Trick or Treat? Pollinator attraction in Vanilla pompona (Orchidaceae)

ABSTRACTNatural pollination of species belonging to the pantropical orchid genus Vanilla remains poorly understood. Based on sporadic records, euglossine bees have been observed visiting flowers of Neotropical Vanilla species. Our research aimed at better understanding the pollinator attraction mechanism of the Neotropical species Vanilla pompona, a crop wild relative with valuable traits for vanilla crop improvement programs. Using video footage, we identified floral visitors and examined their behavior. The flowers of V. pompona attracted Eulaema cingulata males, which distinctively displayed two behaviors: floral scent collection and nectar search; with the latter leading to pollen removal. Morphological measurements of floral and visitor traits showed that other Eulaema species may also act as potential pollinators. Additionally, we recorded natural fruit set in three populations and over a period of two years, tested for nectar presence and analyzed floral fragrances through gas chromatography - mass spectrometry. We observed a low natural fruit set (2.42%) and did not detect nectar. Twenty floral volatile compounds were identified, with the dominant compound trans-carvone oxide previously found to attract Eulaema cingulata males. We hypothesize a dual attraction of Eulaema cingulata males to V. pompona flowers, based on floral fragrance reward as the primary long-distance attraction, and food deception for successful pollen removal. Further research confirming this hypothesis is recommended to develop appropriate conservation policies for Vanilla crop wild relatives, which are the primary reserves of this crop’s genetic variation.

Bee-Mediated Selection Favors Floral Sex Specialization in a Heterantherous Species: Strategies to Solve the Pollen Dilemma

Animal-pollinated plants show a broad variation in floral morphology traits and gametophyte production within populations. Thus, floral traits related to plant reproduction and sexuality are usually exposed to pollinator-mediated selection. Such selective pressures may be even stronger in heterantherous and pollen flowers, in which pollen contributes to both bee feeding and pollination, overcoming the “pollen dilemma” or the inability to perform both functions simultaneously. We describe the phenotypic gender and sexual organ morphology of flowers in two populations of Macairea radula (Melastomataceae), a heterantherous and buzz-pollinated species with pollen flowers. We estimated selection gradients on these traits through female and male fitness components. Both populations showed sizeable phenotypic gender variation, from strict hermaphrodites to increased femaleness or maleness. We found a continuous variation in style and stamen size, and this variation was correlated with corresponding shape values of both sexual organs. We detected bee-mediated selection towards short and long styles through seed number and towards intermediate degrees of heteranthery through pollen removal in one population, and selection towards increased maleness through pollen dispersal in both populations. Our results suggest that bee-mediated selection favors floral sex specialization and stylar dimorphism in M. radula, optimizing reproductive success and solving the pollen dilemma.

Are flowers tuned to buzzing pollinators? Variation in the natural frequency of stamens with different morphologies and its relationship to bee vibrations

During buzz pollination, bees use vibrations to remove pollen from flowers. Vibrations at the natural frequency of pollen-carrying stamens are amplified through resonance, resulting in higher-amplitude vibrations. Because pollen release depends on vibration amplitude, bees could increase pollen removal by vibrating at the natural frequency of stamens. Yet, few studies have characterized the natural frequencies of stamens and compared them to frequencies of buzz-pollinating bees. Here we use laser Doppler vibrometry to characterise natural frequencies of stamens of six buzz-pollinated Solanum taxa of contrasting stamen morphology. We also compare the fundamental frequency of bumblebee buzzes produced on two Solanum species with different natural frequencies. We found that stamen morphology and plant identity explain variation in natural frequency of stamens. Our results show that medium-sized pollinators, such as bumblebees, produce buzzes of frequencies higher than the natural frequency of most (5/6) of the Solanum species we studied. However, the observed natural frequency of Solanum stamens is at the low end of the range of frequencies produced by other buzz-pollinating bees. Thus, our findings suggest that in some buzz pollination interactions, but not others, stamen resonance may play a role in mediating pollen release.

Floral vibrations by buzz-pollinating bees achieve higher frequency, velocity and acceleration than flight and defence vibrations

Vibrations play an important role in insect behaviour. In bees, vibrations are used in a variety of contexts including communication, as a warning signal to deter predators and during pollen foraging. However, little is known about how the biomechanical properties of bee vibrations vary across multiple behaviours within a species. In this study, we compared the properties of vibrations produced by Bombus terrestris audax (Hymenoptera: Apidae) workers in three contexts: during flight, during defensive buzzing, and in floral vibrations produced during pollen foraging on two buzz-pollinated plants (Solanum, Solanaceae). Using laser vibrometry, we were able to obtain contactless measures of both the frequency and amplitude of the thoracic vibrations of bees across the three behaviours. Despite all three types of vibrations being produced by the same power flight muscles, we found clear differences in the mechanical properties of the vibrations produced in different contexts. Both floral and defensive buzzes had higher frequency and amplitude velocity, acceleration, and displacement than the vibrations produced during flight. Floral vibrations had the highest frequency, amplitude velocity and acceleration of all the behaviours studied. Vibration amplitude, and in particular acceleration, of floral vibrations has been suggested as the key property for removing pollen from buzz-pollinated anthers. By increasing frequency and amplitude velocity and acceleration of their vibrations during vibratory pollen collection, foraging bees may be able to maximise pollen removal from flowers, although their foraging decisions are likely to be influenced by the presumably high cost of producing floral vibrations.

Accurate position exchange of stamen and stigma by movement in opposite direction resolves the herkogamy dilemma in a protandrous plant, Ajuga decumbens (Labiatae)

Herkogamy is an effective way to reduce sexual interference. However, the separation of stigma and anther potentially leads to a conflict because the pollen may be placed in a location on the pollinator different from the point of stigma contact, which can reduce pollination accuracy. Floral mechanisms aiming to resolve this conflict have seldom been explored. The floral biology of protandrous Ajuga decumbens was studied to uncover how the herkogamy dilemma can be resolved. Flower anthesis was divided into male, middle, female and wilting phases. The positions of stigma and stamen were dissimilar in different flower development stages. We measured the distance of the stamen and stigma to the lower corolla lip at different floral phases, which was the pollinators’ approaching way. The pollen viability, stigma receptivity, pollen removal and pollen deposition on stigma were investigated at different phases. During the male phase, the dehisced anthers were lower than the stigma, located at the pollinators’ approaching way, and dispersed most pollen with high viability. As the flower developed, the anthers moved upwards, making way for pollen deposition during the female phase. Meanwhile, the stigma becomes receptive by moving into the way and consequently was deposited with sufficient pollen. The position exchange of the stamen and stigma created a dynamic herkogamy at the floral phase with different sexual functions. This floral mechanism effectively avoided sexual interference and maintained pollination accuracy. In Ajuga, the movement herkogamy might be of adaptive significance in response to the changes in the pollination environment.

Staminodes influence pollen removal and deposition rates in nectar-rewarding self-incompatible Phanera yunnanensis (Caesalpinioideae)

Staminodes are sterile stamens that produce no pollen, exhibit diverse structures and perform various functions. Flowers of Phanera yunnanensis possess three fertile stamens with large anthers and long filaments, and seven staminodes with tiny anthers and short filaments. To investigate the adaptive significance of staminodes in this species, we studied effects of staminode removal on pollen removal and deposition, flower visitation rate and fruit set in Xishuangbanna, south-western China. Four species of nectar-foraging pollinators visited flowers, mostly Amegilla zonata and Apis cerana (2.80 ± 0.15 and 1.76 ± 0.41 visits h−1 per flower, respectively). Staminode removal did not affect fruit set, but increased visitation by A. cerana by 2.6-fold, reduced visitation by A. zonata by 68% and increased the pollen removal rate for both pollinators (all effects were significant). Staminode removal significantly reduced pollen deposition rate for A. zonata, but not for A. cerana. These results suggest that the staminodes of P. yunnanensis filter which insects act as pollinators and affect pollen removal and deposition rates. By reducing pollen removal rates, staminodes may implement a pollen-dispensing schedule that spreads pollen dispersal from individual flowers over multiple pollinators. By altering pollen deposition rates, staminodes may influence reproductive fitness in other ways.

Feasibility of Ceiling-Mounted Assist Device of Air-Purifier for Removal of Airborne Allergic Pollen Grains

Feasibility of a ceiling-mounted assist device of the air-purifier for removal of airborne allergenic pollen grains is investigated by both turbulent flow and particle-tracking calculations. The device is mounted straight above the air-purifier and it collects suspended pollen grains in the exhaust flow of the air-purifier. It is found from the turbulent flow calculation that the flow rate of the assist device should be larger than that of the air-purifier. Otherwise the upward air flows around the assist device, and pollen grains move along the surrounding flow; they are never removed from the air. We also found about 40% improvement of the pollen removal efficiency by installing the assist device.