Macrolides and alcohols as scent gland constituents of the Madagascan frog Mantidactylus femoralis and their intraspecific diversity

Natural ten-membered lactones: sources, structural diversity, biological activity, and intriguing future

Covering: 2012 to 2022Ten-membered lactones (TMLs) are an interesting and diverse group of natural polyketides that are abundant in fungi and, to a lesser extent, in bacteria, marine organisms, and insects. TMLs are known for their ability to exhibit a wide spectrum of biological activity, including phytotoxic, cytotoxic, antifungal, antibacterial, and others. However, the random discovery of these compounds by scientific groups with various interests worldwide has resulted in patchy information about their distribution among different organisms and their biological activity. Therefore, despite more than 60 years of research history, there is still no common understanding of the natural sources of TMLs, their structural type classification, and most characteristic biological activities. The controversial nomenclature, incorrect or erroneous structure elucidation, poor identification of producing organisms, and scattered information on the biological activity of compounds - all these factors have led to the problems with dereplication and the directed search for TMLs. This review consists of two parts: the first part (Section 2) covers 104 natural TMLs, published between 2012 and 2022 (after the publishing of the previous review), and the second part (Section 3) summarizes information about 214 TMLs described during 1964-2022 and as a result highlights the main problems and trends in the study of these intriguing natural products.

Identification and determination of the absolute configuration of amorph-4-en-10β-ol, a cadinol-type sesquiterpene from the scent glands of the African reed frog Hyperolius cinnamomeoventris

Hyperolid reed frogs are one of the few families of Anurans known to possess glands that emit volatile compounds used in chemical communication. Hyperolius cinnamomeoventris, a model species, possesses a gular gland on its vocal sac that emits chemicals, and sends visual and auditory signals during calling. Previous investigations have shown that the glandular compounds are typically macrocyclic lactones. However, in this work, we show that another major constituent of the male specific gland is (10R,1S,6R,7R,10R)-amorph-4-ene-10β-ol [(1R,4R,4aR,8aS)-4-isopropyl-1,6-dimethyl-1,2,3,4,4a,7,8,8a-octahydronaphthalen-1-ol]. This compound was synthesized for the first time and has the opposite configuration to amorph-4-ene-10β-ol known from plants. A short synthesis using an organocatalytic approach through a tandem Mannich/intramolecular Diels-Alder reaction led to a mixture of cadinols, which was used for the assignment of the natural cadinol structures and their stereoisomers.

Cinnamomeoventrolide - Double Bond Regioisomerism in Frog Semiochemicals

Frogs of the families Mantellinae and Hyperoliidae possess male specific femoral or gular glands that are used during courtship. These glands release volatile compounds, e. g. the macrocyclic lactone gephyromantolide A (2,6,10-trimethyl-6-undecen-11-olide) in the case of Gephyromantis boulengeri (Mantellinae). During the analysis of the volatiles of Hyperolius cinnamomeoventris (Hyperoliidae) we detected an unknown compound A, which we called cinnamomeoventrolide, whose mass spectrum showed high similarity with the spectrum of gephyromantolide A. Nevertheless, slight spectral differences led to the proposal of a regioisomer of gephyromantolide A as a structure for A, 2,6,10-trimethyl-5-undecen-11-olide. A versatile synthesis of this compound was developed to allow access to all four stereoisomers from a single chiral starting material, the so-called (S)-Roche ester, using ring-closing metathesis as a key step. With these stereoisomers, the absolute configuration of the natural product was established to have the (2R,10S)-configuration by GC on a chiral phase. The configuration of natural gephyromantolide A is the opposite. Both frogs seem to use a similar biosynthetic pathway to access the target compounds, differing in the stereochemistry of the reduction steps, and requiring an additional isomerization in case of G. boulengeri. This unique regioisomeric differentiation of double bonds in semiochemicals has so far only been observed in insects. The compounds are likely to play a role in species-recognition of the frogs.

Population diversification in the frog Mantidactylus bellyi on an isolated massif in northern Madagascar based on genetic, morphological, bioacoustic and ecological evidence

In the processes that give rise to new species, changes first occur at the population level. But with the continuous nature of the divergence process, change in biological properties delimiting the shift from “individuals of divergent populations” towards “individuals of distinct species”, as well as abiotic factors driving the change, remain largely ambivalent. Here we study diversification processes at the population level in a semi-aquatic frog, Mantidactylus (Brygoomantis) bellyi, across the diverse vegetation types of Montagne d’Ambre National Park (MANP), Madagascar. Genetic diversity was assessed with seven newly developed microsatellite markers as well as mitochondrial DNA sequences and concordance with patterns of ecological, morphological, and bioacoustic divergence evaluated. We found M. bellyi lacking mitochondrial differentiation within MANP, while microsatellite datasets partitioned them into three highly differentiated, geographically separated subpopulations (with indications for up to five subpopulations). The molecular grouping–primarily clustering individuals by geographic proximity–was coincident with differences in mean depth and width of waters, suggesting a possible role of fluvial characteristics in genetic exchange in this stream-breeding species. Genetic clustering not consistent with differences in call properties, except for dominant call frequencies under the two-subpopulations model. Morphological divergence was mostly consistent with the genetic clustering; subpopulations strongly differed by their snout-vent length, with individuals from high-elevation subpopulations smaller than those from populations below 1000 m above sea level. These results exemplify how mountains and environmental conditions might primarily shape genetic and morphological divergence in frog populations, without strongly affecting their calls.

Chemical Diversity of Volatile Macrocylic Lactones from Frogs

For a long time, frogs were believed to communicate primarily via the acoustic channel, but during the last decades it became obvious that various lineages also use chemical communication. In this Account we present our research on the identification of volatile lactones from Madagascan Mantellidae and African Hyperoliidae frogs. Both possess male specific glands that can disseminate a range of volatile compounds. Key constituents are macrocyclic lactones. They show high variability in structure and occurrence. We focus here on the synthetic approaches we have used to clarify constitution and configuration of the glandular compounds. Key synthetic methods are ring-closing metathesis and nucleophilic epoxide opening. Often, but not always, the natural compounds occurs in amounts that excludes their investigation by NMR spectroscopy. Instead, we use GC/MS analysis, GC/IR, microreactions, and synthesis to identify such components. Several aspects of our work will be described giving some insight in our scientific approach.

1 Introduction

2 Macrocylic Lactones from the Fatty Acid Biosynthetic Pathway

3 Unsaturated Lactones

4 Terpenoid Lactones

5 Macrolide Occurrence

6 Conclusions

The Role of Total Synthesis in Structure Revision and Elucidation of Decanolides (Nonanolides)

Ten-membered lactones are commonly observed structures of natural products. They are mostly fungal metabolites, which often act as plant pathogens, but recently ten-membered lactones were identified as pheromones of frogs and termites. Although modern spectroscopic methods are nowadays routinely used to elucidate the structure of natural products, structural assignments of ten-membered lactones often remain incomplete or are surprisingly often erroneous. Most errors concern the absolute configuration. The examples discussed in this chapter demonstrate that enantioselective total synthesis is not only an efficient tool for corroborating or revising a proposed structure, but that the synthesis of different stereoisomers as references for gas chromatographic investigations can be a vital part of the structure elucidation process if only minute amounts of material are available. As a method of outstanding importance for the synthesis of ten-membered lactones olefin metathesis has emerged. Most of the examples discussed herein use one or more olefin metathesis reactions as key steps.

Direct deposition GC/IR techniques in natural product identification

Covering: up to the end of 2019 The application of coupled gas chromatography/direct deposition Fourier transform infrared (GC/DD-FTIR) spectroscopy in the field of volatile natural product identification yields orthogonal information to GC/MS analysis about functional groups and C-C double bonds. This underused method facilitates structure elucidation of trace compounds and can be combined with modern DFT-calculation to further improve the usefulness of the obtained results.

'Barcode fishing' for archival DNA from historical type material overcomes taxonomic hurdles, enabling the description of a new frog species

Taxonomic progress is often hindered by intrinsic factors, such as morphologically cryptic species that require a broad suite of methods to distinguish, and extrinsic factors, such as uncertainties in the allocation of scientific names to species. These uncertainties can be due to a wide variety of factors, including old and poorly preserved type specimens (which contain only heavily degraded DNA or have lost important diagnostic characters), inappropriately chosen type specimens (e.g. juveniles without diagnostic characters) or poorly documented type specimens (with unprecise, incorrect, or missing locality data). Thanks to modern sequencing technologies it is now possible to overcome many such extrinsic factors by sequencing DNA from name-bearing type specimens of uncertain assignment and assigning these to known genetic lineages. Here, we apply this approach to frogs of the Mantidactylus ambreensis complex, which was recently shown to consist of two genetic lineages supported by concordant differentiation in mitochondrial and nuclear genes. These lineages co-occur on the Montagne d'Ambre Massif in northern Madagascar but appear to have diverged in allopatry. We use a recently published bait set based on three mitochondrial markers from all known Malagasy frog lineages to capture DNA sequences from the 127-year-old holotype of Mantidactylus ambreensis Mocquard, 1895. With the obtained sequences we are able to assign the name M. ambreensis to the lowland lineage, which is rather widespread in the rainforests of northern Madagascar, leaving the microendemic high-elevation lineage on Montagne d'Ambre in north Madagascar in need of description. We describe this species as Mantidactylus ambony sp. nov., differing from M. ambreensis in call parameters and a smaller body size. Thus, using target enrichment to obtain DNA sequence data from this old specimen, we were able to resolve the extrinsic (nomenclatural) hindrances to taxonomic resolution of this complex. We discuss the broad-scale versatility of this 'barcode fishing' approach, which can draw on the enormous success of global DNA barcoding initiatives to quickly and efficiently assign type specimens to lineages.

An insight into the skin glands, dermal scales and secretions of the caecilian amphibian Ichthyophis beddomei

The caecilian amphibians are richly endowed with cutaneous glands, which produce secretory materials that facilitate survival in the hostile subterranean environment. Although India has a fairly abundant distribution of caecilians, there are only very few studies on their skin and secretion. In this background, the skin of Ichthyophis beddomei from the Western Ghats of Kerala, India, was subjected to light and electron microscopic analyses. There are two types of dermal glands, mucous and granular. The mucous gland has a lumen, which is packed with a mucous. The mucous-producing cells are located around the lumen. In the granular gland, a lumen is absent; the bloated secretory cells, filling the gland, are densely packed with granules of different sizes which are elegantly revealed in TEM. There is a lining of myo-epithelial cells in the peripheral regions of the glands. Small flat disk-like dermal scales, dense with squamulae, are embedded in pockets in the dermis, distributed among the cutaneous glands. 1-4 scales of various sizes are present in each scale pocket. Scanning electron microscopic observation of the skin surface revealed numerous glandular openings. The skin gland secretions, exuded through the pores, contain fatty acids, alcohols, steroid, hydrocarbons, terpene, aldehyde and a few unknown compounds.

Structural and bio-functional assessment of the postaxillary gland in Nidirana pleuraden (Amphibia: Anura: Ranidae)

BACKGROUND

Owing to their incomplete adaptation to the terrestrial environment, amphibians possess complex cutaneous glandular systems. The skin glands not only regulate water loss and respiratory gas and salt exchange, but are also involved in defense against predators and microorganisms, social communication, and reproduction. These glands are distributed throughout the integument, but can accumulate in specific regions, forming visible outgrowths known as macroglands. Some macroglands are sexually dimorphic and mediate intersexual communication and reproductive success. The postaxillary gland is a sexually dimorphic macrogland in Nidirana pleuraden. Its biological function and its morphological and histochemical characteristics are unclear. In the present study, we describe the structure and ultrastructure of the postaxillary gland, and explore its main function.

RESULTS

The postaxillary gland has a thinner epidermis than the dorsal region of N. pleuraden. In addition to ordinary serous glands (OSG), type I and II mucous gland (I MG & II MG), a type of specialized mucous gland (SMG) is also found to constitute the postaxillary gland. The SMG is larger than other gland types, and consists of high columnar mucocytes with basal nuclei arranged radially toward a lumen. SMGs are positive to periodic acid-Schiff stain and stained blue in Masson's trichrome stain. A discontinuous myoepithelial sheath lacking innervation encircles SMG mucocytes, and the outlets of such glands are X- or Y-shaped. Transmission electron microscopy reveals abundant secretory granules in SMG, which are biphasic, composed of an electron-opaque outer ring and a less electron-dense core. Lipid droplets, and organelles, such as rough endoplasmic reticulum and Golgi stacks, are located in the supranuclear cytoplasm of the mucocytes in SMG. Female N. pleuraden exhibits chemotaxis toward homogenate of the postaxillary gland, but male does not. On treatment with trypsin, this sexual attraction disappears.

CONCLUSIONS

The postaxillary gland of N. pleuraden is a male-specific macrogland that consists primarily of SMGs, together with OSGs, I MGs and II MGs. Other than their extremely large size, SMGs structurally and histochemically resemble many reported specialized gland types in amphibian sexually dimorphic skin glands. Secretions of the postaxillary gland are proteinaceous sexual pheromones, which are believed to attract females at male calling intermissions.

Multiple Independent Recruitment of Sodefrin Precursor-Like Factors in Anuran Sexually Dimorphic Glands

Chemical signaling in animals often plays a central role in eliciting a variety of responses during reproductive interactions between males and females. One of the best-known vertebrate courtship pheromone systems is sodefrin precursor-like factors (SPFs), a family of two-domain three-finger proteins with a female-receptivity enhancing function, currently only known from salamanders. The oldest divergence between active components in a single salamander species dates back to the Late Paleozoic, indicating that these proteins potentially gained a pheromone function earlier in amphibian evolution. Here, we combined whole transcriptome sequencing, proteomics, histology, and molecular phylogenetics in a comparative approach to investigate SPF occurrence in male breeding glands across the evolutionary tree of anurans (frogs and toads). Our study shows that multiple families of both terrestrially and aquatically reproducing frogs have substantially increased expression levels of SPFs in male breeding glands. This suggests that multiple anuran lineages make use of SPFs to complement acoustic and visual sexual signaling during courtship. Comparative analyses show that anurans independently recruited these proteins each time the gland location on the male's body allowed efficient transmission of the secretion to the female's nares.

Sympatric lineages in the Mantidactylus ambreensis complex of Malagasy frogs originated allopatrically rather than by in-situ speciation

Madagascar's biota is characterized by a high degree of microendemism at different taxonomic levels, but how colonization and in-situ speciation contribute to the assembly of local species communities has rarely been studied on this island. Here we analyze the phylogenetic relationships of riparian frogs of the Mantidactylus ambreensis species complex, which is distributed in the north of Madagascar and was originally described from Montagne d'Ambre, an isolated mountain of volcanic origin, currently protected within Montagne d'Ambre National Park (MANP). Data from mitochondrial DNA, and phylogenomic data from FrogCap, a sequence capture method, independently confirm that this species complex is monophyletic within the subgenus Ochthomantis, and identify two main clades within it. These two clades are separated by 5.6-6.8% pairwise distance in the mitochondrial 16S rRNA gene and co-occur in MANP, with one distributed at high elevations (940-1375 m a.s.l.) and the other at lower elevations (535-1010 m a.s.l.), but show almost no haplotype sharing in the nuclear RAG1 gene. This occurrence in syntopy without admixture confirms them as independent evolutionary lineages that merit recognition as separate species, and we here refer to them as high-elevation (HE) and low-elevation (LE) lineage; they will warrant taxonomic assessment to confidently assign the name ambreensis to one or the other. Populations of the M. ambreensis complex from elsewhere in northern Madagascar all belong to the LE lineage, although they do occur over a larger elevational range than in Montagne d'Ambre (285-1040 m a.s.l.). Within LE there are several phylogroups (LE1-LE4) of moderately deep divergence (1.5-2.8% in 16S), but phylogroup LE4 that occurs in MANP has a deeply nested phylogenetic position, as recovered separately by mitochondrial and sequence capture datasets. This suggests that HE and LE did not diverge by a local fission of lower and upper populations, but instead arose through a more complex biogeographic scenario. The branching pattern of phylogroups LE1-LE4 shows a clear south-to-north phylogeographic pattern. We derive from these results a testable hypothesis of vicariant speciation that restricted the HE lineage to MANP and the LE candidate species to a climatic refugium further south, with subsequent northwards range expansion and secondary colonization of MANP by LE. These results provide an example for complex assembly of local microendemic amphibian faunas on Madagascar.

Identification and Synthesis of Luteolide, a Highly Branched Macrolide Semiochemical from the Mantellid Frog Gephyromantis luteus

Luteolide is a 10-membered aliphatic macrolactone, (4 R,8 S,9 S)-4,8-dimethylundecan-9-olide ((-) -17), released by the femoral gland of males of the mantellid frog Gephyromantis luteus. Its structure was established using NMR, MS, and chiral GC and confirmed by stereoselective synthesis of different stereoisomers. Among the approximately 20 current macrolides known from the Mantellidae, luteolide is the first example of a volatile macrolide furnishing three stereogenic centers and an ethyl side chain.

Frogolide - An Unprecedented Sesquiterpene Macrolactone from Scent Glands of African Frogs

Some amphibians use chemical signals in addition to optical and acoustical signals to transmit information. Males of mantellid frogs from Madagascar and hyperoliid frogs from Africa emit complex, species- and sex-specific bouquets of volatiles from their femoral or gular glands. We report here on the identification, synthesis, and determination of the absolute configuration of a macrocyclic lactone occurring in several species of both families, (S)-3,7,11-dodec-6,10-dien-12-olide (S-14, frogolide). Macrolides are a preferred compound class of frog volatiles. Nevertheless, frogolide is the first macrocyclic lactone obviously derived from the terpene pathway, in contrast to known frog macrolides that are usually formed via the fatty acid biosynthetic pathway.

A synthetic dodecanolide library for the identification of putative semiochemicals emitted by mantellid frogs

Aliphatic macrolides are used by various organisms as semiochemicals or pheromones, e.g. by the endemic Madagascan frog family Mantellinae. Volatile, methyl-branched aliphatic macrolides occur in various species, but their identification by MS is difficult, while isolation is not possible due to the low amounts produced. Therefore, we synthesized a library of methyl-branched 11-dodecanolides to identify unknown macrolides secreted by Gephyromantis moseri, a mantelline frog restricted to the rainforest and characterized by largely terrestrial habitats. The syntheses used metal-organic epoxide opening and ring-closing-metathesis as key steps. All library members 2-, 4-, 6-, 8-, and 10-methyl-11-dodecanolide were thus identified as part of the secretion by comparison of their mass spectra and their gas chromatographic retention indices. These compounds comprise new natural products.

Mass Spectrometry of Aliphatic Macrolides, Important Semiochemicals or Pheromones

Macrolides are a relatively common structural motif prevalent in Nature. However, the structures of these large ring lactones have been relatively difficult to elucidate via NMR spectroscopy due to the minute amounts of compounds that are sometimes obtainable from natural sources. Thus, GC-MS analysis of individual macrolactones has become the method of choice for the structural identification of these compounds. Here we discuss the mass spectrometric behavior of aliphatic macrolides, evaluating spectra from numerous compounds of various ring size, including derivatives containing methyl branches as well as double bonds. The specific fragmentation of these macrolactones under electron impact conditions allows for the development of a general rule set aimed at the identification of similar compounds by mass spectrometry. In addition, the mass spectra of dimethyl disulfide adducts of unsaturated macrolides are discussed. The mass spectra of almost 50 macrolides are presented.

Identification, synthesis and mass spectrometry of a macrolide from the African reed frog Hyperolius cinnamomeoventris

The contents of the gular glands of the male African reed frog Hyperolius cinnamomeoventris consist of a mixture of aliphatic macrolides and sesquiterpenes. While the known macrolide gephyromantolide A was readily identified, the structure of another major component was suggested to be a tetradecen-13-olide. The synthesis of the two candidate compounds (Z)-5- and (Z)-9-tetradecen-13-olide revealed the former to be the naturally occurring compound. The synthesis used ring-closing metathesis as key step. While the Hoveyda–Grubbs catalyst furnished a broad range of isomeric products, the (Z)-selective Grubbs catalyst lead to pure (Z)-products. Analysis by chiral GC revealed the natural frog compound to be (5Z,13S)-5-tetradecen-13-olide (1). This compound is also present in the secretion of other hyperoliid frogs as well as in femoral glands of male mantellid frogs such as Spinomantis aglavei. The mass spectra of the synthesized macrolides as well as their rearranged isomers obtained during ring-closing metathesis showed that it is possible to assign the location of the double bond in an unsaturated macrolide on the basis of its EI mass spectrum. The occurrence of characteristic ions can be explained by the fragmentation pathway proposed in the article. In contrast, the localization of a double bond in many aliphatic open-chain compounds like alkenes, alcohols or acetates, important structural classes of pheromones, is usually not possible from an EI mass spectrum. In the article, we present the synthesis and for the first time elucidate the structure of macrolides from the frog family Hyperoliidae.

Reassignment of the absolute configuration of plakinidone from the sponge consortium Plakortis halichondrioides-Xestospongia deweerdtae using a combination of synthesis and a chiroptical approach

Recent work by Wu et al. in connection with the first synthesis of the marine natural product plakinidone revealed that the most salient feature of its purported structure, a six-membered perlactone moiety, was in fact a five-membered lactone, i.e. a 3-methyl-4-hydroxy-2(5H)-furanone or tetronic acid ring. With the planar structure of plakinidone confidently revised, we undertook a new investigation to unambiguously establish its absolute configuration. Upon preparing two stable derivatives 1 and 5 from a sample of naturally occurring plakinidone extracted from the sponge association Plakortis halichondriodes-Xetospongia deweerdtae, the absolute configuration was assigned by synthesis and vibrational and electronic circular dichroism (VCD and ECD) measurements in combination with density functional theory calculations at the B3LYP/aug-cc-pVDZ/PCM(CH3CN) level of theory. Our combined efforts and the agreement between the experimental and calculated VCD/ECD spectra of 1 revealed that the absolute configuration of plakinidone was in fact (11S,17R) and not the formerly reported (11S,17S) diastereomer assigned by Wu et al.. Therefore, we propose that natural plakinidone is accurately represented by structure 12.

Ontogenetic Development of the Derived Olfactory System of the Mantellid Frog Mantidactylus betsileanus

The nasal cavity of Mantidactylus betsileanus, a frog of the Madagascar-Comoroan endemic family Mantellidae, is characterized by a unique internal architecture. Unlike the state commonly observed in anurans, the two discernible olfactory subsystems of M. betsileanus (the main olfactory organ and the vomeronasal organ) are anatomically separated from each other, suggesting an enhanced functional differentiation. Here we evaluate the ontogenetic formation of this extraordinary anatomical state based on a histological study of a developmental series of M. betsileanus. The olfactory system of premetamorphic tadpoles, and most of its changes during metamorphosis, resembles that of other anurans. At the end of metamorphosis however, a growing obstruction of the passage between main olfactory organ and vomeronasal organ takes place, leading to the deviant morphological state previously described for adults. The late appearance of this atypical anatomical feature in the course of ontogeny agrees with the phylogenetic hypothesis of the observed obstruction representing a derived state for these frogs. From a functional point of view, the apparent autonomy of the vomeronasal organ is possibly linked to the presence of clade-specific femoral glands that are known to produce pheromones and that likewise are fully expressed in adults only. Anat Rec, 299:943-950, 2016. © 2016 Wiley Periodicals, Inc.

Odorous secretions in anurans: morphological and functional assessment of serous glands as a source of volatile compounds in the skin of the treefrog Hypsiboas pulchellus (Amphibia: Anura: Hylidae)

Serous (granular or venom) glands occur in the skin of almost all species of adult amphibians, and are thought to be the source of a great diversity of chemical compounds. Despite recent advances in their chemistry, odorous volatile substances are compounds that have received less attention, and until now no study has attempted to associate histological data with the presence of these molecules in amphibians, or in any other vertebrate. Given the recent identification of 40 different volatile compounds from the skin secretions of H. pulchellus (a treefrog species that releases a strong odour when handled), we examined the structure, ultrastructure, histochemistry, and distribution of skin glands of this species. Histological analysis from six body regions reveals the presence of two types of glands that differ in their distribution. Mucous glands are homogeneously distributed, whereas serous glands are more numerous in the scapular region. Ultrastructural results indicate that electron-translucent vesicles observed within granules of serous glands are similar to those found in volatile-producing glands from insects and also with lipid vesicles from different organisms. Association among lipids and volatiles is also evidenced from chemical results, which indicate that at least some of the volatile components in H. pulchellus probably originate within the metabolism of fatty acids or the mevalonate pathway. As odorous secretions are often considered to be secreted under stress situations, the release of glandular content was assessed after pharmacological treatments, epinephrine administrated in vivo and on skin explants, and through surface electrical stimulation. Serous glands responded to all treatments, generally through an obvious contraction of myoepithelial cells that surround their secretory portion. No response was observed in mucous glands. Considering these morpho-functional results, along with previous identification of volatiles from H. pulchellus and H. riojanus after electrical stimulation, we suggest that the electron-translucent inclusions found within the granules of serous glands likely are the store sites of volatile compounds and/or their precursors. Histochemical and glandular distribution analyses in five other species of frogs of the hylid tribe Cophomantini, revealed a high lipid content in all the species, whereas a heterogeneous distribution of serous glands is only observed in species of the H. pulchellus group. The distribution pattern of serous glands in members of this species group, and the odorous volatile secretions are probably related to defensive functions.

The use of the lactone motif in chemical communication

A wide variety of organisms communicate via the chemical channel using small molecules. A structural feature quite often found is the lactone motif. In the present paper, the current knowledge on such lactones will be described, concentrating on the structure, chemistry, function, biosynthesis and synthesis of these compounds. Lactone semiochemicals from insects, vertebrates and bacteria, which this article will focus on, are particularly well investigated. In addition, some ideas on the advantageous use of lactones as volatile signals, which promoted their evolutionary development, will be discussed.

Stereoselective synthesis of trans-THF rings using an oxidative cyclisation-radical deoxygenation sequence: application to the formal synthesis of trans-(2R,5R)-linalool oxide

Permanganate-mediated oxidative cyclisation-Barton–McCombie radical deoxygenation sequence is applied to 1,5-dienes to achieve stereoselective synthesis of trans-2,5-disubstituted THF rings.

The anuran vocal sac: a tool for multimodal signalling

Although in anurans the predominant mode of intra- and intersexual communication is vocalization, modalities used in addition to or instead of acoustic signals range from seismic and visual to chemical. In some cases, signals of more than one modality are produced through or by the anuran vocal sac. However, its role beyond acoustics has been neglected for some time and nonacoustic cues such as vocal sac movement have traditionally been seen as an epiphenomenon of sound production. The diversity in vocal sac coloration and shape found in different species is striking and recently its visual properties have been given a more important role in signalling. Chemosignals seem to be the dominant communication mode in newts, salamanders and caecilians and certainly play a role in the aquatic life phase of anurans, but airborne chemical signalling has received less attention. There is, however, increasing evidence that at least some terrestrial anuran species integrate acoustic, visual and chemical cues in species recognition and mate choice and a few secondarily mute anuran species seem to fully rely on volatile chemical cues produced in glands on the vocal sac. Within vertebrates, frogs in particular are suitable organisms for investigating multimodal communication by means of experiments, since they are tolerant of disturbance by observers and can be easily manipulated under natural conditions. Thus, the anuran vocal sac might be of great interest not only to herpetologists, but also to behavioural biologists studying communication systems.

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Our view of anuran communication has changed drastically during the last few decades.

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Anurans use diverse communication strategies often directly related to the vocal sac.

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Distinctive vocal sac shapes, sizes and colours suggest functional diversity.

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Evidence increases that the vocal sac also plays a role in chemical signalling.

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The anuran vocal sac may be of great interest for studies on communication systems.

Take time to smell the frogs: vocal sac glands of reed frogs (Anura: Hyperoliidae) contain species-specific chemical cocktails

Males of all reed frog species (Anura: Hyperoliidae) have a prominent, often colourful, gular patch on their vocal sac, which is particularly conspicuous once the vocal sac is inflated. Although the presence, shape, and form of the gular patch are well-known diagnostic characters for these frogs, its function remains unknown. By integrating biochemical and histological methods, we found strong evidence that the gular patch is a gland producing volatile compounds, which might be emitted while calling. Volatile compounds were confirmed by gas chromatography–mass spectrometry in the gular glands in 11 species of the hyperoliid genera Afrixalus, Heterixalus, Hyperolius, and Phlyctimantis. Comparing the gular gland contents of 17 specimens of four sympatric Hyperolius species yielded a large variety of 65 compounds in species-specific combinations. We suggest that reed frogs might use a complex combination of at least acoustic and chemical signals in species recognition and mate choice.