is limited, a bee population is potentially vulnerable to the same diseases or parasites. If a threat strikes such a population, the impact can be devastating.

By contrast, in a population with greater genetic diversity, a disease may affect only part of the population, while another part remains resistant. In practical terms, genetic diversity reduces the likelihood that a catastrophic event will wipe out an entire colony or apiary.

The role of the queen in genetic diversity

The queen’s mating behaviour actively promotes genetic diversity. During her mating flights, the queen mates with numerous drones. As these drones (some of which may originate from a radius of 10 to 15 km) have diverse genetic characteristics, this diversity is reflected in the queen’s female offspring.

These mating flights, which take place over several days, result in the collection of sperm from 10 to 20 drones. The queen stores this sperm in her spermatheca for the rest of her life.

Once back in the hive, she begins her egg-laying activity at a rate of about 2,000 eggs per day. The queen may fertilise or not fertilise each egg, thereby determining its sex. When she fertilises an egg, she draws from her large reserve of stored sperm. This means that although all bees in the colony share the same mother—the queen—the female offspring (workers) have different fathers. They are therefore both full sisters and half-sisters …

Drones, by contrast, have only a mother and no father. From a strictly genetic point of view, one should therefore not speak of the “father” of a bee, but rather of its “grandmother”: the drone is, in effect, a grandmother.

Royal jelly, the maker of queens

In bees, queens and workers are both females … So how do larvae become workers or queens when both possess chromosomes from the mother (the laying queen) and from a father (via drone sperm)?

The answer lies in nutrition with royal jelly, a food that is normally given to all larvae only during their first three days of life.

In most cases, feeding with royal jelly stops after three days, resulting in a worker (or, in the case of unfertilised eggs, a drone). As previously noted, the colony itself decides when it needs a new queen. When this is the case, a female larva continues to receive royal jelly beyond the usual three days.

This changes the way genes are “marked”, that is, how they are expressed. In particular, worker genes are inhibited, which by default allows queen genes to be expressed.

How certain breeding practices can weaken bee populations

Genetic diversity is important for bees, as it is for all species. Yet large-scale imports of foreign bee strains, the increasing use of hybrids selected primarily for honey yield rather than adaptation to local conditions, and migratory beekeeping, which contributes to genetic homogenisation, may in the long term seriously threaten the genetic heritage of bees. This, in turn, reduces their capacity to adapt to major challenges such as ecological crises or climate change.

 

Source: https://reinesdabeilles.fr/2018/01/18/introduction-a-la-genetique-des-abeilles/