Scientific review | Research applied to beekeeping
Scientific review | Research applied to beekeeping
The problem observed in the apiary
A queen lays regularly, yet empty cells are scattered among areas of capped brood. Immediate questions arise: is the queen defective, is there a health problem or a genetic cause? A photograph of the comb alone cannot provide the answer.
Brood compactness is a useful indicator, not a diagnosis. Interpretation requires information about queen age, actual laying, eggs and larvae in cells, health and environmental conditions. One possible explanation is complementary sex determination, controlled by the csd gene.
How the csd gene works
In honey bees, the complementary sex determiner (csd) gene helps determine sexual development. A diploid individual heterozygous at the csd locus can develop as a female; a haploid individual from an unfertilised egg develops as a male. Diploid homozygosity at csd can instead result in diploid males.
Workers generally recognise and remove diploid males during early larval development. Their removal leaves empty cells and can produce a patchy or “mosaic” brood pattern. This can increase when queens and drones share csd alleles that permit homozygosity in offspring.
Why instrumental insemination is part of this research
A study published in Apidologie on 7 April 2026 presented non-lethal genotyping of queens and drones before instrumental insemination. Queen exuviae and small drone-leg samples enabled identification of specific csd variants.
The researchers deliberately planned matched-allele matings to produce diploid males and investigate the biological system. Seven queens began laying and showed the expected patchy pattern. This is an important experimental demonstration, not proof of widespread commercial adoption or a sufficient basis for visually identifying queens with genetic problems.
Not all patchy brood is caused by csd
Differential diagnosis is essential. Patchiness can also result from pathogen-associated larval mortality, nutritional shortages, Varroa and virus damage, temperature problems, colony management or queen reproductive quality. The observed pattern also depends on when cells are examined.
To begin assessment, note whether empty cells previously contained eggs or larvae, follow several brood cycles, check health and compare colonies of similar age and management. If the problem persists, select investigations according to symptoms rather than jumping straight to “inbreeding”.
Implications for a breeding programme
Pedigree control helps identify maternal and paternal lines and plan crosses with clear objectives. Intensive selection and controlled mating nevertheless require attention to genetic diversity, especially when few colonies contribute to the next generation.
Maintaining csd allele diversity matters: selecting for productivity alone is insufficient if the reproductive gene pool becomes too narrow. Instrumental insemination controls mating; it does not automatically guarantee better fitness. Quality must subsequently be assessed through daughter colonies, fertility and other traits in different environments.
What about naturally mated daughter queens?
A daughter mating with drones from different areas and a more genetically diverse population may encounter csd combinations different from those of her original colony. This can greatly reduce matched-mating risk, with the irregular brood pattern likely disappearing in the new colony.
Local mating conditions, drone population genetics and the actual cause of the defect remain decisive. Genetic hypotheses must therefore be distinguished from apiary observations.
ESA Regine’s perspective
For ESA Regine, Ligustica selection is a progressive, verifiable process: choosing colonies, identifying lines, planning crosses where appropriate, observing daughters and comparing results. Genetics helps formulate more precise questions; it does not remove colonies’ biological complexity.
We therefore do not present mating control as a universal solution. Diagnostic caution and preservation of diversity are part of breeding quality measured over time.
Frequently asked questions
Does mosaic brood prove inbreeding? No. It is nonspecific; matching csd alleles are only one possible explanation.
Does an instrumentally inseminated queen necessarily produce more compact brood? No. It depends on the breeding stock, reproductive quality, health and environment. Instrumental insemination is valuable for controlling crosses and evaluating progeny.
Scientific sources
Claire — ESA Regine editorial team
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