Multimodal floral cues resembling both generalized food sources and oviposition sites in Isotrema manshuriense (Aristolochiaceae) pollinated by flies.

Abstract

The genus Aristolochia is renowned for its kettle-like trap flowers that temporarily retain visiting insects, primarily using trapping trichomes. Most Aristolochia species attract fly pollinators through floral mimicry, by imitating their oviposition sites and/or food sources. In this study, we investigated the potential mimicry system of Isotrema manshuriense, a species formerly classified as Aristolochia manshuriensis that lacks trapping trichomes. We identified its pollinators and analysed its multimodal floral cues. Anthesis was divided into three sexual phases: female phase, interphase, and male phase. Insects entrapped in flowers were collected and identified. Limb secretion from female-phase flowers was analysed using GC-MS for metabolite profiling. Visual and olfactory cues of female- and male-phase flowers were investigated by measuring the spectral reflectance of spatial flower parts. External and internal floral volatile compounds were analysed during the day and at night. Floral scent profiles were compared to those of other species and potential models through non-metric multidimensional scaling. Most flower visitors were female flies, with pollinators in the Fanniidae, Anthomyiidae, and Lonchaeidae. There was no significant difference in number of retained insects between female- and male-phase flowers. Limb secretion contained various amino acids and organic acids. Spatial and temporal variations were observed in perianth coloration, including UV reflectance, and floral scent composition. Notably, external floral scents resembled those of cucurbit fruits, whereas internal scents were more similar to those of rewarding flowers. I. manshuriense exhibits unique floral traits, distinct from other brood site-mimicking Isotrema and Aristolochia species, while sharing the saprophagous nature of pollinators. This suggests that I. manshuriense incorporates multiple models, positioning flowers as phenotypic intermediates between generalized food sources and oviposition sites.