Beetle visitations, and associations with quantitative variation of attractants in floral odors of Homalomena propinqua (Araceae)

Can Pheromones Contribute to Phylogenetic Hypotheses? A Case Study of Chrysomelidae

Pheromones mediate species-level communication in the search for mates, nesting, and feeding sites. Although the role of pheromones has long been discussed by various authors, their existence was not proven until the mid-twentieth century when the first sex pheromone was identified. From this finding, much has been speculated about whether this communication mechanism has acted as a regulatory agent in the process of speciation, competition, and sexual selection since it acts as an intraspecific barrier. Chrysomelidae is one of the major Phytophaga lineages, with approximately 40,000 species. Due to this immense diversity the internal relationships remain unstable when analyzed only with morphological data, consequently recent efforts have been directed to molecular analyses to establish clarity for the relationships and found their respective monophyly. Therefore, our goals are twofold 1) to synthesize the current literature on Chrysomelidae sex pheromones and 2) to test whether Chrysomelidae sex pheromones and their chemical structures could be used in phylogenetic analysis for the group. The results show that, although this is the first analysis in Chrysomelidae to use pheromones as a phylogenetic character, much can be observed in agreement with previous analyses, thus confirming that pheromones, when known in their entirety within lineages, can be used as characters in phylogenetic analyses, bringing elucidation to the relationships and evolution of organisms.

How the volatile organic compounds emitted by corpse plant change through flowering

The corpse plant (Amorphophallus titanum) is so named because it produces a pungent, foul odor when flowering. Little is known about how the emitted volatiles change throughout the two-day flowering period. In this study, the comprehensive monitoring of the presence and change in volatile molecules during the female and the male flowering phases of A. titanum was conducted, and the plant temperature was monitored. A total of 422 volatile features were detected over the entire sampling period, of which 118 features were statistically significantly different between the pre-flowering and both flowering phases, and an additional 304 features were found present throughout the flowering period. A total of 45 molecules could be assigned putative names. The volatile profile of A. titanum changes over the two-day flowering period, with the S-containing molecules and aldehydes dominant in the female flowering phase, and the alcohols and hydrocarbons dominant in the male flowering phase. The two-dimensional gas chromatography time-of-flight mass spectrometry (GC × GC-TOFMS) enabled us to identify 32 new molecules produced by A. titanum. Each of these molecules alone, and in combination, likely contribute to the different odors emitted during the flowering phase of A. titanum.

The Genome-Wide Identification of Long Non-Coding RNAs Involved in Floral Thermogenesis in Nelumbo nucifera Gaertn

The sacred lotus (Nelumbo nucifera Gaertn.) can maintain a stable floral chamber temperature when blooming, despite ambient temperature fluctuations; however, the long non-coding RNAs (lncRNAs) involved in floral thermogenesis remain unclear. In the present study, we obtain comprehensive lncRNAs expression profiles from receptacles at five developmental stages by strand-specific RNA sequencing to reveal the lncRNAs regulatory mechanism of the floral thermogenesis of N. nucifera. A total of 22,693 transcripts were identified as lncRNAs, of which approximately 44.78% had stage-specific expression patterns. Subsequently, we identified 2579 differential expressed lncRNAs (DELs) regulating 2367 protein-coding genes mainly involved in receptacle development and reproductive process. Then, lncRNAs with floral thermogenesis identified by weighted gene co-expression network analysis (WGCNA) were mainly related to sulfur metabolism and mitochondrial electron transport chains. Meanwhile, 70 lncRNAs were predicted to act as endogenous target mimics (eTMs) for 29 miRNAs and participate in the regulation of 16 floral thermogenesis-related genes. Our dual luciferase reporter assays indicated that lncRNA LTCONS_00068702 acted as eTMs for miR164a_4 to regulate the expression of TrxL2 gene. These results deepen our understanding of the regulation mechanism of floral thermogenesis by lncRNAs and accumulate data for further research.

Reproductive biology of Syagrus coronata (Arecaceae): sex-biased insect visitation and the unusual case of scent emission by peduncular bracts

Several monoecious species of palms have developed complex strategies to promote cross-pollination, including the production of large quantities of floral resources and the emission of scents that are attractive to pollinators. Syagrus coronata constitutes an interesting model with which to understand the evolution of plant reproductive strategies in a monoecious species adapted to seasonally dry forests. We monitored blooming phenology over 1 year, during which we also collected and identified floral visitors and putative pollinators. We identified potential floral visitor attractants by characterizing the scent composition of inflorescences as well as of peduncular bracts, during both male and female phases, and the potential for floral thermogenesis. Syagrus coronata produces floral resources throughout the year. Its inflorescences are predominantly visited by a diverse assortment of small-sized beetles, whose richness and abundance vary throughout the different phases of anthesis. We did not find evidence of floral thermogenesis. A total of 23 volatile compounds were identified in the scent emitted by the inflorescences, which did not differ between male and female phases; whereas the scent of the peduncular bracts was composed of only 4-methyl guaiacol, which was absent in inflorescences. The composition of floral scent chemistry indicates that this palm has evolved strategies to be predominantly pollinated by small-sized weevils. Our study provides rare evidence of a non-floral scent emitting structure involved in pollinator attraction, only the second such case specifically in palms. The peculiarities of the reproductive strategy of S. coronata might play an important role in the maintenance of pollination services and pollen dispersion.

Biosynthetic and Functional Color-Scent Associations in Flowers of Papaver nudicaule and Their Impact on Pollinators

Despite increasing evidence for biosynthetic connections between flower pigments and volatile compounds, examples of such relationships in polymorphic plant species remains limited. Herein, color-scent associations in flowers from Papaver nudicaule (Papaveraceae) have been investigated. The spectral reflectance and scent composition of flowers of four color cultivars was determined. We found that pigments and volatiles occur in specific combinations in flowers of P. nudicaule. The presence of indole in the bouquets is strongly associated with the occurrence of yellow pigments called nudicaulins, for which indole is one of the final biosynthetic precursors. Whereas yellow flowers emit an excess of indole, orange flowers consume it during nudicaulin production and lack the substance in their bouquet. By using the honeybee, Apis mellifera, evaluations were made on how color and scent affect the discrimination of these flowers by pollinators. Honeybees were able to discriminate artificial odor mixtures resembling those of the natural flower odors. Bees trained with stimuli combining colors and odors showed an improved discrimination performance. The results indicate that the indole moiety of nudicaulins and emitted indole might be products of the same biochemical pathway. We propose that conserved pathways account for the evolution of color-scent associations in P. nudicaule and that these associations positively affect flower constancy of pollinators.

Diversity of pollination ecology in the Schismatoglottis Calyptrata Complex Clade (Araceae)

Field studies integrating pollination investigations with an assessment of floral scent composition and thermogenesis in tropical aroids are rather few. Thus, this study aimed to investigate the pollination biology of nine species belonging to Schismatoglottis Calyptrata Complex Clade. The flowering mechanism, visiting insect activities, reproductive system, thermogenesis and floral scent composition were examined. Anthesis for all species started at dawn and lasted 25-29 h. Colocasiomyia (Diptera, Drosophilidae) are considered the main pollinators for all the investigated species. Cycreon (Coleoptera, Hydrophilidae) are considered secondary pollinators as they are only present in seven of the nine host plants, despite the fact that they are the most effective pollen carrier, carrying up to 15 times more pollen grains than Colocasiomyia flies. However, the number of Colocasiomyia individuals was six times higher than Cycreon beetles. Chaloenus (Chrysomelidae, Galeuricinae) appeared to be an inadvertent pollinator. Atheta (Coleoptera, Staphylinidae) is considered a floral visitor in most investigated species of the Calyptrata Complex Clade in Sarawak, but a possible pollinator in S. muluensis. Chironomidae midges and pteromalid wasps are considered visitors in S. calyptrata. Thermogenesis in a biphasic pattern was observed in inflorescences of S. adducta, S. calyptrata, S. giamensis, S. pseudoniahensis and S. roh. The first peak occurred during pistillate anthesis; the second peak during staminate anthesis. Inflorescences of all investigated species of Calyptrata Complex Clade emitted four types of ester compound, with methyl ester-3-methyl-3-butenoic acid as a single major VOC (volatile organic compound). The appendix, pistillate zone, staminate zone and spathe emitted all these compounds. A mixed fly-beetle pollination system is considered an ancestral trait in the Calyptrata Complex Clade, persisting in Sarawak taxa, whereas the marked reduction of interpistillar staminodes in taxa from Peninsular Malaysia and especially, Ambon, Indonesia, is probably linked to a shift in these taxa to a fly-pollinated system.

Synopsis of the cyclocephaline scarab beetles (Coleoptera, Scarabaeidae, Dynastinae)

The cyclocephaline scarabs (Scarabaeidae: Dynastinae: Cyclocephalini) are a speciose tribe of beetles that include species that are ecologically and economically important as pollinators and pests of agriculture and turf. We provide an overview and synopsis of the 14 genera of Cyclocephalini that includes information on: 1) the taxonomic and nomenclatural history of the group; 2) diagnosis and identification of immature life-stages; 3) economic importance in agroecosystems; 4) natural enemies of these beetles; 5) use as food by humans; 6) the importance of adults as pollination mutualists; 7) fossil cyclocephalines and the evolution of the group; 8) generic-level identification of adults. We provide an expanded identification key to genera of world Cyclocephalini and diagnoses for each genus. Character illustrations and generic-level distribution maps are provided along with discussions on the relationships of the tribe’s genera.

Innate Receiver Bias: Its Role in the Ecology and Evolution of Plant–Animal Interactions

Receiver bias in plant–animal interactions is here defined as “selection mediated by behavioral responses of animals, where those responses have evolved in a context outside the interactions.” As a consequence, the responses are not necessarily linked to fitness gains in interacting animals. Thus, receiver bias can help explain seemingly maladaptive patterns of behavior in interacting animals and the evolution of plant traits that trigger such behavior. In this review, I discuss principles of receiver bias, show its overlap with mimicry and how it differs from mimicry, and outline examples in different plant–animal interactions. The most numerous and best documented examples of receiver bias occur within plant–pollinator interactions. I elaborate on the ability of some plants to heat up their flowers (i.e., floral thermogenesis) and argue that this trait likely evolved under receiver bias, especially in pollination systems with oviposition mimicry. Further examples include signals in insect-mediated seed dispersal and plant defense through repellence of aphids. These examples show that receiver bias is widespread in different plant–animal interactions. For a broader understanding of the role of receiver bias in those interactions, we need more data on how animals respond to plant signals, the context and evolutionary history of those behaviors, and the evolutionary patterns of plant signals.

Understanding intraspecific variation of floral scent in light of evolutionary ecology

Background and Aims

Among the various floral traits involved in pollinator attraction and potentially under selection mediated by pollinators, floral scent/fragrance has been less investigated than other components of floral phenotype. Whether or not pollinator-mediated selection impacts floral scents depends on the heritability of scent/fragrance and the occurrence of some variation within species. Although most studies have investigated how scent varies among species, growing amounts of data are available on variation at the intraspecific level.

Methods

The results of 81 studies investigating intraspecific variation of floral scents in 132 taxa were reviewed. For each study, whether variation was found in either identity, proportion or absolute quantities of volatile organic compounds (VOCs) was recorded, as well as information with the potential to explain variation, such as methodology, plant origin or pollination biology.

Key Results

Variation was found for almost all investigated species, both among individuals (among and sometimes within populations) and within individuals across different temporal scales. Cases in which such variation is a possible result of pollinator-mediated selection were analysed, by discussing separately selection related to variation in pollinator identity/behaviour among populations or across time, deceit pollination and sex-specific selection. Not surprisingly, in many cases, pollinator-mediated selection alone does not explain the observed variation in floral scent. This led us to review current knowledge on less investigated factors, such as selection mediated by natural enemies, genetic drift and gene flow, environmental constraints, phylogenetic inertia, or biochemical constraints that could be invoked to explain scent variation.

Conclusions

This review highlights the great potential of analysing floral scent variation and including it in integrated studies of floral phenotypes. We also have identified the current gaps in our understanding of this complex signal and we propose several methodological and conceptual future directions in this research area.

Floral characteristics and pollination ecology of Manglietia ventii (Magnoliaceae), a plant species with extremely small populations (PSESP) endemic to South Yunnan of China

Manglietia ventii is a highly endangered plant species endemic to Yunnan province in China, where there are only five known small populations. Despite abundant flowering there is very low fruit and seed set, and very few seedlings in natural populations, indicating problems with reproduction. The causes of low fecundity in M. ventii are not known, largely because of insufficient knowledge of the species pollination ecology and breeding system. We conducted observations and pollination experiments, and analyzed floral scents to understand the pollinator-plant interactions and the role of floral scent in this relationship, as well as the species breeding system. Like the majority of Magnoliaceae, M. ventii has protogynous and nocturnal flowers that emit a strong fragrance over two consecutive evenings. There is a closing period (the pre-staminate stage) during the process of anthesis of a flower, and we characterize the key flowering process as an "open-close-reopen" flowering rhythm with five distinct floral stages observed throughout the floral period of this species: pre-pistillate, pistillate, pre-staminate, staminate, and post-staminate. Flowers are in the pistillate stage during the first night of anthesis and enter the staminate stage the next night. During anthesis, floral scent emission occurs in the pistillate and staminate stages. The effective pollinators were weevils (Sitophilus sp.) and beetles (Anomala sp.), while the role of Rove beetles (Aleochara sp.) and thrips (Thrips sp.) in pollination of M. ventii appears to be minor or absent. The major chemical compounds of the floral scents were Limonene, β-Pinene, α-Pinene, 1,8-Cineole, Methyl-2-methylbutyrate, p-Cymene, Methyl-3-methyl-2-butenoate and 2-Methoxy-2-methyl-3-buten, and the relative proportions of these compounds varied between the pistillate and staminate stages. Production of these chemicals coincided with flower visitation by weevils and beetles. The results of pollination experiments suggest that M. ventii is pollinator-dependent, and low seed set in natural populations is a result of insufficient pollen deposition. Thus, conservation of the species should focus on improving pollination service through the introduction of genetically variable individuals and increase in density of reproducing trees.

Geographical matching of volatile signals and pollinator olfactory responses in a cycad brood-site mutualism

Brood-site mutualisms represent extreme levels of reciprocal specialization between plants and insect pollinators, raising questions about whether these mutualisms are mediated by volatile signals and whether these signals and insect responses to them covary geographically in a manner expected from coevolution. Cycads are an ancient plant lineage in which almost all extant species are pollinated through brood-site mutualisms with insects. We investigated whether volatile emissions and insect olfactory responses are matched across the distribution range of the African cycad Encephalartos villosus. This cycad species is pollinated by the same beetle species across its distribution, but cone volatile emissions are dominated by alkenes in northern populations, and by monoterpenes and a pyrazine compound in southern populations. In reciprocal choice experiments, insects chose the scent of cones from the local region over that of cones from the other region. Antennae of beetles from northern populations responded mainly to alkenes, while those of beetles from southern populations responded mainly to pyrazine. In bioassay experiments, beetles were most strongly attracted to alkenes in northern populations and to the pyrazine compound in southern populations. Geographical matching of cone volatiles and pollinator olfactory preference is consistent with coevolution in this specialized mutualism.

Flowering mechanisms, pollination strategies and floral scent analyses of syntopically co-flowering Homalomena spp. (Araceae) on Borneo

In this study, the flowering mechanisms and pollination strategies of seven species of the highly diverse genus Homalomena (Araceae) were investigated in native populations of West Sarawak, Borneo. The floral scent compositions were also recorded for six of these species. The selected taxa belong to three out of four complexes of the section Cyrtocladon (Hanneae, Giamensis and Borneensis). The species belonging to the Hanneae complex exhibited longer anthesis (53-62 h) than those of the Giamensis and Borneensis complexes (ca. 30 h). Species belonging to the Hanneae complex underwent two floral scent emission events in consecutive days, during the pistillate and staminate phases of anthesis. In species belonging to the Giamensis and Borneensis complexes, floral scent emission was only evident to the human nose during the pistillate phase. A total of 33 volatile organic compounds (VOCs) were detected in floral scent analyses of species belonging to the Hanneae complex, whereas 26 VOCs were found in samples of those belonging to the Giamensis complex. The floral scent blends contained uncommon compounds in high concentration, which could ensure pollinator discrimination. Our observations indicate that scarab beetles (Parastasia gestroi and P. nigripennis; Scarabaeidae, Rutelinae) are the pollinators of the investigated species of Homalomena, with Chaloenus schawalleri (Chrysomelidae, Galeuricinae) acting as a secondary pollinator. The pollinators utilise the inflorescence for food, mating opportunities and safe mating arena as rewards. Flower-breeding flies (Colocasiomyia nigricauda and C. aff. heterodonta; Diptera, Drosophilidae) and terrestrial hydrophilid beetles (Cycreon sp.; Coleoptera, Hydrophilidae) were also frequently recovered from inflorescences belonging to all studied species (except H. velutipedunculata), but they probably do not act as efficient pollinators. Future studies should investigate the post-mating isolating barriers among syntopically co-flowering Homalomena sharing the same visiting insects.

Floral diversity and pollination strategies of three rheophytic Schismatoglottideae (Araceae)

Homoplastic evolution of 'unique' morphological characteristics in the Schismatoglottideae - many previously used to define genera - prompted this study to compare morphology and function in connection with pollination biology for Aridarum nicolsonii, Phymatarum borneense and Schottarum sarikeense. Aridarum nicolsonii and P. borneense extrude pollen through a pair of horned thecae while S. sarikeense sheds pollen through a pair of pores on the thecae. Floral traits of spathe constriction, presence and movement of sterile structures on the spadix, the comparable role of horned thecae and thecae pores, the presence of stamen-associated calcium oxalate packages, and the timing of odour emission are discussed in the context of their roles in pollinator management. Pollinators for all investigated species were determined to be species of Colocasiomyia (Diptera: Drosophilidae).

Thermogenesis, flowering and the association with variation in floral odour attractants in Magnolia sprengeri (Magnoliaceae)

Magnolia sprengeri Pamp. is an ornamentally and ecologically important tree that blooms at cold temperatures in early spring. In this study, thermogenesis and variation in the chemical compounds of floral odours and insect visitation in relation to flowering cycles were studied to increase our understanding of the role of floral thermogenesis in the pollination biology of M. sprengeri. There were five distinct floral stages across the floral cycle of this species: pre-pistillate, pistillate, pre-staminate, staminate and post-staminate. Floral thermogenesis during anthesis and consisted of two distinct peaks: one at the pistillate stage and the other at the staminate stage. Insects of five families visited M. sprengeri during the floral cycle, and sap beetles (Epuraea sp., Nitidulidae) were determined to be the most effective pollinators, whereas bees (Apis cerana, Apidae) were considered to be occasional pollinators. A strong fragrance was released during thermogenesis, consisting of 18 chemical compounds. Although the relative proportions of these compounds varied at different floral stages across anthesis, linalool, 1-iodo-2-methylundecane and 2,2,6-trimethyl-6-vinyltetrahydro-2H-pyran-3-ol were dominant. Importantly, we found that the floral blends released during the pistillate and staminate stages were very similar, and coincided with flower visitation by sap beetles and the two thermogenic episodes. Based on these results, we propose that odour acts as a signal for a reward (pollen) and that an odour mimicry of staminate-stage flowers occurs during the pistillate stage.

Semiochemistry of the Scarabaeoidea

The superfamily Scarabaeoidea comprises a large and diverse monophyletic group. Members share ancestral characteristics, but often exhibit considerable differences in their ecology, physiology, or mating strategies. A large number of species are regarded as pests of crop or amenity plants, while others are beneficial to humans and even may be extremely rare as a result of anthropogenic activities. A significant number of chemical ecology-based studies have been conducted with the Scarabaeoidea in order to characterize semiochemicals influencing their behavior, such as pheromones and plant-derived allelochemicals. These may be used either to control or preserve populations of the beetles, depending upon pest or beneficial status. This paper is a review of the role and identity of the semiochemicals of the Scarabaeoidea, with comments on possible future research and applied opportunities in the field of chemical ecology.

Veratrole biosynthesis in white campion

White campion (Silene latifolia) is a dioecious plant that emits 1,2-dimethoxybenzene (veratrole), a potent pollinator attractant to the nocturnal moth Hadena bicruris. Little is known about veratrole biosynthesis, although methylation of 2-methoxyphenol (guaiacol), another volatile emitted from white campion flowers, has been proposed. Here, we explore the biosynthetic route to veratrole. Feeding white campion flowers with [(13)C9]l-phenylalanine increased guaiacol and veratrole emission, and a significant portion of these volatile molecules contained the stable isotope. When white campion flowers were treated with the phenylalanine ammonia lyase inhibitor 2-aminoindan-2-phosphonic acid, guaiacol and veratrole levels were reduced by 50% and 63%, respectively. Feeding with benzoic acid (BA) or salicylic acid (SA) increased veratrole emission 2-fold, while [(2)H5]BA and [(2)H6]SA feeding indicated that the benzene ring of both guaiacol and veratrole is derived from BA via SA. We further report guaiacol O-methyltransferase (GOMT) activity in the flowers of white campion. The enzyme was purified to apparent homogeneity, and the peptide sequence matched that encoded by a recently identified complementary DNA (SlGOMT1) from a white campion flower expressed sequence tag database. Screening of a small population of North American white campion plants for floral volatile emission revealed that not all plants emitted veratrole or possessed GOMT activity, and SlGOMT1 expression was only observed in veratrole emitters. Collectively these data suggest that veratrole is derived by the methylation of guaiacol, which itself originates from phenylalanine via BA and SA, and therefore implies a novel branch point of the general phenylpropanoid pathway.

The evolution of floral scent and olfactory preferences in pollinators: coevolution or pre-existing bias?

Coevolution is thought to be a major factor in shaping plant-pollinator interactions. Alternatively, plants may have evolved traits that fitted pre-existing preferences or morphologies in the pollinators. Here, we test these two scenarios in the plant family of Araceae and scarab beetles (Coleoptera, Scarabaeidae) as pollinators. We focused on floral volatile organic compounds (VOCs) and production/detection of VOCs by scarab beetles. We found phylogenetic structure in the production/detection of methoxylated aromatics in scarabs, but not plants. Within the plants, most of the compounds showed a well-supported pattern of correlated evolution with scarab-beetle pollination. In contrast, the scarabs showed no correlation between VOC production/detection and visitation to Araceae flowers, with the exception of the VOC skatole. Moreover, many VOCs were found in nonpollinating beetle groups (e.g., Melolonthinae) that are ancestors of pollinating scarabs. Importantly, none of the tested VOCs were found to have originated in pollinating taxa. Our analysis indicates a Jurassic origin of VOC production/detection in scarabs, but a Cretaceous/Paleocene origin of floral VOCs in plants. Therefore, we argue against coevolution, instead supporting the scenario of sequential evolution of floral VOCs in Araceae driven by pre-existing bias of pollinators.