Loidolt, F. | Search bioRxiv

bioRxiv2026

A Polygenic Route to Thermal Melanism and high-elevation adaptation in Honey Bees

Thermal melanism, whereby darker pigmentation occurs in colder environments, is a widespread adaptive pattern, yet its genetic and physiological basis in eusocial insects remains poorly understood. East African honey bees (Apis mellifera) occupy steep elevational gradients in which highland populations are darker, larger, and more cold tolerant than lowland conspecifics. Here, we integrated population genomics, structural variation, reciprocal translocations, and functional genomics to dissect the basis of elevational melanism in honey bees. 139 workers from five East African mountain systems were sampled and phenotyped, and whole-genome resequencing was performed. A genome-wide association study identified 977 SNPs significantly associated with abdominal pigmentation, with a single pronounced peak at the ebony ortholog AmEbony on chromosome CM009931.2. Allelic variation at AmEbony formed three genetic clusters that closely tracked a continuous pigmentation gradient, and 77 highly divergent SNPs (DXY = 1) almost perfectly discriminated dark from light bees. Two large inversions (r7, r9), previously linked to high-elevation adaptation, were detected across additional mountain systems and were enriched in dark, highland genomic backgrounds, but did not replace AmEbony as the primary pigmentation locus. Reciprocal highland-lowland translocations revealed lineage-specific yet convergent expression shifts along detoxification/immune, chemosensory, proteostasis, and cuticle axes. CRISPR/Cas9-mediated disruption of AmEbony in A. m. carnica altered pigmentation and induced coherent changes in head transcriptomes, notably in odorant-binding and redox-related modules. Our results demonstrate that thermal melanism in a eusocial pollinator is governed by a top-heavy polygenic architecture centered on AmEbony, linking mechanistically naturally segregating pigmentation alleles to gene regulatory reprogramming and high-elevation adaptation.

evolutionary biology↗

bioRxiv2026

Octopaminergic signaling contributes to thermal adaptation to elevation in African honey bees (Apis mellifera)

Adaptation to local environments enables species to thrive in diverse and challenging habitats. Steep elevational gradients provide a compelling natural adaptation laboratory, because abiotic conditions change progressively over short geographical differences. Given that elevation can strongly reshape physiology and behavior of insects, neuromodulatory systems offer a promising lens through which to examine elevation-specific adaptation. We challenged the hypothesis that adaptation to elevation involves octopaminergic signaling in honey bees (Apis mellifera), an important pollinator species occupying different elevations along East African mountains. We collected foragers from two distinct elevations at Mount Kenya (1,150 m and 1,900 m above sea level) and analyzed elevation-dependent changes in octopaminergic signaling. Tissue-specific analysis revealed a striking upregulation of all three octopamine {beta} receptor genes in the thoracic flight muscles and elevated octopamine brain concentrations at high elevation. Expression differences in the brain and fat body were rather modest. We subjected CRISPR/Cas9-mediated octopamine {beta}2 receptor knockouts to cold stress to study the function of octopaminergic signaling in thermoregulation. Loss of AmOAR{beta}2 reduced both the slope and amplitude of heating phases, indicating altered thermogenic dynamics. Together, these results identify the octopaminergic system as a central neuromodulatory regulator of thermogenic performance across elevations in honey bees. More broadly, our study highlights how modulation of conserved aminergic signaling pathways can shape physiological resilience to environmental gradients, pointing to a general mechanism by which insects adapt to changing thermal landscapes. Highlights- Bees from high and low elevation differ in expression of octopamine {beta} receptor genes and octopamine brain concentrations - CRISPR/Cas9-mediated octopamine receptor knockout alters thermogenic behavior - Octopaminergic signaling emerges as a key neuromodulator in thermal adaptation to elevation in honey bees Significance statementAnimals living along mountain gradients must cope with rapidly changing temperatures, yet the mechanisms enabling this adaptation remain poorly understood. We show that honey bees from higher elevations have increased brain octopamine levels and enhanced expression of octopamine receptors in heat-producing flight muscles. Using gene editing, we demonstrate that disrupting one key receptor alters how bees generate heat under cold stress. These findings identify octopamine signaling as a central regulator of thermogenesis and reveal a mechanism by which insects adjust to colder environments. More broadly, our results highlight how conserved neuromodulatory systems can fine-tune physiological performance, offering insight into how insects may respond to changing climates and expanding environmental extremes.

physiology↗