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 and mechanical trait divergence within a specialized pollination system remain rare. To address these questions, 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 determined the mechanics of spatial partitioning by examining the precise sites of pollen placement and stigma contact on the bee body. 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 the other two bee taxa (Halictus spp 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. Females visited both oil and nectar-bearing species, yet differential pollen placement further reduced interspecific pollen transfer. These results demonstrate that mechanical isolation via spatial pollen placement on shared female bees is the primary mechanism reducing interspecific pollen transfer, while male behavioral specialization provides a complementary barrier. This combination of floral reward divergence and pollinator sex-specific foraging facilitates coexistence of sympatric congeners.

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