Using weapons instead of perfume – chemical association strategies of the myrmecophilous bug Scolopostethus pacificus (Rhyparochromidae)

A vast diversity of parasites associate with ants. Living in and around ant nests these organisms must overcome ant colony defenses. As ant defensive behavior is mainly mediated by species-specific cuticular hydrocarbons (CHCs) or alarm pheromones, ant-associated parasites can either crack their hosts chemical communication code by modifying their own CHC-profiles or use pro-active strategies like chemical weaponry for distraction and repellency. While the chemical nature of ant-parasite interactions has been intensively studied for highly host specific parasites, the chemical-deceptive strategies of the rather rare ant-resembling Heteropterans are unknown. To gain insight into this system, I studied the bug Scolopostethus pacificus (Barber 1918) which can be found near the nests of the ecologically dominant and aggressive velvety tree ant (Liometopum occidentale, Emery 1895). Using behavioral, chemical and molecular approaches I disentangled the relationship of S. pasificus and its host ant. Chemical profiling of the bug and the ant revealed that the bug does not make use of CHC insignificance or mimicry, but instead uses a cocktail of volatile compounds released from its metathoracic glands that likely moderates encounters with its aggressive host. Feeding trials with armed and artificially disarmed bugs revealed a defensive function of the gland exudates. Targeted molecular gut barcoding showed that S. pasificus does not feed on L. occidentale. These results suggest that chemical weaponry, rather than a chemical code-cracking CHC matching or chemical insignificance, enables S. pasificus to get along with and live in close proximity to its host ant

Using weapons instead of perfume – chemical association strategies of the myrmecophilous bug Scolopostethus pacificus (Rhyparochromidae)

A vast diversity of parasites associate with ants. Living in and around ant nests these organisms must overcome ant colony defenses. As ant defensive behavior is mainly mediated by species-specific cuticular hydrocarbons (CHCs) or alarm pheromones, ant-associated parasites can either crack their hosts chemical communication code by modifying their own CHC-profiles or use pro-active strategies like chemical weaponry for distraction and repellency. While the chemical nature of ant-parasite interactions has been intensively studied for highly host specific parasites, the chemical-deceptive strategies of the rather rare ant-resembling Heteropterans are unknown. To gain insight into this system, I studied the bug Scolopostethus pacificus (Barber 1918) which can be found near the nests of the ecologically dominant and aggressive velvety tree ant (Liometopum occidentale, Emery 1895). Using behavioral, chemical and molecular approaches I disentangled the relationship of S. pasificus and its host ant. Chemical profiling of the bug and the ant revealed that the bug does not make use of CHC insignificance or mimicry, but instead uses a cocktail of volatile compounds released from its metathoracic glands that likely moderates encounters with its aggressive host. Feeding trials with armed and artificially disarmed bugs revealed a defensive function of the gland exudates. Targeted molecular gut barcoding showed that S. pasificus does not feed on L. occidentale. These results suggest that chemical weaponry, rather than a chemical code-cracking CHC matching or chemical insignificance, enables S. pasificus to get along with and live in close proximity to its host ant

Molecular evolution of gland cell types and chemical interactions in animals

Across the Metazoa, the emergence of new ecological interactions has been enabled by the repeated evolution of exocrine glands. Specialized glands have arisen recurrently and with great frequency, even in single genera or species, transforming how animals interact with their environment through trophic resource exploitation, pheromonal communication, chemical defense and parental care. The widespread convergent evolution of animal glands implies that exocrine secretory cells are a hotspot of metazoan cell type innovation. Each evolutionary origin of a novel gland involves a process of ‘gland cell type assembly’: the stitching together of unique biosynthesis pathways; coordinated changes in secretory systems to enable efficient chemical release; and transcriptional deployment of these machineries into cells constituting the gland. This molecular evolutionary process influences what types of compound a given species is capable of secreting, and, consequently, the kinds of ecological interactions that species can display. Here, we discuss what is known about the evolutionary assembly of gland cell types and propose a framework for how it may happen. We posit the existence of ‘terminal selector’ transcription factors that program gland function via regulatory recruitment of biosynthetic enzymes and secretory proteins. We suggest ancestral enzymes are initially co-opted into the novel gland, fostering pleiotropic conflict that drives enzyme duplication. This process has yielded the observed pattern of modular, gland-specific biosynthesis pathways optimized for manufacturing specific secretions. We anticipate that single-cell technologies and gene editing methods applicable in diverse species will transform the study of animal chemical interactions, revealing how gland cell types are assembled and functionally configured at a molecular level

De novo biosynthesis of simple aromatic compounds by an arthropod (Archegozetes longisetosus)

The ability to synthesize simple aromatic compounds is well known from bacteria, fungi and plants, which all share an exclusive biosynthetic route—the shikimic acid pathway. Some of these organisms further evolved the polyketide pathway to form core benzenoids via a head-to-tail condensation of polyketide precursors. Arthropods supposedly lack the ability to synthesize aromatics and instead rely on aromatic amino acids acquired from food, or from symbiotic microorganisms. The few studies purportedly showing de novo biosynthesis via the polyketide synthase (PKS) pathway failed to exclude endosymbiotic bacteria, so their results are inconclusive. We investigated the biosynthesis of aromatic compounds in defence secretions of the oribatid mite Archegozetes longisetosus. Exposing the mites to a diet containing high concentrations of antibiotics removed potential microbial partners but did not affect the production of defensive benzenoids. To gain insights into benzenoid biosynthesis, we fed mites with stable-isotope labelled precursors and monitored incorporation with mass spectrometry. Glucose, malonic acid and acetate, but not phenylalanine, were incorporated into the benzenoids, further evidencing autogenous biosynthesis. Whole-transcriptome sequencing with hidden Markov model profile search of protein domain families and subsequent phylogenetic analysis revealed a putative PKS domain similar to an actinobacterial PKS, possibly indicating a horizontal gene transfer

Temperature Affects Chemical Defense in a Mite-Beetle Predator-Prey System

Temperature influences all biochemical and biophysiological processes within an organism. By extension, it also affects those ecological interactions that are mediated by gland-produced chemical compounds, such as reservoir-based chemical defense. Herein, we investigate how environmental temperature affects the regeneration of defensive secretions and influences the efficacy of chemical defense in a model predator-prey system: the oribatid mite Archegozetes longisetosus and the predaceous rove beetle Stenus juno. Through a combination of chemical analyses, non-linear regression modeling and theoretical simulations we show that the amount of defensive secretion responded to temperature in a unimodal optimum curve: the regeneration rate followed a positive, linear relationship up to 35 °C, but rapidly broke down beyond this temperature (“tipping point” effect). Using functional response simulations, there is an initially positive dampening effect on the predation rate when regeneration is optimal, but at higher temperatures chemical defense does not counteract the previously described effects of elevated predatory pressure. In a larger context, our results demonstrate the need to integrate relevant environmental factors in predator-prey modeling approaches

Temperature Affects Chemical Defense in a Mite-Beetle Predator-Prey System

Temperature influences all biochemical and biophysiological processes within an organism. By extension, it also affects those ecological interactions that are mediated by gland-produced chemical compounds, such as reservoir-based chemical defense. Herein, we investigate how environmental temperature affects the regeneration of defensive secretions and influences the efficacy of chemical defense in a model predator-prey system: the oribatid mite Archegozetes longisetosus and the predaceous rove beetle Stenus juno. Through a combination of chemical analyses, non-linear regression modeling and theoretical simulations we show that the amount of defensive secretion responded to temperature in a unimodal optimum curve: the regeneration rate followed a positive, linear relationship up to 35 °C, but rapidly broke down beyond this temperature (“tipping point” effect). Using functional response simulations, there is an initially positive dampening effect on the predation rate when regeneration is optimal, but at higher temperatures chemical defense does not counteract the previously described effects of elevated predatory pressure. In a larger context, our results demonstrate the need to integrate relevant environmental factors in predator-prey modeling approaches

In search of cues: dung beetle attraction and the significance of volatile composition of dung

Volatile organic compounds (VOCs) provide animals with multiple cues about location, type, and condition of valuable resources such as food. In particular, odour cues are often essential for the localization and discrimination of resources with patchy distribution. Dung beetles (Scarabaeoidea) rely on such scented resources to locate food for their own diet and to provision their progeny. Despite the beetles’ mostly generalist choice across dung types, several studies showed that the beetles prefer some dung types over others. Yet, the importance of VOCs for dung localization and differentiation remains unclear. In this study, we used six single chemical components (indole, skatole, phenol, butyric acid, 2-butanone, and p-cresol), two different blends of these components, and six different dung types for a detailed behavioural analysis of dung beetles. We found very little specialization of beetle species towards specific VOCs. We found that dung baits and baits with synthetic compounds attracted similar communities of dung beetles, but the visitors of synthetic baits exhibited much lower diversity and abundance. The analysis of dung scent profiles of six types of dung revealed both, unique patterns in composition and ubiquitous components such as p-cresol. However, when we used a six-component blend of synthetic compounds, it turned out to be as attractive as three of the most attractive dung types in the field. Our findings highlight the significance of key VOCs, but, moreover, that dung beetles use a blend of specific components for resource localization

Molecular evolutionary trends and biosynthesis pathways in the Oribatida revealed by the genome of Archegozetes longisetosus

Background Oribatid mites are a specious order of microarthropods within the subphylum Chelicerata, compromising about 11,000 described species. They are ubiquitously distributed across different microhabitats in all terrestrial ecosystems around the world and were among the first animals colonizing terrestrial habitats as decomposers and scavengers. Noted for their biosynthesis capacities and biochemical diversity, the majority of oribatid mites possess a pair of exocrine opisthonotal oil-glands used for chemical defense and communication. Genomic resources are lacking for oribatids despite their species richness and ecological importance. Results We used a comparative genomic approach to investigate the developmental, sensory and glandular biosynthetic gene repertoire of the clonal, all-female oribatid mite species Archegozetes longisetosus Aoki, a model species used by numerous laboratories for the past 30 years. Here, we present a 190-Mb genome assembly constructed from Nanopore MinION and Illumina sequencing platforms with 23,825 predicted protein-coding genes. Genomic and transcriptional analyses revealed patterns of reduced body segmentation and loss of segmental identity gene abd-A within Acariformes, and unexpected expression of key eye development genes in these eyeless mites across developmental stages. Consistent with the soil dwelling lifestyle, investigation of the sensory genes revealed a species-specific expansion of gustatory receptors, the largest chemoreceptor family in the genome used in olfaction, and evidence of horizontally transferred enzymes used in cell wall degradation of plant and fungal matter, both components of the Archegozetes longisetosus diet. Using biochemical and genomic data, we were able to delineate the backbone biosynthesis of monoterpenes, an important class of compounds found in the major exocrine gland system of Oribatida – the oil glands. Conclusions With the Archegozetes longisetosus genome, we now have the first high-quality, annotated genome of an oribatid mite genome. Given the mite’s strength as an experimental model, the new sequence resources provided here will serve as the foundation for molecular research in Oribatida and will enable a broader understanding of chelicerate evolution

De novo biosynthesis of simple aromatic compounds by an arthropod (Archegozetes longisetosus)

The ability to synthesize simple aromatic compounds is well known from bacteria, fungi and plants, which all share an exclusive biosynthetic route—the shikimic acid pathway. Some of these organisms further evolved the polyketide pathway to form core benzenoids via a head-to-tail condensation of polyketide precursors. Arthropods supposedly lack the ability to synthesize aromatics and instead rely on aromatic amino acids acquired from food, or from symbiotic microorganisms. The few studies purportedly showing de novo biosynthesis via the polyketide synthase (PKS) pathway failed to exclude endosymbiotic bacteria, so their results are inconclusive. We investigated the biosynthesis of aromatic compounds in defence secretions of the oribatid mite Archegozetes longisetosus. Exposing the mites to a diet containing high concentrations of antibiotics removed potential microbial partners but did not affect the production of defensive benzenoids. To gain insights into benzenoid biosynthesis, we fed mites with stable-isotope labelled precursors and monitored incorporation with mass spectrometry. Glucose, malonic acid and acetate, but not phenylalanine, were incorporated into the benzenoids, further evidencing autogenous biosynthesis. Whole-transcriptome sequencing with hidden Markov model profile search of protein domain families and subsequent phylogenetic analysis revealed a putative PKS domain similar to an actinobacterial PKS, possibly indicating a horizontal gene transfer