Abstract: Co-flowering species in sympatry sharing a common pollinator can mitigate potential reproductive interference through a combination of ethological and mechanical barriers. However, empirical studies integrating both sex-specific foraging behavior and mechanical trait divergence within a specialized pollination system have been rarely conducted. To bridge this gap, the floral rewards, floral visitors, foraging behavior, visitation frequency, pollen-transfer efficiency, and reproductive isolation were compared between two sympatric Lysimachia species: oil-flowered L. congestiflora and nectar-bearing L. stenosepala, which share the oil-collecting bee Macropis omeiensis. Furthermore, we examined the mechanics of spatial partitioning by detecting the precise sites of pollen placement and stigma contact on the bee body for both species. In L. congestiflora, female M. omeiensis bees collected floral oil and pollen, and their pollen-transfer efficiency (pollen deposition/pollen removal) was higher than that of other bees (Halictus sp. and Lasioglossum occidens). While in L. stenosepala, both female and male M. omeiensis foraged only for nectar, acting as efficient pollinators. Pollen placement sites on the female oil bee were ventral in L. congestiflora and on the head in L. stenosepala. Male M. omeiensis bees visited the nectar-bearing flower, but not the oil flower. Female bees visited both oil-offering and nectar-bearing species; yet, differential pollen placement further reduced interspecific pollen transfer. These results suggest that mechanical isolation via spatial pollen placement on shared female bees is a key mechanism for reducing interspecific pollen transfer, while the specialized behavior of male bees provides a complementary barrier. This combination of floral reward divergence and sex-specific foraging behavior facilitates the coexistence of sympatric congeners.

Key words: Lysimachia-Macropis pollination mutualism, Macropis omeiensis, male bee, oil-collecting bee, oil flower, pollen placement, reproductive interference