Honeybee Genetics: Polyandry, Breeding, Inbreeding Risk

Honeybees don't inherit sex the way mammals do. They're haplodiploid: queens and workers develop from fertilised eggs and are diploid, carrying two sets of 16 chromosomes (2n=32), while drones develop from unfertilised eggs and are haploid, carrying just one set (n=16). This single fact ripples through almost everything unusual about bee genetics and colony social structure, including relatedness patterns that don't map neatly onto our human intuitions: full sisters (sharing the same father) are related to each other at r=0.75 — more closely related than human siblings — while half-sisters (different fathers, same mother) are related at only r=0.25.

A queen typically mates with somewhere between 12 and 18 drones during her mating flight(s), storing sperm from all of them for use throughout her life. This extreme polyandry (multiple mating) isn't incidental — genetic diversity within a colony has a measurable payoff for colony-level resilience. In modelled colonies, those with 15 or more distinct patrilines (father lineages) show a roughly 34% higher three-year survival rate than colonies with fewer than 7 patrilines (about 88% versus under 35%). The proposed mechanism is straightforward: a genetically diverse workforce is less likely to be uniformly vulnerable to any single disease strain or environmental stressor, since different patrilines carry different resistance traits.

Selective breeding programmes for honeybees typically target traits with a real, heritable genetic basis, most notably hygienic behaviour — the tendency of workers to detect and remove diseased or mite-infested brood before it can spread infection further. A colony model tracking roughly 170 functional loci across 9 trait clusters might assign hygienic behaviour a heritability of h²=0.85 and Varroa Sensitive Hygiene (VSH), a related and increasingly important trait, h²=0.78. High heritability estimates like these (whether from a model or genuine published apicultural genetics research) are exactly what makes selective breeding for disease resistance a realistic long-term strategy, rather than one that has to rely purely on chemical treatment.

Sex in honeybees is determined at a single gene locus known as the csd locus, and this creates a specific, well-documented genetic hazard: if a fertilised egg happens to be homozygous at this locus (inheriting the same csd allele from both parents), it develops into a diploid male rather than a female worker or queen. Colonies detect and eat these diploid male larvae shortly after hatching, since they're non-viable as functional colony members — a real and substantial energetic waste. In small or isolated populations with limited allelic diversity at the csd locus, diploid males become more common; models often flag diploid male frequency above 5% of male brood as a 'critical inbreeding' warning, corresponding to a broader inbreeding coefficient above roughly F=0.30, associated with a genuine fitness collapse in the population. This is a real risk for geographically or genetically isolated bee populations, not just a modelling artefact.

Commercial and hobbyist beekeeping recognises several named genetic lines or 'breeds,' each associated with a rough package of trait tendencies — gentleness, honey yield, winter hardiness, swarming tendency, and disease resistance among them. Commonly referenced lines include Italian, Carniolan, Buckfast, Nordic Dark, Cape, and dedicated VSH (Varroa Sensitive Hygiene) lines, each representing a different tradeoff a beekeeper might choose depending on climate and priorities. As with any livestock breed description, these trait profiles are generalisations rather than guarantees for any individual colony. To see genetic diversity and inbreeding risk actually play out across simulated generations, try the Beehive Colony: Agent-Based Model simulation's genetics module.