Multiple Mating Behaviour in Honey Bees... A Mechanism Compensating for Inbreeding
The mating of a queen honeybee to several and in some cases 'many' individual drones has been noted in various texts in the past. However many treatments of the phenomena have been limited to just mentioning it, in an almost embarrassed way owing to old fashioned ideas on human morality that were (and still are) propagated by many religious institutions.
Polyandry or multiple mating serves an essential function in honeybees. It is a mechanism that helps to avoid inbreeding. Generally the mating of closely related individuals results in inbreeding depression. This is characterised by spotty or 'pepperpot' brood pattern, reduced vigour, slow growth, reduced survivability and increased susceptibility to pests and diseases.
However, we should not fall into the trap of assuming that small colonies are necessarily weak or that docile and calm colonies are automatically inbred. Vigour may be a character of the F1 hybrid, but a smaller colony with calm, long lived and hard working bees can also produce honey in similar quantity, and do so under poor conditions that other strains cannot work under.
Genes exist in multiple forms, known as alleles. Normal females have two
different alleles for the sex gene and are therefore heterozygous. One of these alleles is derived from the chromosomes of an egg and the other from the chromosome of a sperm. If two identical sex
alleles align, (homozygous) it results in a diploid drone which is a lethal condition that workers are able to recognise soon after an egg hatches and they remove the offending larvae and eat it, providing a useful protein supplement. The empty cells caused by such removals, creates a spotty brood pattern (sometimes called 'shotgun' brood).
It has been variably estimated that there are between 6 and 19 different sex alleles in honey bee populations [Adams et.al. 1977]. Multiple mating and the number of sex alleles in the gene pool increases the probability of out-crossing, thus improving brood viability. Multiple mating also reduces the production of diploid males within colonies.
There are an estimated 6 to 19 sex alleles [more info coming]
Multiple mating and the haplo-diploid reproductive system also effect the degree of relatedness between sister groups within a colony. Workers fathered by different drones are half-sisters and have 25% of their genes in common. The workers who have the same father, have 75% of their genes in common and are thus known as super-sisters. Full sisters can exist, but are rare as the workers need to be fathered by brother drones. Such full-sisters have 50% of their genes in common.
The individual workers within subfamilies are able to recognise their relatedness and this 'kin recognition' influences interactions and behaviour. Workers that are rearing a queen may choose a larva that is more closely related to itself [Breed, 1985] or a worker may preferentially feed a close related worker larva rather than one from another sister group. Workers that are closely related to the queen may swarm with her.
The out-crossing mating habits exhibited in drone assembly mating increase genetic diversity enabling colonies to display a
wide range of behaviours and thus adapt and thrive in changing circumstances and different environments.
The converse of this is that stable populations that have evolved in particular circumstances are able to mate more closely both in sexual and geographic senses, because their characteristics 'fit' the pertaining circumstances. This is line breeding in the best possible manner as multiple matings along with an increased mating frequency that tends to occur in strains that can exist under severe conditions. The adaptability feature of outcrossing matings still exists, but is only available spasmodically under weather conditions that allow drone assembly.
There has been some suggestion that behavioral differences among subfamilies may fit some subgroups rather better to a particular task than others. Thus polyandry provides the different groups of bees for the so called 'division of labour' the sister groups that are themselves best adapted to the task in question naturally gravitating towards that task and fulfilling it. Whilst this notion seems attractive, I believe that much more work needs to be done on honey bees and other social species before sensible conclusions can be derived.
It has also been suggested that one advantage of having a wide range of genotypes within a population could be reduced transmission of parasites as some subsets would be more able than others to resist the parasite. However, this has not been demonstrated in honeybees. Indeed, John Harbo has found increased viability of the parasite Varroa Jacobsoni in genetically diverse colonies, when compared to genetically uniform colonies.
Although this page was written by myself it contains information that has been absorbed into it from an American Bee
Journal article entitled "Multiple Mating - A Mechanism to avoid Inbreeding" Author, at present unknown and a paper about the evolution of multiple mating by Kellie Palmer and Ben Oldroyd.