USE IN BREEDING AND SELECTION

a) Presentation of the technique

Instrumental insemination is a relatively common technique in beekeeping breeding and selection, with the first mention dating back to 1923 (work by Quinn; Woyke, 1968). It allows fertilization to be carried out with precise control over the male lineage used. It is therefore employed in two contexts: in a scientific approach to gain a better understanding of honey-bee genetics, or in a breeding context to enable more efficient selection and dissemination of a given genetic line.

b) Results obtained and comparison with conventional insemination

Queens inseminated with a single drone display specific characteristics. The first concerns the insemination volume, around 2 to 3 microlitres, compared with 8 to 10 microlitres in conventional insemination, as well as a lower number of spermatozoa, approximately three times fewer than in classical insemination (Bolten & Harbo, 1982). Since the quantity of sperm stored in the spermatheca is the primary factor explaining egg-laying duration and thus queen longevity (Baer et al., 2016; Al-Lawati, 2009), queens inseminated with a single drone have a shorter lifespan than conventionally inseminated queens. Their overwintering may therefore be more delicate, although it is possible (Nolan, 1932; Bienkowska, 2011; Jungels, 2014).

Insemination with a single unrelated drone can produce colonies whose development level is generally similar to that of colonies headed by conventionally inseminated queens during the first months (Harbo, 1999). However, the use of inbred inseminations can drastically reduce colony development (patchy brood, other symptoms, etc.).

Another characteristic of queens produced by this technique concerns their progeny: the resulting workers share the same mother and the same father. Consequently, all workers within the colony are full sisters (no subfamilies or half-sisters as in colonies headed by queens inseminated by multiple drones). This results in highly homogeneous colonies. Likewise, if daughter queens are grafted from queens inseminated with a single drone, the same homogeneity is observed. It is also possible to inseminate several similar queens using semen from a single drone through dilution, in order to work with colonies of very similar phenotype (Harbo, 1986).

c) Use in a selection approach

Single-drone insemination has been used for several years by some beekeepers in selection programmes. It may serve two objectives. First, it facilitates the detection of certain traits that are difficult to identify within a colony. A trait or behaviour expressed by 100% of the workers in a colony is easier to observe than one expressed only by a subfamily (approximately 8–10% of workers in a conventional colony). For traits governed by multiple genes, the inheritance mechanism is also easier to study.

A further advantage lies in improving the quality of progeny testing by producing more homogeneous daughter queens. Performance variability among daughter queens is reduced. The relatedness coefficient between two full-sister queens is 0.75, compared with 0.50 for half-sisters derived from a conventionally inseminated queen.

The use of inbred single-drone inseminations, such as aunt-nephew inseminations, allows both increased homogeneity of offspring (facilitating trait detection) and enhanced expression of certain difficult-to-observe traits, including recessive ones (Jungels, 2014). This approach is often oriented toward the search for varroa-resistant bees, as varroa resistance is a complex trait with a poorly understood inheritance mechanism (Harris, 2009). P. Jungels, a professional beekeeper in Luxembourg, has been using inbred single-drone insemination for about ten years in the breeding of varroa-resistant bees (Jungels, 2014).

Queens inseminated with a single drone are produced in spring (May–June). Varroa levels are assessed in August of the same year. Queens overwintered in Mini-Plus units are used the following year for grafting queens intended for natural mating in a Buckfast environment or for producing new single-drone-inseminated queens. Inbred single-drone queens (nephew) are generally studied outside the main selection scheme. The genetic material of interest isolated using this technique is reincorporated once sufficient knowledge has been gained.

Other beekeepers follow similar approaches. However, due to the highly uncertain overwintering success of these queens, they never form the core of a selection programme, but rather serve as complementary selection pathways to the main programme based on conventionally inseminated queens.

A recent study nevertheless moderates the interest of this technique for selection. Kistler et al. showed that the exclusive use of such queens within a selection scheme does not yield greater genetic progress and significantly increases the risk of inbreeding in the selected population.

d) Effects of inbreeding (insemination with related drones)

In honey bees, sex is determined by a single gene with several dozen different alleles. Heterozygous individuals develop into female workers or queens. Homozygous individuals become diploid males, which are eliminated by workers at the larval stage. As the level of inbreeding increases in a selection scheme, the probability of producing homozygous individuals for sex determination increases. This results in brood patterns where a significant proportion may be removed by the bees, reaching up to 50% in the case of fertilization by a single drone carrying an allele also present in the queen (Brückner, 1979). Such patchy brood has also been observed in colonies headed by queens inseminated in aunt-nephew crosses (P. Jungels, personal communication).

However, these visible inbreeding effects disappear in the next generation when subsequent fertilization occurs naturally. If the local drone population is sufficiently genetically diverse, the probability of renewed homozygosity is low. If a queen is inbred but produces non-inbred workers, these workers may in some way compensate for the queen’s inbreeding with respect to production parameters such as honey or wax (Bienefeld, 1989).

USE FOR SCIENTIFIC PURPOSES

For researchers, single-drone insemination makes it possible in particular to study variation in specific traits, the genetic basis of observed differences, and to facilitate selection (Rothenbuhler, 1960). It has been widely used in genetic research, for example in backcross experiments to study inheritance patterns of traits such as susceptibility to American foulbrood (Rothenbuhler, 1964).

The drones used for insemination may have varying degrees of relatedness to the queen. They may originate from different lineages (e.g. nephews produced by a sister queen) or from the queen’s own gametes (Harbo, 1986; Oldroyd, 1992; Kubasek, 1980).

 

REFERENCES

(References retained as in the original text)

Authors:

• Benjamin Basso (INRAE)

• Matthieu Guichard (Agroscope)

Publication: 27.11.2023

 https://itsap.asso.fr/articles/insemination-a-un-male-le-point-sur-cette-pratique