Noise Tolerant Photonic Bowtie Grating Environmental Sensor.

Resonant photonic refractive index sensors have made major advances based on their high sensitivity and contact-less readout capability, which is advantageous in many areas of science and technology. A major issue for the technological implementation of such sensors is their response to external influences, such as vibrations and temperature variations; the more sensitive a sensor, the more susceptible it also becomes to external influences. Here, we introduce a novel bowtie-shaped sensor that is highly responsive to refractive index variations while compensating for temperature changes and mechanical (linear and angular) vibrations. We exemplify its capability by demonstrating the detection of salinity to a precision of 0.1%, corresponding to 2.3 × 10-4 refractive index units in the presence of temperature fluctuations and mechanical vibrations. As a second exemplar, we detected bacteria growth in a pilot industrial environment. Our results demonstrate that it is possible to translate high sensitivity resonant photonic refractive index sensors into real-world environments.

Convergent evolutionary patterns of heterostyly across angiosperms support the pollination-precision hypothesis.

Since the insights by Charles Darwin, heterostyly, a floral polymorphism with morphs bearing stigmas and anthers at reciprocal heights, has become a model system for the study of natural selection. Based on his archetypal heterostylous flower, including regular symmetry, few stamens and a tube, Darwin hypothesised that heterostyly evolved to promote outcrossing through efficient pollen transfer between morphs involving different areas of a pollinator's body, thus proposing his seminal pollination-precision hypothesis. Here we update the number of heterostylous and other style-length polymorphic taxa to 247 genera belonging to 34 families, notably expanding known cases by 20%. Using phylogenetic and comparative analyses across the angiosperms, we show numerous independent origins of style-length polymorphism associated with actinomorphic, tubular flowers with a low number of sex organs, stamens fused to the corolla, and pollination by long-tongued insects. These associations provide support for the Darwinian pollination-precision hypothesis as a basis for convergent evolution of heterostyly across angiosperms.

Nectar cardenolides and floral volatiles mediate a specialized wasp pollination system.

Specialization in plant pollination systems can arise from traits that function as filters of flower visitors. This may involve chemical traits such as floral volatiles that selectively attract favoured visitors and non-volatile nectar constituents that selectively deter disfavoured visitors through taste or longer-term toxic effects or both. We explored the functions of floral chemical traits in the African milkweed Gomphocarpus physocarpus, which is pollinated almost exclusively by vespid wasps, despite having nectar that is highly accessible to other insects such as honeybees. We demonstrated that the nectar of wasp-pollinated G. physocarpus contains cardenolides that had greater toxic effects on Apis mellifera honeybees than on Vespula germanica wasps, and also reduced feeding rates by honeybees. Behavioural experiments using natural compositions of nectar compounds showed that these interactions are mediated by non-volatile nectar chemistry. We also identified volatile compounds with acetic acid as a main component in the floral scent of G. physocarpus that elicited electrophysiological responses in wasp antennae. Mixtures of these compounds were behaviourally effective for attraction of V. germanica wasps. The results show the importance of both volatile and non-volatile chemical traits as filters that lead to specialization in plant pollination systems.

The economy of pollen dispersal in flowering plants.

Mating success of flowering plants depends strongly on the efficiencies of pollen removal from flowers and its subsequent dispersal to conspecific stigmas. We characterized the economy of pollen dispersal in flowering plants by analysing pollen fates and their correlates for 228 species. The mean percentage of pollen removed from flowers (removal efficiency) varied almost twofold according to the type of pollen-dispersal unit, from less than 45% for orchids and milkweeds with solid pollinia, to greater than 80% for species with granular monads or sectile (segmented) pollinia. The mean percentage of removed pollen reaching stigmas (pollen transfer efficiency, PTE) varied from 2.4% for species with separate monads to 27.0% for orchids with solid pollinia. These values tended to be higher in plants with single pollinator species and in those with non-grooming pollinators. Nectar production increased removal efficiency, but did not influence PTE. Among types of pollen-dispersal units, the net percentage of produced pollen that was dispersed to stigmas varied negatively with removal efficiency and positively with PTE, indicating the relative importance of the latter for overall pollen economy. These findings confirm the key importance of floral traits, particularly pollen packaging, for pollen dispersal outcomes and highlight the under-appreciated pollination efficiency of non-grooming pollinators.

Effect of fuchsin fixation of pollen on DNA barcode recovery.

Pollen grains attached to insects are a valuable source of ecological information which can be used to reconstruct visitation networks. Morphological pollen identification relies on light microscopy with pollen usually stained and mounted in fuchsin jelly, which is also used to remove pollen from the bodies of insects. Pollen embedded in fuchsin jelly could potentially be used for DNA barcoding and metabarcoding (large-scale taxonomic identification of complex mixed samples) and thus provide additional information for pollination networks. In this study, we determine whether fuchsin-embedded pollen can be used for downstream molecular applications. We evaluate the quality of plant barcode (ITS) sequences amplified from DNA extracted from both fresh (untreated) pollen, and pollen which had been embedded in fuchsin jelly. We show that the addition of fuchsin to DNA extraction does not impact DNA barcode sequence quality during short-term storage. DNA extractions from both untreated and fuchsin-treated pollen produced reliable barcode sequences of high quality. Our findings suggest that pollen which has been collected, stained, and embedded in fuchsin jelly for preliminary microscopy work can be used within several days for downstream genetic analysis, though the quality of DNA from pollen stored in fuchsin jelly for extended periods is yet to be established.

The enantiostylous floral polymorphism of Barberetta aurea (Haemodoraceae) facilitates wing pollination by syrphid flies.

BACKGROUND AND AIMS: Sexual polymorphisms of flowers have traditionally been interpreted as devices that promote cross-pollination, but they may also represent adaptations for exploiting particular pollination niches in local environments. The cross-pollination function of enantiostyly, characterized by flowers having either left- or right-deflected styles, has been uncertain in some lineages, such as the Haemodoraceae, because the positioning of stamens and styles is not always completely reciprocal among morphs. METHODS: We examined the floral biology of populations of the poorly known species Barberetta aurea (Haemodoraceae) across its native range in South Africa to establish the general features of its enanatiostylous reproductive system and the agents and mechanism of pollen transfer. RESULTS: We confirmed that B. aurea has a system of dimorphic enantiostyly. Style morph ratios varied among populations sampled, but with an overall tendency to being equal. Crossing experiments demonstrated that B. aurea is fully self-compatible, that intra- and inter-morph crosses are equally fertile and that it is wholly dependent on pollinator visits for seed production. Pollination is mainly by syrphid flies that transfer the sticky pollen via their wings, which contact the anthers and stigma precisely as they hover during approach and feeding. The majority of syrphid fly visitors feed on a film of highly concentrated nectar situated at the base of ultraviolet-absorbent 'nectar guides'. Because one of the three stamens is deflected in the same direction as the style, we predicted a high likelihood of intra-morph pollination, and this was corroborated by patterns of transfer of coloured dye particles in cage experiments involving syrphid flies. CONCLUSIONS: Barbaretta aurea exhibits dimorphic enantiostyly and, in contrast to most enantiostylous species, which are pollinated by bees, its flowers are specialized for pollination by syrphid flies. The lack of complete reciprocity of the enantiostylous arrangement of sexual organs facilitates both inter- and intra-morph pollen transfer on the wings of these flies.

Steven D. Johnson.

Interview with Steve Johnson, who works on plant-pollinator interactions at the University of KwaZulu-Natal in South Africa.

Beyond pollen:ovule ratios: Evolutionary consequences of pollinator dependence and pollination efficiency for pollen and ovule production in angiosperms.

PREMISE: The relative per-flower production of ovules and pollen varies broadly with angiosperm mating systems, with outcrossing types commonly producing more pollen grains per ovule than selfing types. The evolutionary causes of this variation are contentious, especially the relevance of pollination risk. Resolution of this debate may have been hampered by its focus on pollen:ovule (P:O) ratios rather than on the evolution of pollen and ovule numbers per se. METHODS: Using published mean ovule and pollen counts, we analyzed associations with the proportion of removed pollen that reaches stigmas (pollen-transfer efficiency) and differences between pollinator-dependent and autogamous forms within and among species. Analyses involved Bayesian methods that simultaneously considered variation in pollen and ovule numbers and accounted for phylogenetic relatedness. We also assessed the utility of P:O ratios as mating-system proxies and their association with female outcrossing rates. RESULTS: Median pollen number declined consistently with pollen-transfer efficiency among species, whereas median ovule number did not. Similarly, in both intraspecific and interspecific analyses, pollinator-dependent plants produced more pollen than autogamous plants, whereas ovule production did not differ statistically. Distributions of P:O ratios overlapped extensively for self-incompatible and self-compatible species and for different mating-system classes, and P:O ratios correlated weakly with outcrossing rate. CONCLUSIONS: Our findings demonstrate that pollinator dependence and pollination efficiency commonly influence the evolution of pollen number per flower but have more limited effects on ovule number. P:O ratios provide ambiguous, possibly misleading, information about mating systems, especially when compared among clades.

'Bleeding' flowers of Ceropegia gerrardii (Apocynaceae-Asclepiadoideae) mimic wounded insects to attract kleptoparasitic fly pollinators.

Kleptomyiophily, where flowers imitate wounded insects to attract 'kleptoparasitic' flies as pollinators, is one of the most specialized types of floral mimicry and often involves physical trapping devices. However, the diversity of pollinators and functional floral traits involved in this form of mimicry remain poorly understood. We report a novel example of kleptomyiophily in the nontrapping flowers of Ceropegia gerrardii and explore the floral traits responsible for attracting pollinators. The pollinators, reproductive biology and floral traits (including epidermal surfaces, spectral reflectance and the composition of nectariferous petal secretions and scent) were investigated. Attractive volatiles were identified using electrophysiological and behavioural experiments. Ceropegia gerrardii was predominantly pollinated by kleptoparasitic Desmometopa spp. (Milichiidae) flies. The flower corollas extrude a protein- and sugar-containing secretion, similar to the haemolymph of wounded insects, on which the flies feed. Floral scent was chemically similar to that of injured honey bees. Four out of 24 electrophysiologically active compounds, all released by injured honey bees, were identified as key players in pollinator attraction. Our results suggest that C. gerrardii flowers chemically mimic wounded honey bees to attract kleptoparasitic flies and reward them with a secretion similar to the haemolymph on which they would normally feed.

The lady's 'slippery' orchid: functions of the floral trap and aphid mimicry in a hoverfly-pollinated Phragmipedium species in Brazil.

BACKGROUND AND AIMS: Trap flowers are fascinating cases of adaptation, often linked to oviposition-site mimicry systems. Some trap flowers do not imprison pollinators for a pre-determined period, but rather force them to move through a specific path, manipulating their movements in a way that culminates in pollen transfer, often as they leave through a secondary opening. METHODS: We investigated the previously unknown pollination system of the lady's slipper orchid Phragmipedium vittatum and assessed the function of micro-morphological traits of its trap flowers. KEY RESULTS: Our observations revealed that P. vittatum is pollinated by females of two hoverfly species (Syrphidae). Eggs laid by flies on or near raised black spots on the flowers indicate that the orchid mimics aphids which serve as food for their aphidophagous larvae. Dark, elevated aphid-like spots appear to attract the attention of hoverflies to a slipping zone. This region has downward projecting papillate cells and mucilage secretion that promote slipperiness, causing potential pollinators to fall into the labellum. They then follow a specific upward route towards inner aphid-like spots by holding onto upward oriented hairs that aid their grip. As hoverflies are funnelled by the lateral constriction of the labellum, they pass the stigma, depositing pollen they may be carrying. Later, they squeeze under one of the articulated anthers which places pollen smears onto their upper thorax. Then, they depart through one of the narrow lateral holes by holding onto hairs projecting from the petals. CONCLUSIONS: This study confirms the system of aphid mimicry in Phragmipedium and highlights the sophisticated micro-morphological traits used by trap flowers in pollinator attraction, trapping, guidance and release, thus promoting precise pollen transfer.

Bird pollination.

Johnson gives an overview of bird pollinators and the plant species they pollinate.

Antibiotic-functionalized gold nanoparticles for the detection of active ß-lactamases.

Antimicrobial resistance (AMR) continues to threaten the effective treatment and prevention of bacterial infections. The spread of resistant infections is accelerated by the lack of fast and cost-effective tests for the detection of AMR at the point-of-care. We aimed to address this challenge by developing a diagnostic tool to detect one of the major forms of AMR, the ß-lactamase enzymes. Antibiotic-functionalized gold nanoparticles (AuNPs) have been successfully developed for the detection of ß-lactamases in challenging biological media, namely undiluted urine. Furthermore, these tools are compatible with samples containing a urine sample preservative (boric acid) or hematuria (blood). The functionalized AuNPs interact with the active ß-lactamases, resulting in the hydrolysis of the surface-bound antibiotics, which then inhibits binding of the AuNPs to a capture protein (a penicillin-binding protein) to indicate the presence of active ß-lactamases. We successfully integrated the antibiotic-functionalized AuNPs into a new lateral flow assay (LFA), which can be used to detect active ß-lactamases down to the detection limit of 11 nM. While we demonstrate the use of antibiotic-functionalized AuNPs in an LFA format to provide a novel method of detecting active ß-lactamases, these functionalized AuNPs are amenable to a range of alternative diagnostic technologies and could lead to vital point-of-care diagnostics for the early detection of multi-drug resistant infections.

Biomechanics of nectar feeding explain flower orientation in plants pollinated by long-proboscid flies.

The function of flower orientation is much debated, with adaptation to pollinator mouthparts being a particularly compelling explanation, but also one that has lacked empirical support from broad-scale comparative studies. The two families of long-proboscid fly pollinators show similar hovering behaviour while feeding on nectar but differ in the biomechanics of their proboscides which can be up to 80 mm in length: Tabanidae have a fixed forward-pointing proboscis while Nemestrinidae can swivel their proboscis downwards. We predicted that this difference has implications for the evolution of flower orientation. We established the flower angles of 156 South African plant species specialised for pollination by long-proboscid flies. Using a phylogenetically corrected analysis, we found that flowers pollinated by Tabanidae tend to be horizontally orientated, while those pollinated by Nemestrinidae tend to be more variable in orientation and more often vertically orientated. These results confirm the importance of pollinator biomechanics for the evolution of floral traits and highlight a potential mechanism of reproductive isolation between sympatric plant species pollinated by different long-proboscid fly families.

Why honeybees are poor pollinators of a mass-flowering plant: Experimental support for the low pollen quality hypothesis.

PREMISE: Honeybees dominate the flower-visitor assemblages of many plant species, yet their efficiency in terms of the quality of pollen delivered to stigmas is largely unknown. We investigated why honeybees are poor pollinators of Aloe ferox, a self-incompatible succulent treelet with large numbers of flowers. Honeybees are very frequent visitors to flowers of this species, yet contribute very little to seed production. METHODS: We assessed pollen loads on honeybees, studied their visitation behavior, selectively excluded birds from plants to determine direct effects of bees on pollen deposition, seed set, and ovule abortion, and used a novel "split-pollinator" method to test whether honeybees deposit mainly low-quality self pollen. For the latter, we captured honeybees, and with their existing pollen loads, used them to either pollinate virgin flowers on the plant on which they were caught or to pollinate virgin flowers on different plants. RESULTS: Honeybees cumulatively deposit as much pollen on stigmas as do birds, but our experiments showed that the pollen deposited by honeybees is mostly low-quality self pollen that leads to substantial ovule discounting and depressed seed set. CONCLUSIONS: Lack of movement among A. ferox plants during individual honeybee foraging bouts is the most likely explanation for their deposition of low-quality self pollen on stigmas. The "split-pollinator" method is a simple and cost-effective technique to test the quality of pollination.

Rodent responses to volatile compounds provide insights into the function of floral scent in mammal-pollinated plants.

Flowers pollinated by mammals have evolved in many plant families. Several scent compounds that attract bats to flowers have been identified, but the chemical ecology of pollination mutualisms between plants and ground-dwelling mammals is poorly understood. Rodents are key pollinators in South Africa and rely heavily on olfaction to locate food. Our aim was to identify compounds that may function to attract rodents to flowers. Eighteen volatile compounds, including 14 that are prominent in the scent of rodent-pollinated flowers, were used in choice experiments involving wild-caught individuals of four native rodent species. Rodents were generally attracted to oxygenated aliphatic compounds, specifically ketones and esters, but not to some aromatic compounds common in floral scents of insect-pollinated species, nor to a sulfide compound that is attractive to bats. Associative conditioning using sugar solution as a reward had only weak effects on the attractiveness of compounds to rodents. The attractive effect of some compounds disappeared when they were blended with compounds that did not attract rodents. We conclude that aliphatic ketones and esters are likely to play a key role in attracting rodents to flowers. Deployment of these compounds may allow plants to exploit rodent sensory bias that evolved in other contexts such as intra-specific communication and searching for seeds. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Florivory can facilitate rain-assisted autogamy in a deceptive tropical orchid.

Florivores and rainfall generally have negative impacts on plant fecundity. However, in some cases, they can mediate fruit set. Some plants face severe pollen-limited fecundity and any additional fruit set, even if from self-pollination, can be advantageous. This is the case in some tropical deceptive orchids, such as the threatened Cyrtopodium hatschbachii. Here we test the hypothesis that florivory of the anther cap would facilitate rain-assisted autogamy in this species. In the field, we followed flowers in which the anther cap was removed by the orthopteran Stenopola sp. and found cases where pollinia self-deposited after rainfall and in one case this resulted in swelling of the column typical of fruit development. This event comprised 33% of all fruit set in the population in 2019. We then experimentally varied anther cap removal and rainfall in a factorial design and found increased fruit set in the group with cap removal (simulated florivory) followed by rain. The water absorption by pollinia makes them heavier, causing the stipe to bend. The droplet of water on the stigma then shrinks and pulls the pollinia back onto the stigma, causing self-pollination. Seeds from self-pollination have considerable viability and may allow population persistence, given that bee-mediated cross-pollination is uncertain and even absent in some years. Our study provides a unique example of how two unrelated factors (i.e., florivory and rain) that are detrimental alone may together promote fruit set.

A global-scale expert assessment of drivers and risks associated with pollinator decline.

Pollinator decline has attracted global attention and substantial efforts are underway to respond through national pollinator strategies and action plans. These policy responses require clarity on what is driving pollinator decline and what risks it generates for society in different parts of the world. Using a formal expert elicitation process, we evaluated the relative regional and global importance of eight drivers of pollinator decline and ten consequent risks to human well-being. Our results indicate that global policy responses should focus on reducing pressure from changes in land cover and configuration, land management and pesticides, as these were considered very important drivers in most regions. We quantify how the importance of drivers and risks from pollinator decline, differ among regions. For example, losing access to managed pollinators was considered a serious risk only for people in North America, whereas yield instability in pollinator-dependent crops was classed as a serious or high risk in four regions but only a moderate risk in Europe and North America. Overall, perceived risks were substantially higher in the Global South. Despite extensive research on pollinator decline, our analysis reveals considerable scientific uncertainty about what this means for human society.

Fly Pollination of Kettle Trap Flowers of Riocreuxiatorulosa (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)-ß-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.

Optimising backscatter from multiple beam interference.

Optical sensing applications are usually reliant on the intensity of the measured signal. For remote sensing applications where a target is probed with a laser beam, the sensitivity will be limited by the amount of backscattered light returned from the target to the detector. We demonstrate a method of increasing the signal returned to the detector by illuminating the target with a number of independently controlled beams, where both the position and phase are optimised. We show an improvement in the backscattered signal that is proportional to the number of beams used. The method is demonstrated within a laser microphone, measuring audio signal due to vibrations in surfaces, showing a significant improvement in the signal-to-noise of the measurement.