Pollination by long‐proboscid horseflies and its implications for reproductive isolation among coflowering Satyrium orchids in South Africa

PREMISE: Floral adaptations to pollinators can drive lineage diversification and promote coexistence of species. We investigated the reproductive biology of Satyrium longicolle, a South African orchid that we hypothesized to belong to a long-proboscid horsefly pollination guild and examined overlap of pollinators and floral traits among coflowering sympatric Satyrium species. METHODS: We observed pollinators across different sites and years, carried out experimental pollinations, and measured spectral reflectance, scent chemistry, floral dimensions and nectar properties for both S. longicolle, and sympatric congeners and other species belonging to the horsefly pollination guild. Spectral reflectance was analyzed in appropriate insect vision models. RESULTS: Satyrium longicolle has weakly scented flowers that are pollinated diurnally by long-proboscid Philoliche horseflies, which ignored the strongly scented flowers of sympatric moth-pollinated congeners. The cream UV-reflecting flowers of S. longicolle and other guild members differed in color, but not achromatic contrast, from the white UV-absorbing flowers of hawkmoth-pollinated species. Although spur lengths and nectar properties are similar among the sympatric Satyrium species, spurs of S. longicolle are horizontal, while spurs of hawkmoth-pollinated congeners are pendant, coinciding with different foraging behaviors of the respective pollinators. Spur length of S. longicolle co-varied geographically with horsefly proboscis length. The orchid is self-compatible but reliant on pollinators for seed production. Crosses with a sympatric moth-pollinated congener resulted in seeds with embryos. CONCLUSIONS: The novel evolution of a specialized horsefly pollination system within Satyrium contributes to phenotypic diversity in the genus and results in strong ethological reproductive isolation with sympatric moth-pollinated congeners.