Fly Pollination of Kettle Trap Flowers of Riocreuxia torulosa (Ceropegieae-Anisotominae): A Generalized System of Floral Deception

Elaborated kettle trap flowers to temporarily detain pollinators evolved independently in several angiosperm lineages. Intensive research on species of Aristolochia and Ceropegia recently illuminated how these specialized trap flowers attract particular pollinators through chemical deception. Morphologically similar trap flowers evolved in Riocreuxia; however, no data about floral rewards, pollinators, and chemical ecology were available for this plant group. Here we provide data on pollination ecology and floral chemistry of R. torulosa. Specifically, we determined flower visitors and pollinators, assessed pollen transfer efficiency, and analysed floral scent chemistry. R. torulosa flowers are myiophilous and predominantly pollinated by Nematocera. Pollinating Diptera included, in order of decreasing abundance, male and female Sciaridae, Ceratopogonidae, Scatopsidae, Chloropidae, and Phoridae. Approximately 16% of pollen removed from flowers was successfully exported to conspecific stigmas. The flowers emitted mainly ubiquitous terpenoids, most abundantly linalool, furanoid (Z)-linalool oxide, and (E)-β-ocimene—compounds typical of rewarding flowers and fruits. R. torulosa can be considered to use generalized food (and possibly also brood-site) deception to lure small nematocerous Diptera into their flowers. These results suggest that R. torulosa has a less specific pollination system than previously reported for other kettle trap flowers but is nevertheless specialized at the level of Diptera suborder Nematocera.

Male flowers of Aconitum compensate for toxic pollen with increased floral signals and rewards for pollinators

Many plants require animal pollinators for successful reproduction; these plants provide pollinator resources in pollen and nectar (rewards) and attract pollinators by specific cues (signals). In a seeming contradiction, some plants produce toxins such as alkaloids in their pollen and nectar, protecting their resources from ineffective pollinators. We investigated signals and rewards in the toxic, protandrous bee-pollinated plant Aconitum napellus, hypothesizing that male-phase flower reproductive success is pollinator-limited, which should favour higher levels of signals (odours) and rewards (nectar and pollen) compared with female-phase flowers. Furthermore, we expected insect visitors to forage only for nectar, due to the toxicity of pollen. We demonstrated that male-phase flowers emitted more volatile molecules and produced higher volumes of nectar than female-phase flowers. Alkaloids in pollen functioned as chemical defences, and were more diverse and more concentrated compared to the alkaloids in nectar. Visitors actively collected little pollen for larval food but consumed more of the less-toxic nectar. Toxic pollen remaining on the bee bodies promoted pollen transfer efficiency, facilitating pollination.

The role of hummingbirds in the evolution and diversification of Bromeliaceae: unsupported claims and untested hypotheses

At least half of the 3600 species of Bromeliaceae are pollinated by hummingbirds. There is little doubt that the four to 12 evolutionary shifts towards and c. 32 shifts away from hummingbird pollination opened new evolutionary spaces for bromeliad diversification, and that hummingbird pollination has led to increased bromeliad diversification rates. However, the mechanisms leading to these increased rates remain unclear. We here propose that there are four main types of mechanisms that may increase diversification rates of hummingbird-pollinated bromeliad clades: (1) bromeliad speciation through adaptation to different hummingbird species; (2) increased allopatric speciation in hummingbird-pollinated clades due to lower pollen transfer efficiency compared with other pollinators; (3) differential speciation rates in hummingbird-pollinated clades dependent on of flowering phenology and hummingbird behaviour; and (4) higher speciation rates of bromeliads in montane environments (where hummingbird pollination predominates) due to topographic population fragmentation. To date, none of these hypotheses has been appropriately tested, partly due to a lack of data, but also because research so far has focused on documenting the pattern of increased diversification in hummingbird-pollinated clades, implicitly assuming that this pattern supports an underlying mechanism while ignoring the fact that several competing mechanisms may be considered. The aim of the present review is to increase awareness of these mechanisms and to trigger research aimed at specifically testing them. We conclude that much additional research on the roles of hummingbird behaviour and gene flow between bromeliad species is needed to elucidate their contribution to the evolution of diversity in bromeliads and other plant families.

Pollinator effectiveness and importance between female and male mining bee ( Andrena )

Bees are often considered to be effective pollinators in both agricultural and natural ecosystems but could be ineffective pollinators in that they collect large quantities of pollen for food provision but deliver little to stigmas. Male bees do not collect pollen to feed larvae, and their pollination role has been underappreciated. Here we compare pollination effectiveness, visit frequency and pollen foraging behaviour between female and male individuals of a mining bee, Andrena emeishanica , visiting a nectariferous spring flower ( Epimedium pubescens ). Female bees were observed to forage for both pollen and nectar, but male bees foraged only for nectar. Female bees had large hairy hind tibiae with conspicuous scopae, and nearly 90% of the pollen grains they collected went onto the hind legs. Male bees removed less pollen from anthers than female bees but deposited more pollen on stigmas per visit. The higher pollen transfer efficiency of male bees was due to 48.4% of pollen grains remaining ungroomed on the thorax and abdomen, available for stigma contact, but their visitation rate to flowers was much lower. Our results indicate that male solitary bees could transfer more pollen on the stigma per visit but were less important (transferred less pollen in total, because they made fewer visits per unit time) than females.

Some Unresolved Issues of Measuring the Efficiency of Pollinators: Experimentally Testing and Assessing the Predictive Power of Different Methods

Knowledge of efficiency of pollinators is valuable in the derivation of (i) the degree of mutualism specialization of a flower visitor in the natural plant communities, (ii) the optimum number of pollinators needed for the maximum pollination in a plant population, and (iii) the pollinator risk assessment in the sustainable agriculture. Earlier researchers used many direct and indirect methods for measuring the pollination efficiency (PE) of flower visitors. However, a great ambiguity exists in the usage of this terminology that necessitated its fresh scrutiny. I tested the available three standard methods afresh to find the efficiency of pollinators. These included comparing the (i) number of pollen grains removed and deposited by the visitors; (ii) seed set resulting from a single and the multiple visits of the visitors; and (iii) “pollen transfer efficiency (PTE)” derived from the foraging behavior and abundances of the visitors. Observations were recorded on the visitors of four plant species in an agroecosystem of Northwest India. These plants represented a wide variety of the floral types across the angiosperms. The first two methods, namely, the “number of pollen grains removed and deposited” and the “seed set resulting from a single and the multiple visits,” were appropriate in finding differences between the efficiency ranks of the pollinators of those flowers where the number of deposited pollen grains was less than the number of ovules in the ovary. However, these two methods completely failed in situations where exactly reverse condition of pollen grains and ovules existed. Thus, these two methods of measuring the PE of visitors had limited approach and lacked a universal application over the entire angiosperm taxa. On the other hand, use of “pollen transfer efficiency”, derived from the foraging behavior and abundance of the visitors, seemed to have an edge over the other two methods as this was helpful in finding differences between the efficiency ranks of the pollinators of plants in all the three situations tested in this study. However, validation of all the three methods through the plant reproductive potential seemed to be an integral confirmatory step for drawing inferences about the efficiency of pollinators.

Flowering phenology and trap pollination of the rare endemic plant Ceropegia thaithongiae in montane forest of northern Thailand

Although Ceropegia species are well known for their complex pitfall flowers that temporarily imprison their pollinators, various aspects of their pollination ecology are still unknown. This study investigated flowering phenology, functional floral traits, and insect visitation in a natural population of a rare endemic lithophyte, Ceropegia thaithongiae Kidyoo. Flowering of C. thaithongiae was not synchronous but staggered. Anthesis lasted mostly 1–2 days, but its duration was shorter in flowers with a pollinium inserted into the stigmatic chamber. Several different insects visited flowers, but only chloropid and milichiid flies were effective pollinators. At anthesis, the epithelial osmophores on the corolla lobes emitted a floral scent that was simple in composition. Nectar of high viscosity was exuded from the nectaries hidden behind the guide rails. When transported by an insect, the pollinarium was attached to bristles on the mouthparts. Size and shape of the thin pellucid margin of the pollinium enable it to fit into the stigmatic chamber in a lock-and-key arrangement. The pollen transfer efficiency was 6.8%. The plant’s staggered flowering and pollination-induced ending of anthesis are advantageous in decreasing competition for pollinators when flower-visiting insects are scarce.