Beijing — In the pollination evolution of angiosperms, oil flowers represent a highly unique group. Approximately 2000 species across 11 families secrete lipids (rather than traditional nectar) through specialized oil glands as floral rewards. Oil-offering plants rely heavily on specialized oil-collecting bees for pollination, with females using specialized brushes on their legs to collect lipids for nest-building and larval provisioning. Among them, Macropis (Melittidae) is a classic genus of oil-collecting bees known for their strict oligolecty on the genus Lysimachia (Primulaceae). However, for decades, studies in Europe and North America generally assumed that Macropis bees primarily associate with oil-secreting loosestrifes, where males also patrol for mates. Prior to this work, specialized visits by Macropis bees—especially males—to non-oil, nectar-bearing Lysimachia species had not been documented.
As the global epicenter of diversification for Lysimachia , Asia has long lacked in-depth studies on its oil flower-oil bee mutualism. Recently, a research team led by Prof. Shuang-Quan Huang from the School of Life Sciences at Central China Normal University published a study in the Journal of Systematics and Evolution (JSE) entitled “Differentiation of floral rewards and pollen placement minimize reproductive interference between two sympatric Lysimachia species sharing an oil-collecting bee” (DOI: 10.1111/jse.70094). Located in the Asian center of Lysimachia oil flower evolution, this study provides the first empirical verification in Asia of the ecological value of male Macropis bees as high-efficiency pollinators, while revealing the isolation and coexistence mechanisms between two sympatric congeners—the oil flower Lysimachia congestiflora and the nectar-bearing Lysimachia stenosepala —when sharing the specialized oil bee Macropis omeiensis .
First Verification in Asia: Macropis Bees Specialize on Nectariferous Lysimachia
In sharp contrast to historical European observations where male Macropis bees seek mates on oil-secreting loosestrifes, the research team observed at the Jiucaiping Nature Reserve in Guizhou that male Macropis omeiensis completely ignore the oil-offering L. congestiflora . Instead, males exclusively visit, feed on nectar, and patrol for mates on the sympatric nectar-bearing congener L. stenosepala . This is the first empirical confirmation in Asia of the specialized utilization of nectar-producing Lysimachia flowers by Macropis bees.
Male Bees: High-Efficiency Pollinators
Long dismissed as mere “mating machines” with little role in pollination, male bees were re-evaluated through quantitative measurements by the research team. Male Macropis bees not only exhibited high visitation frequencies, but because they do not groom pollen into specialized leg baskets, the pollen grains adhering to their bodies remain loose and readily transferable. Quantitative experiments revealed that male M. omeiensis bees exhibited high visitation frequencies and superior single-visit pollen-transfer efficiencies, proving to be the primary and most effective pollinators for the nectar-bearing L. stenosepala . This finding fills a major knowledge gap regarding the functional role of male oil bees.
Reward Divergence and Spatial Precision Prevent Interference
In their interactions with the shared pollinator M. omeiensis , the oil flower L. congestiflora provides oils and pollen, while the nectar-bearing L. stenosepala offers nectar. This extreme reward divergence causes male bees to remain strictly confined to the nectar-bearing plants. Meanwhile, female bees must “cross-over” to forage (collecting oils and pollen from oil flowers for brood care, and nectar from nectar flowers for metabolic energy). To avoid reproductive interference, the two sympatric species have evolved precise mechanical isolation: L. congestiflora deposits pollen exclusively on the female’s ventral surface, thorax, and legs, whereas L. stenosepala neatly places pollen on the female's head. Furthermore, the stigma contact sites match these pollen placement positions, demonstrating precise, differential utilization of the pollinator’s body space by the two sympatric species.
This study represents the first comprehensive investigation of the Lysimachia - Macropis mutualism in the subtropical region. It broadens our understanding of Macropis behavior and provides the first proof of their specialized visits to nectar-bearing Lysimachia alongside the crucial pollination contributions of males. The findings demonstrate how closely related sympatric plants utilize reward divergence, sex-specific pollinator behaviors, and precise spatial pollen partitioning to resolve the evolutionary puzzle of avoiding reproductive interference and maintaining coexistence.
Significance for Plant-Pollinator Evolution
Since Stefan Vogel’s pioneering research over half a century ago, the oil flower–oil bee mutualism has been known to involve at least 11 angiosperm families and 3 bee families. Given that female oil bees require nectar for personal metabolic needs, they may frequently visit nearby co-flowering nectar plants, potentially risking interspecific pollen transfer. To our knowledge, this is the first documented case where the sexes of a single bee species functionally divide labor to act as primary pollinators for two sympatric, closely related plant species. Incorporating the pollination contributions of male bees offers a fresh perspective on how co-flowering plants sharing a single pollinator minimize reproductive interference. Future field studies quantifying natural interspecific pollen transfer and associated fitness costs will be crucial for clarifying the evolutionary stability of this specialized mutualism.
Original Source:
Bai-Zhu Li and Shuang-Quan Huang. 2026. Differentiation of floral rewards and pollen placement minimize reproductive interference between two sympatric Lysimachia species sharing an oil-collecting bee. Journal of Systematics and Evolution. https://onlinelibrary.wiley.com/doi/10.1111/jse.70094
Keywords:
Lysimachia – Macropis pollination mutualism, Macropis omeiensis , male bee, oil‐collecting bee, oil flower, pollen placement, reproductive interference.
Author's Institution:
Institute of Evolution and Ecology, School of Life Sciences, Central China Normal University, Wuhan, Hubei, China
About JSE:
Journal of Systematics and Evolution (JSE) is a peer-reviewed, bi-monthly international English-language journal dedicated to the understanding of biodiversity and evolution. Papers in any of the following and related fields will
be considered: systematics and biodiversity conservation, paleobiology and biogeography, adaptation, speciation and domestication, evolutionary genomics and Evo-Devo, theory and methods in evolutionary biology.
Journal of Systematics and Evolution
Differentiation of floral rewards and pollen placement minimize reproductive interference between two sympatric Lysimachia species sharing an oil-collecting bee.
13-Aug-2026