Ivory-eyed drones: what a mutation teaches us about honey bee genetics
Why a small mutation can teach us so much
Honey bee breeding considers complex traits such as gentleness, hygienic behaviour, colony development and productivity. Occasionally, however, a very rare genetic variant makes it possible to see an evolutionary process that is normally difficult to document with exceptional clarity. This is the case of ivory-eyed drones studied by a group of researchers and beekeepers in the United States.
The study, published in July 2026 in Molecular Biology and Evolution, followed a recessive mutation from its identification in a queen through to its disappearance, reconstructing its genetic, physiological and behavioural consequences. It does not describe a useful production trait to select for: it provides a concrete example of how natural selection works in honey bees.
Haplodiploidy: why drones are a special case
In honey bees, workers and queens are generally diploid, whereas drones normally develop from unfertilised eggs and possess a single set of chromosomes. This system, known as haplodiploidy, allows recessive genetic variants to be expressed directly in males: there is no second copy of the same gene capable of masking their effect.
In females, by contrast, a recessive mutation can remain hidden in the heterozygous state. This difference has important implications when considering maternal lines and the origin of drones. It does not, however, mean that every recessive trait is harmful or that haplodiploidy prevents every unfavourable mutation from persisting.
The initial observation: ivory-eyed drones
Working with beekeepers, the researchers identified a normal-looking queen that produced both ordinary-eyed drones and others with abnormal ivory pigmentation. Her diploid daughters did not express the phenotype. The colony was monitored, and drone production observed over several seasons.
Ivory-eyed drones were less common than expected from simple one-to-one segregation of a recessive allele. Approximately one in four emerged drones had ivory eyes, consistent with possible adverse effects on early survival, but this alone does not establish exactly when the losses occurred.
A mutation associated with the cinnabar gene
Genomic analysis and other molecular methods identified a small sequence change in cinnabar, a gene involved in metabolic pathways essential to eye pigmentation. The researchers examined gene expression, proteins and metabolites to link the molecular alteration to the appearance of the eyes.
This multiomics approach goes beyond a simple visual correlation. The observed variant disrupted production of the substances responsible for pigmentation. Yet an obvious phenotype such as eye colour does not encompass all the mutation's effects: the most consequential effects emerged from behavioural studies.
Drone size was not the problem
Ivory-eyed drones showed no significant differences from their normal-eyed brothers in the main morphological and reproductive measurements analysed. Simply examining the body or certain sperm parameters would therefore not have fully identified the problem.
In orientation tests, however, ivory-eyed drones had considerable difficulty finding their colony. In one trial, ten days after transfer to an isolated hive, only one of the 51 introduced ivory-eyed drones remained, compared with 83 of 192 normal drones. In another trial involving release at a distance, no ivory-eyed drones returned, whereas some controls did. The authors interpret these results as a severe orientation deficit.
Why the mutation disappeared
A drone that cannot orient itself and complete its flights properly has fewer reproductive opportunities. In the reported observations, the mutation did not persist in the study population and disappeared with the death of the carrier queen and her sons. This is an example of purifying selection, which reduces transmission of unfavourable variants.
The authors also discuss possible mortality during early development. This hypothesis should not be treated as an established fact: the behavioural evidence is more direct, whereas the precise dynamics of embryonic losses require further investigation.
What is the relationship with the csd gene?
The cinnabar mutation studied in these drones is distinct from csd, a gene involved in sex determination. These are two different genetic issues. Although both involve haploids, diploids and allelic variants, this does not justify diagnosing brood defects on the basis of a vague notion of “inbreeding”.
In genetic improvement, complex traits, risks associated with genetic variability and individual Mendelian defects should be considered separately. Pedigrees help formulate hypotheses and plan crosses, but verifiable observations are needed to establish which process is actually occurring.
A case study is not a trial across many colonies
To assess this work properly, its central biological unit must be identified: the mutation was found in the offspring of a single carrier queen. Numerous drones subsequently provided genetic, anatomical and behavioural measurements; nevertheless, this was neither a comparison of dozens of independent families nor a breeding trial involving Ligustica lines. The defensible conclusion concerns the mechanism observed for this particular variant. It cannot establish the frequency of other harmful mutations in Italian breeding operations, the quality of a strain or the breeding value of every family displaying abnormalities.
The difference from exploratory studies with three or five colonies per treatment lies in the scientific question. This study does not claim to compare two management practices: as a genetic case study, it reconstructs the connection between an identifiable mutation and a rare phenotype using converging molecular techniques. Replication at the individual level helps compare the two phenotypes within the offspring studied; generalising to populations and lines would require additional independent cases.
How the genetic association was verified
The researchers followed the appearance of ivory-eyed drones during the 2022 and 2023 seasons, documenting an imbalance between phenotypes relative to the expected one-to-one Mendelian proportion in three trials: Fisher's exact test gave p = 0.03. This result supports the loss or non-expression of part of the phenotype, but does not by itself identify the biological stage of any mortality. The authors considered alternatives, including worker egg laying, and the relatedness data did not support that explanation.
To map the variant, a genome-wide association study was performed on 50 ivory-eyed drones and 50 normal-eyed brothers, alongside other mapping strategies. The strongest association involved a change in the coding sequence of cinnabar on chromosome 4: a small insertion/deletion that interferes with correct RNA processing and protein synthesis. Gene expression, proteome and ommochrome-pathway metabolite analyses provided convergent evidence. The interpretation is supported by agreement between genetic and biochemical data rather than a single statistical value.
What evidence supports the orientation deficit?
Ten reproductive and morphological parameters, including body measurements and sperm characteristics, showed no statistically significant differences between ivory-eyed drones and their normal brothers in the reported trials. A negative result does not prove perfect biological equivalence: it means that no difference was detected with the samples and methods used. Observations of orientation flights, however, revealed a substantial discrepancy in return to the colony. Ten days after the experimental transfer, 1 of 51 ivory-eyed drones and 83 of 192 normal drones remained: these are observed fractions under that protocol, not universal probabilities of drone survival.
A second line of evidence concerning light responses and phototaxis is consistent with a visual deficit. It remains appropriate to distinguish directly recorded homing behaviour from natural mating success, which was not measured for every individual. For queen breeders, the methodological lesson is this: an easily visible trait can indicate broader functional effects, while a favourable morphological examination or sperm assessment does not automatically guarantee orientation ability and reproductive success.
Mendelian genetics, quantitative selection and the csd gene
The cinnabar gene is involved in a highly specific single-gene phenotype. Traits such as production, hygienic behaviour, gentleness and colony development instead depend on polygenic networks and interactions with the environment. The percentages observed in ivory-eyed drones cannot be used to estimate the heritability of production traits or to attribute diagnostic powers to a pedigree that it does not possess. Selection against unfavourable variants is also distinct from positive selection for complex breeding performance: it requires different indicators, experimental units and statistical methods.
It is also essential to distinguish cinnabar from the csd locus, which is involved in sex determination and the possible production of diploid males under particular allelic combinations. Irregular brood is not evidence of an eye mutation or a csd problem without further investigation. In organising a breeding programme, this research supports caution when interpreting exceptional phenotypes, the importance of traceability and genetic variability, and cooperation with qualified laboratories; it does not justify new procedures for indiscriminately eliminating lines.
Implications for ESA Regine and queen breeders
In work with Apis mellifera ligustica, monitoring paternal and maternal lines is important precisely because it allows matings to be studied and offspring followed. The ivory-eyed drone case nevertheless reminds us that a good programme cannot rely on a few laboratory indicators or a pedigree alone. Behaviour, function and colony performance also matter.
ESA Regine did not conduct this research and does not claim to be able to identify or eliminate every unfavourable genetic variant. The practical message is simpler: use genetics to make better decisions, preserve diversity and continue to assess what the animals actually express.
Frequently asked questions
Is a normal-looking drone necessarily a good breeder? No. Some problems emerge only through behavioural, physiological or genetic measurements.
Does haplodiploidy automatically eliminate harmful mutations? No. It can expose recessive variants to selection in males, but their fate depends on the severity of the effect, frequency, matings and population structure.
Scientific sources
Claire – ESA Regine editorial team
Comments