The process of pollinator-driven evolution is best studied at the level of populations and among closely related plant species. Araceae provide a plant model for investigating plant–pollinator interactions, but few studies have investigated intraspecific variation in their pollination systems. Typhonium brownii (Araceae) is distributed widely across 2000 km from tropical to temperate latitudes in Australia, yet the existence of intraspecific variation and ecotypes has not been investigated. Typhonium brownii from five regions, potentially representing distinct taxa, and populations of the sister species, T. eliosurum, were studied to explore pollinator and floral trait divergence. We characterize significant intraspecific floral trait variation in T. brownii, indicating the existence of a species complex, despite the taxa trapping similar Coleoptera (Staphylinidae, Scarabaeidae). Although all T. brownii showed similar temperature increases in the appendix, there were significant shifts in the timing and pattern of thermogenic and anthesis rhythms between regions (taxa), and all T. brownii taxa had distinct scent compositions, with T. sp. aff. brownii being the most dissimilar to other taxa. In contrast, T. eliosurum inflorescences almost exclusively trapped Diptera (Sphaeroceridae, Psychodidae), had modest temperature increases confined to the staminate zone and had a distinct scent profile which differed from all T. brownii taxa; this scent was confirmed in field bioassays to be important for pollinator attraction. Prevalent volatile organic compounds (VOCs) emitted by T. eliosurum and T. brownii taxa included the common dung constituents skatole, indole and p-cresol. Typhonium eliosurum and T. brownii taxa further differed significantly in morphology and trapping mechanisms, particularly the fly-pollinated T. eliosurum. It is possible that a subset of ubiquitous VOCs identified in T. eliosurum and T. brownii taxa attract local communities of dung-seeking flies and beetles, and that floral morphological features are more important for trapping different insect orders in these dung mimics.
Background Flowers which imitate insect oviposition sites likely represent the most widespread form of floral mimicry, exhibit the most diverse floral signals and are visited by two of the most speciose and advanced taxa of insect – beetles and flies. Detailed comparative studies on brood-site mimics pollinated exclusively by each of these insect orders are lacking, limiting our understanding of floral trait adaptation to different pollinator groups in these deceptive systems. Methods Two closely related and apparent brood-site mimics Typhonium angustilobum and T. wilbertii (Araceae) observed to trap these distinct beetle and fly pollinator groups were used to investigate potential divergence in floral signals and traits most likely under pollinator-mediated selection. Trapped pollinators were identified and their relative abundances enumerated, and thermogenic, visual, and chemical signals and morphological traits were examined using thermocouples and qRT-PCR, reflectance, gas chromatography-mass spectrometry, floral measurements and microscopy. Key Results T. angustilobum and T. wilbertii were functionally specialised to trap saprophagous Coleoptera and Diptera, respectively. Both species shared similar colour and thermogenic traits and contained two highly homologous AOX genes (AOX1a and AOX1b) most expressed in the thermogenic tissue and stage (unlike pUCP). Scent during the pistillate stage differed markedly - T. angustilobum emitted a complex blend of sesquiterpenes, and T. wilbertii, a dung mimic, high relative amounts of skatole, p-cresol, and irregular terpenes. Species differed significantly in floral morphology related to trapping mechanisms. Conclusions Functional specialisation and pollinator divergence were not associated with differences in anthesis rhythm and floral thermogenic or visual signals between species, but with significant differences in floral scent and morphological features, suggesting these floral traits are critical for the attraction and filtering of beetle or fly pollinators in these two brood-site mimics.
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