Evidence for social parasitism of early insect societies by Cretaceous rove beetles

The genomic and cellular basis of biosynthetic innovation in rove beetles

How evolution at the cellular level potentiates macroevolutionary change is central to understanding biological diversification. The >66,000 rove beetle species (Staphylinidae) form the largest metazoan family. Combining genomic and cell type transcriptomic insights spanning the largest clade, Aleocharinae, we retrace evolution of two cell types comprising a defensive gland-a putative catalyst behind staphylinid megadiversity. We identify molecular evolutionary steps leading to benzoquinone production by one cell type via a mechanism convergent with plant toxin release systems, and synthesis by the second cell type of a solvent that weaponizes the total secretion. This cooperative system has been conserved since the Early Cretaceous as Aleocharinae radiated into tens of thousands of lineages. Reprogramming each cell type yielded biochemical novelties enabling ecological specialization-most dramatically in symbionts that infiltrate social insect colonies via host-manipulating secretions. Our findings uncover cell type evolutionary processes underlying the origin and evolvability of a beetle chemical innovation.

The genomic and cellular basis of biosynthetic innovation in rove beetles

How evolution at the cellular level potentiates change at the macroevolutionary level is a major question in evolutionary biology. With >66,000 described species, rove beetles (Staphylinidae) comprise the largest metazoan family. Their exceptional radiation has been coupled to pervasive biosynthetic innovation whereby numerous lineages bear defensive glands with diverse chemistries. Here, we combine comparative genomic and single-cell transcriptomic data from across the largest rove beetle clade, Aleocharinae. We retrace the functional evolution of two novel secretory cell types that together comprise the tergal gland-a putative catalyst behind Aleocharinae's megadiversity. We identify key genomic contingencies that were critical to the assembly of each cell type and their organ-level partnership in manufacturing the beetle's defensive secretion. This process hinged on evolving a mechanism for regulated production of noxious benzoquinones that appears convergent with plant toxin release systems, and synthesis of an effective benzoquinone solvent that weaponized the total secretion. We show that this cooperative biosynthetic system arose at the Jurassic-Cretaceous boundary, and that following its establishment, both cell types underwent ∼150 million years of stasis, their chemistry and core molecular architecture maintained almost clade-wide as Aleocharinae radiated globally into tens of thousands of lineages. Despite this deep conservation, we show that the two cell types have acted as substrates for the emergence of adaptive, biochemical novelties-most dramatically in symbiotic lineages that have infiltrated social insect colonies and produce host behavior-manipulating secretions. Our findings uncover genomic and cell type evolutionary processes underlying the origin, functional conservation and evolvability of a chemical innovation in beetles.

The oldest case of paedomorphosis in rove beetles and description of a new genus of Paederinae from Cretaceous amber (Coleoptera: Staphylinidae)

The ecology of extinct species from the Cretaceous is largely unknown. Morphological features of specimens preserved in amber can help to reveal habitats and evolutionary strategies that occurred in fossil lineages. An unusually small rove beetle (Staphylinidae) from the subfamily Paederinae with a Y-shaped suture on the head and modified tarsi and antennae is newly described here as Midinudon juvenis Tokareva & Żyła gen. et sp. nov. We hypothesise that such a combination of characters represents the earliest example of paedomorphosis in Staphylinidae and discuss other possible reasons that could explain the small size and morphological modifications of the new species. We provide the results of total-evidence phylogenetic analysis and discuss the relationships of Midinudon juvenis Tokareva & Żyła gen. et sp. nov. within Paederinae.

Early evolution of wing scales prior to the rise of moths and butterflies

Although scales are a defining and conspicuous feature of moths and butterflies (Lepidoptera),^1-3 their earliest evolution predates the group but is shrouded by a dearth of fossil evidence. Herein, we report two new species in mid-Cretaceous Kachin amber, representing lineages closely related to Lepidoptera: one represents the extinct Tarachoptera, with dense scales on the fore- and hindwings, while the other is an early lineage of caddisflies, with a hindwing covered by a single layer of angustifoliate scales. A novel phylogenetic analysis of 174 morphological characters and 73 extant and fossil representatives of Mecopterida demonstrates a monophyletic origin of scales in the common ancestor of Tarachoptera, Trichoptera, and Lepidoptera; that Tarachoptera are monophyletic but their scale morphology is plesiomorphic for the whole group; and that scales were lost early in caddisfly evolution before reappearing multiple times within the clade. Collectively, these fossils provide clarity into the origin and early evolution of scales before their diversification among the moths and butterflies.

Evolutionary history of interactions among terrestrial arthropods

The study of terrestrial arthopod fossils preserved with microscopic fidelity in amber and as permineralized replicas has been revolutionized by CT scanning. Fine preservation facilitates phylogenetic interpretation of fossils, but molecular divergence-time models still commonly use insufficient fossil calibrations, skewing estimates away from the direct (i.e. fossil, morphological) evidence. Interactions among terrestrial arthropods (predation, parasitoidism; phoresy, social symbionts) are briefly reviewed from the fossil record. Predation is the oldest and most widespread, originating with arachnids since probably the Silurian. The first phoretic arthropods were probably mites (Acari). Parasitoidism extends to the early Jurassic ~200 mya, with four main episodes proposed by [1•]. 100-myo Burmese amber, the most diverse Cretaceous paleobiota, is unique for our understanding of insect eusociality and interrelationships among terrestrial arthropods. Eusocial insect colonies are ecological sinks for thousands of symbiont species; ages of the major eusocial groups and some of their nest symbionts are discussed. Fossilized arthropod interrelationships in Miocene Dominican amber are presented as visual exemplars.

Parallel evolutionary paths of rove beetle myrmecophiles: replaying a deep-time tape of life

The rise of ants over the past ~100 million years reshaped the biosphere, presenting ecological challenges for many organisms, but also opportunities. No insect group has been so adept at exploiting niches inside ant colonies as the rove beetles (Staphylinidae) - a global clade of>64,000 predominantly free-living predators from which numerous socially parasitic 'myrmecophile' lineages have emerged. Myrmecophilous staphylinids are specialized for colony life through changes in behavior, chemistry, anatomy, and life history that are often strikingly convergent, and hence potentially adaptive for this symbiotic way of life. Here, we examine how the interplay between ecological pressures and molecular, cellular, and neurobiological mechanisms shape the evolutionary trajectories of symbiotic lineages in this ancient, convergent system.

Ecological radiations of insects in the Mesozoic

The Mesozoic is a key era for the rise of the modern insect fauna. Among the most important evolutionary events in Mesozoic insects are the radiation of holometabolous insects, the origin of eusocial and parasitoid insects, diversification of pollinating insects, and development of advanced mimicry and camouflage. These events are closely associated with the diversification of insect ecological behaviors and colonization of new ecospaces. At the same time, insects had evolved more complex and closer ecological associations with various plants and animals. Mesozoic insects played a key and underappreciated ecological role in reconstructing and maintaining terrestrial ecosystems. A greater understanding of the history of insects may help to mitigate future changes in insect diversity and abundance.

Termite nest evolution fostered social parasitism by termitophilous rove beetles

Colonies of social insects contain large amounts of resources often exploited by specialized social parasites. Although some termite species host numerous parasitic arthropod species, called termitophiles, others host none. The reason for this large variability remains unknown. Here, we report that the evolution of termitophily in rove beetles is linked to termite nesting strategies. We compared one-piece nesters, whose entire colony life is completed within a single wood piece, to foraging species, which exploit multiple physically separated food sources. Our epidemiological model predicts that characteristics related to foraging (e.g., extended colony longevity and frequent interactions with other colonies) increase the probability of parasitism by termitophiles. We tested our prediction using literature data. We found that foraging species are more likely to host termitophilous rove beetles than one-piece nesters: 99.6% of known termitophilous species were associated with foraging termites, whereas 0.4% were associated with one-piece nesters. Notably, the few one-piece nesting species hosting termitophiles were those having foraging potential and access to soil. Our phylogenetic analyses confirmed that termitophily primarily evolved with foraging termites. These results highlight that the evolution of complex termite societies fostered social parasitism, explaining why some species have more social parasites than others.

A new ant-butterfly symbiosis in the forest canopy fills an evolutionary gap

Myrmecophilous butterflies can establish complex symbiotic relationships with ants. A caterpillar wandering among the brood of the aggressive ponerine ant Neoponera villosa was found inside the core of a nest built in the myrmecophytic bromeliad Aechmea bracteata. This is the first caterpillar found living inside a ponerine ant nest. Its DNA barcode was sequenced, and an integrative approach was used to identify it as Pseudonymphidia agave, a poorly known member of the subtribe Pachythonina in the riodinid tribe Nymphidiini. The cuticle of the tank-like caterpillar lacks projections or tubercles and is covered dorsally by specialized flat setae that form an armor of small plates. Ant-organs potentially related to caterpillar-ant signaling, such as perforated cupola organs and tentacle nectary organs, are present. These morphological traits, together with evidence of social integration (direct contact with host brood, protective morphology, slow movement, no host aggressiveness), suggest that P. agave is a symbiotic, social parasite of N. villosa, preying on its host brood. However, several knowledge gaps remain, including oviposition site, dependence on bromeliad association, steps to colony integration, and larval diet through development. Carnivory has been reported in all known members of the subtribe Pachythonina (caterpillars prey on honeydew-producing hemipterans) suggesting a shift to myrmecophagy inside the ant nests as a possible evolutionary transition.

Multiple phenotypic traits as triggers of host attacks towards ant symbionts: body size, morphological gestalt, and chemical mimicry accuracy

Background

Ant colonies are plagued by a diversity of arthropod guests, which adopt various strategies to avoid or to withstand host attacks. Chemical mimicry of host recognition cues is, for example, a common integration strategy of ant guests. The morphological gestalt and body size of ant guests have long been argued to also affect host hostility, but quantitative studies testing these predictions are largely missing. We here evaluated three guest traits as triggers of host aggression—body size, morphological gestalt, and accuracy in chemical mimicry—in a community of six Eciton army ant species and 29 guest species. We quantified ant aggression towards 314 guests in behavioral assays and, for the same individuals, determined their body size and their accuracy in mimicking ant cuticular hydrocarbon (CHC) profiles. We classified guests into the following gestalts: protective, myrmecoid, staphylinid-like, phorid-like, and larval-shaped. We expected that (1) guests with lower CHC mimicry accuracy are more frequently attacked; (2) larger guests are more frequently attacked; (3) guests of different morphological gestalt receive differing host aggression levels.

Results

Army ant species had distinct CHC profiles and accuracy of mimicking these profiles was variable among guests, with many species showing high mimicry accuracy. Unexpectedly, we did not find a clear relationship between chemical host similarity and host aggression, suggesting that other symbiont traits need to be considered. We detected a relationship between the guests’ body size and the received host aggression, in that diminutive forms were rarely attacked. Our data also indicated that morphological gestalt might be a valuable predictor of host aggression. While most ant-guest encounters remained peaceful, host behavior still differed towards guests in that ant aggression was primarily directed towards those guests possessing a protective or a staphylinid-like gestalt.

Conclusion

We demonstrate that CHC mimicry accuracy does not necessarily predict host aggression towards ant symbionts. Exploitation mechanisms are diverse, and we conclude that, besides chemical mimicry, other factors such as the guests’ morphological gestalt and especially their body size might be important, yet underrated traits shaping the level of host hostility against social insect symbionts.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-021-00427-8.

Total-evidence backbone phylogeny of Aleocharinae (Coleoptera: Staphylinidae)

Phylogenetic studies of Aleocharinae rove beetles, arguably one of the least known and the largest insect lineages, are compromised by its enormous taxonomic diversity. DNA, a powerful resource for phylogenetics, is not available for numerous extant aleocharine species. We provide a broad comparative morphological study of Aleocharinae to frame molecular datasets for total-evidence analyses. Using full-body dissections and slide-mounting techniques for light microscopy supplemented by scanning electron microscopy, we constructed a morphological matrix across all major taxa focused on non-inquiline tribes of Aleocharinae and outgroups. Phylogenetic analyses of this matrix concatenated with earlier published DNA loci and including exemplar taxa lacking molecular data, resolved outstanding controversies and, among other novelties, showed that: the Habrocerinae + Trichophyinae clade is sister group to Aleocharinae; Hypocyphtini are sister to the rest of the "higher Aleocharinae"; Taxicerini are sister to Aleocharini; Hoplandriini and Placusini are nested within a polyphyletic Oxypodini; Hoplandriini are sister to Meoticina; and Actocharini are nested within Liparocephalini. For the first time, morphological synapomorphies are identified for some large clades of Aleocharinae. In addition, 1252 high-resolution microphotographs of aleocharine structures are made available online with the entire matrix for future research.

Tachyporinae Revisited: Phylogeny, Evolution, and Higher Classification Based on Morphology, with Recognition of a New Rove Beetle Subfamily (Coleoptera: Staphylinidae)

Simple Summary

The rove beetle subfamily Tachyporinae has been suggested to be polyphyletic for the last half century but there are no previous studies conducting phylogenetic analysis on this group specifically. Here, the most comprehensive tachyporine phylogeny is shown, which again rejects the monophyly of Tachyporinae and its largest tribe Tachyporini. A revised classification of Tachyporinae is proposed here based on observation of morphological characters and their phylogenetic analyses. This backbone phylogeny will be a framework for further evolutionary and ecological studies.

Abstract

Tachyporinae are one of the most phylogenetically problematic subfamilies in the mega-diverse rove beetle family Staphylinidae. Despite its high diversity and abundance in forest micro-environments, with over 1600 species worldwide, several previous studies had refuted the monophyly of this subfamily and its largest tribe, Tachyporini. Based on the results of morphology-based phylogenetic analyses and direct examination of specimens encompassing two extinct and all forty extant genera, a new suprageneric classification of Tachyporinae is proposed here, with the removal of the tribe Mycetoporini into a newly recognized subfamily Mycetoporinae stat. nov. Four tribes with two subtribes are arranged within Tachyporinae sensu nov.: Tachyporini sensu nov. (Tachyporina stat. nov., sensu nov. and Euconosomatina stat. rev., sensu nov.), Vatesini sensu nov., Deropini, and Tachinusini stat. rev., sensu nov. (= Megarthropsini syn. nov.). Urolitus syn. nov. is placed as a junior synonym of Sepedophilus. Additionally, Palporus stat. nov. is raised to a distinct genus from a subgenus of Tachyporus sensu. nov., and †Mesotachyporus syn. nov. is synonymized with the latter. Mycetoporine Bobitobus stat. rev. is resurrected from synonymy with Lordithon sensu nov., and considered as a valid genus. My revised classification provides a novel framework for taxonomic inventories and ecological studies of these groups.

Termite colonies from mid-Cretaceous Myanmar demonstrate their early eusocial lifestyle in damp wood

Insect eusociality is characterized by cooperative brood care, reproductive division of labour and multiple generations of adults within a colony. The morphological specializations of the different termite castes from Burmese amber were recently reported, indicating the termites possessed advanced sociality in the mid-Cretaceous. Unfortunately, all the reported Cretaceous termites are individually preserved, which does not cover the behaviours of the cooperative brood care and multiple generations of adults in the nests of the Cretaceous termites. Herein, we report three eusocial aggregations from colonies of the oldest known Stolotermitidae, Cosmotermes gen. nov., in 100 Ma mid-Cretaceous Burmese amber. One large aggregation, comprising 8 soldiers, 56 workers/pseudergates and 25 immatures of different instars, additionally presents the behaviours of cooperative brood care and overlapping generations. Furthermore, taphonomic evidence indicates Cosmotermes most probably dwelled in damp/rotting wood, which provides a broader horizon of the early societies and ecology of the eusocial Cosmotermes.

New insects feeding on dinosaur feathers in mid-Cretaceous amber

Due to a lack of Mesozoic fossil records, the origins and early evolution of feather-feeding behaviors by insects are obscure. Here, we report ten nymph specimens of a new lineage of insect, Mesophthirus engeli gen et. sp. nov. within Mesophthiridae fam. nov. from the mid-Cretaceous (ca. 100 Mya) Myanmar (Burmese) amber. This new insect clade shows a series of ectoparasitic morphological characters such as tiny wingless body, head with strong chewing mouthparts, robust and short antennae having long setae, legs with only one single tarsal claw associated with two additional long setae, etc. Most significantly, these insects are preserved with partially damaged dinosaur feathers, the damage of which was probably made by these insects' integument-feeding behaviors. This finding demonstrates that feather-feeding behaviors of insects originated at least in mid-Cretaceous, accompanying the radiation of feathered dinosaurs including early birds.

Evidence from amber for the origins of termitophily

Fossil morphology is often used to infer the ecology of extinct species. In a recent report in Current Biology, Cai and colleagues [1] described an extinct rove beetle, Cretotrichopsenius burmiticus, from two specimens in mid-Cretaceous Burmese amber (∼99 million years old). Based on morphology and the taxonomic group to which the specimens belong, the authors proposed that Cretotrichopsenius was a termitophile - a socially parasitic symbiont of termite colonies. Moreover, the new taxon was claimed to represent the oldest "unequivocal" termitophile so far discovered, pushing back the known evolutionary history of termitophily by ∼80 million years, close to the origin of termite eusociality. Cretotrichopsenius is certainly an important discovery for understanding the evolutionary steps leading to this type of social insect symbiosis. However, we issue a caveat here concerning the authors' assertion that Cretotrichopsenius was truly termitophilous. Additionally, we question the authors' representation of a previously published, likely-termitophilous rove beetle in Burmese amber [2].

Associations between Fossil Beetles and Other Organisms

The present work reveals plant and animal associates of 16 families and subfamilies of fossil beetles that have been preserved in amber from Mexico, the Dominican Republic, and Myanmar. The associates include mites, pseudoscorpions, spiders, insect parasites and predators, fungi, angiosperm parts, vertebrates, and nematodes. The presence of these fossil associates can be attributed to the rapid preservation of organisms in resin, thus maintaining natural associations almost “in situ”. Examples of present-day associations similar to those of the fossils show that specific behavioral patterns are often far more ancient than the specific lineages involved.

A Mesozoic clown beetle myrmecophile (Coleoptera: Histeridae)

Complex interspecies relationships are widespread among metazoans, but the evolutionary history of these lifestyles is poorly understood. We describe a fossil beetle in 99-million-year-old Burmese amber that we infer to have been a social impostor of the earliest-known ant colonies. Promyrmister kistneri gen. et sp. nov. belongs to the haeteriine clown beetles (Coleoptera: Histeridae), a major clade of 'myrmecophiles'-specialized nest intruders with dramatic anatomical, chemical and behavioral adaptations for colony infiltration. Promyrmister reveals that myrmecophiles evolved close to the emergence of ant eusociality, in colonies of stem-group ants that predominate Burmese amber, or with cryptic crown-group ants that remain largely unknown at this time. The clown beetle-ant relationship has been maintained ever since by the beetles host-switching to numerous modern ant genera, ultimately diversifying into one of the largest radiations of symbiotic animals. We infer that obligate behavioral symbioses can evolve relatively rapidly, and be sustained over deep time.

Insect societies fight back: the evolution of defensive traits against social parasites

Insect societies face many social parasites that exploit their altruistic behaviours or their resources. Due to the fitness costs these social parasites incur, hosts have evolved various behavioural, chemical, architectural and morphological defence traits. Similar to bacteria infecting multicellular hosts, social parasites have to successfully go through several steps to exploit their hosts. Here, we review how social insects try to interrupt this sequence of events. They can avoid parasite contact by choosing to nest in parasite-free locales or evade attacks by adapting their colony structure. Once social parasites attack, hosts attempt to detect them, which can be facilitated by adjustments in colony odour. If social parasites enter the nest, hosts can either aggressively defend their colony or take their young and flee. Nest structures are often shaped to prevent social parasite invasion or to safeguard host resources. Finally, if social parasites successfully establish themselves in host nests, hosts can rebel by killing the parasite brood or by reproducing in the parasites' presence. Hosts of social parasites can therefore develop multiple traits, leading to the evolution of complex defence portfolios of co-dependent traits. Social parasites can respond to these multi-level defences with counter-adaptations, potentially leading to geographical mosaics of coevolution.This article is part of the Theo Murphy meeting issue 'Evolution of pathogen and parasite avoidance behaviours'.

Response to "Evidence from amber for the origins of termitophily"

In a recent Current Biology paper [1], we reported the oldest, morphologically specialized, and obligate termitophiles, Cretotrichopsenius burmiticus (Figure 1, left), from mid-Cretaceous Burmese amber, about 99 million years old. Cretotrichopsenius, belonging to the obligately termitophilous rove beetle tribe Trichopseniini, display the protective horseshoe-crab-shaped body typical of many extant termitophiles. However, the termitophilous lifestyle of Cretotrichopsenius is being questioned by Yamamoto et al.[2] based on their representation of the termitophile-related features and premature and presumptive phylogenetic placement of Cretotrichopsenius within Trichopseniini. We stand by our interpretation that Cretotrichopsenius are obligate termitophiles, and Mesosymbion[3], a member of the largely free-living Mesoporini, are not necessarily termitophilous.

Cretaceous origin of the unique prey-capture apparatus in mega-diverse genus: stem lineage of Steninae rove beetles discovered in Burmese amber

Stenus is the largest genus of rove beetles and the second largest among animals. Its evolutionary success was associated with the adhesive labial prey-capture apparatus, a unique apomorphy of that genus. Definite Stenus with prey-capture apparatus are known from the Cenozoic fossils, while the age and early evolution of Steninae was hardly ever hypothesized. Our study of several Cretaceous Burmese amber inclusions revealed a stem lineage of Steninae that possibly possesses the Stenus-like prey-capture apparatus. Phylogenetic analysis of extinct and extant taxa of Steninae and putatively allied subfamilies of Staphylinidae with parsimony and Bayesian approaches resolved the Burmese amber lineage as a member of Steninae. It justified the description of a new extinct stenine genus Festenus with two new species, F. robustus and F. gracilis. The Late Cretaceous age of Festenus suggests an early origin of prey-capture apparatus in Steninae that, perhaps, drove the evolution towards the crown Stenus. Our analysis confirmed the well-established sister relationships between Steninae and Euaesthetinae and resolved Scydmaeninae as their next closest relative, the latter having no stable position in recent phylogenetic studies of rove beetles. Close affiliation of Megalopsidiinae, a subfamily often considered as a sister group to Euaesthetinae + Steninae clade, is rejected.

Deep-Time Convergence in Rove Beetle Symbionts of Army Ants

Recent adaptive radiations provide striking examples of convergence [1-4], but the predictability of evolution over much deeper timescales is controversial, with a scarcity of ancient clades exhibiting repetitive patterns of phenotypic evolution [5, 6]. Army ants are ecologically dominant arthropod predators of the world's tropics, with large nomadic colonies housing diverse communities of socially parasitic myrmecophiles [7]. Remarkable among these are many species of rove beetle (Staphylinidae) that exhibit ant-mimicking "myrmecoid" body forms and are behaviorally accepted into their aggressive hosts' societies: emigrating with colonies and inhabiting temporary nest bivouacs, grooming and feeding with workers, but also consuming the brood [8-11]. Here, we demonstrate that myrmecoid rove beetles are strongly polyphyletic, with this adaptive morphological and behavioral syndrome having evolved at least 12 times during the evolution of a single staphylinid subfamily, Aleocharinae. Each independent myrmecoid clade is restricted to one zoogeographic region and highly host specific on a single army ant genus. Dating estimates reveal that myrmecoid clades are separated by substantial phylogenetic distances-as much as 105 million years. All such groups arose in parallel during the Cenozoic, when army ants diversified into modern genera [12] and rose to ecological dominance [13, 14]. This work uncovers a rare example of an ancient system of complex morphological and behavioral convergence, with replicate beetle lineages following a predictable phenotypic trajectory during their parasitic adaptation to host colonies.