Arthropod-Microbiota Integration: Its Importance for Ecosystem Conservation

Recent reports indicate that the health of our planet is getting worse and that genuine transformative changes are pressing. So far, efforts to ameliorate Earth's ecosystem crises have been insufficient, as these often depart from current knowledge of the underlying ecological processes. Nowadays, biodiversity loss and the alterations in biogeochemical cycles are reaching thresholds that put the survival of our species at risk. Biological interactions are fundamental for achieving biological conservation and restoration of ecological processes, especially those that contribute to nutrient cycles. Microorganism are recognized as key players in ecological interactions and nutrient cycling, both free-living and in symbiotic associations with multicellular organisms. This latter assemblage work as a functional ecological unit called "holobiont." Here, we review the emergent ecosystem properties derived from holobionts, with special emphasis on detritivorous terrestrial arthropods and their symbiotic microorganisms. We revisit their relevance in the cycling of recalcitrant organic compounds (e.g., lignin and cellulose). Finally, based on the interconnection between biodiversity and nutrient cycling, we propose that a multicellular organism and its associates constitute an Ecosystem Holobiont (EH). This EH is the functional unit characterized by carrying out key ecosystem processes. We emphasize that in order to meet the challenge to restore the health of our planet it is critical to reduce anthropic pressures that may threaten not only individual entities (known as "bionts") but also the stability of the associations that give rise to EH and their ecological functions.

Diet is not the primary driver of bacterial community structure in the gut of litter-feeding cockroaches

BACKGROUND

Diet is a major determinant of bacterial community structure in termite guts, but evidence of its importance in the closely related cockroaches is conflicting. Here, we investigated the ecological drivers of the bacterial gut microbiota in cockroaches that feed on lignocellulosic leaf litter.

RESULTS

The physicochemical conditions determined with microsensors in the guts of Ergaula capucina, Pycnoscelus surinamensis, and Byrsotria rothi were similar to those reported for both wood-feeding and omnivorous cockroaches. All gut compartments were anoxic at the center and showed a slightly acidic to neutral pH and variable but slightly reducing conditions. Hydrogen accumulated only in the crop of B. rothi. High-throughput amplicon sequencing of bacterial 16S rRNA genes documented that community structure in individual gut compartments correlated strongly with the respective microenvironmental conditions. A comparison of the hindgut microbiota of cockroaches and termites from different feeding groups revealed that the vast majority of the core taxa in cockroaches with a lignocellulosic diet were present also in omnivorous cockroaches but absent in wood-feeding higher termites.

CONCLUSION

Our results indicate that diet is not the primary driver of bacterial community structure in the gut of wood- and litter-feeding cockroaches. The high similarity to the gut microbiota of omnivorous cockroaches suggests that the dietary components that are actually digested do not differ fundamentally between feeding groups.

When Darwin's Special Difficulty Promotes Diversification in Insects

Eusociality, Darwin's special difficulty, has been widely investigated but remains a topic of great debate in organismal biology. Eusocial species challenge existing theories, and the impact of highly integrated societies on diversification dynamics is controversial with opposing assertions and hypotheses in the literature. Here, using phylogenetic approaches in termites-the first group that has evolved eusociality-we assessed the fundamental prediction that eusocial lineages have higher diversification rates than non-eusocial clades. We found multiple lines of evidence that eusociality provided higher diversification as compared to non-eusociality. This is particularly exacerbated for eusocial species with "true" workers as compared to species with "false" workers. Because most species with "true" workers have an entirely prokaryotic microbiota, the latter feature is also related to higher diversification rates, but it should be investigated further, notably in relation to angiosperm diversification. Overall, this study suggests that societies with "true" workers are not only more successful at ecological timescales but also over millions of years, which further implies that both organism- and species-level traits act on species selection.

The evolution of sociality in termites from cockroaches: A taxonomic and phylogenetic perspective

Despite multiple studies and advances, sociality still puzzles evolutionary biologists in numerous ways, which might be partly addressed with the advent of sociogenomics. In insects, the majority of sociogenomic studies deal with Hymenoptera, one of the two groups that evolved eusociality with termites. But, to fully grasp the evolution of sociality, studies must obviously not restrict to eusocial lineages. Multiple kinds of social system transitions have been recorded and they all bring complementary insights. For instance, cockroaches, the closest relatives to termites, display a wide range of social interactions and evolved convergently subsocial behaviors (i.e., brood care). In this context, we emphasize the need for natural history, taxonomic, and phylogenetic studies. Natural history studies provide the foundations on which building hypotheses, whereas taxonomy provides the taxa to sample to test these hypotheses, and phylogenetics brings the historical framework necessary to test evolutionary scenarios of sociality evolution.

Gut Bacterial Community of the Xylophagous Cockroaches Cryptocercus punctulatus and Parasphaeria boleiriana

Cryptocercus punctulatus and Parasphaeria boleiriana are two distantly related xylophagous and subsocial cockroaches. Cryptocercus is related to termites. Xylophagous cockroaches and termites are excellent model organisms for studying the symbiotic relationship between the insect and their microbiota. In this study, high-throughput 454 pyrosequencing of 16S rRNA was used to investigate the diversity of metagenomic gut communities of C. punctulatus and P. boleiriana, and thereby to identify possible shifts in symbiont allegiances during cockroaches evolution. Our results revealed that the hindgut prokaryotic communities of both xylophagous cockroaches are dominated by members of four Bacteria phyla: Bacteroidetes, Firmicutes, Proteobacteria, and Actinobacteria. Other identified phyla were Spirochaetes, Planctomycetes, candidatus Saccharibacteria (formerly TM7), and Acidobacteria, each of which represented 1–2% of the total population detected. Community similarity based on phylogenetic relatedness by unweighted UniFrac analyses indicated that the composition of the bacterial community in the two species was significantly different (P < 0.05). Phylogenetic analysis based on the characterized clusters of Bacteroidetes, Spirochaetes, and Deltaproteobacteria showed that many OTUs present in both cockroach species clustered with sequences previously described in termites and other cockroaches, but not with those from other animals or environments. These results suggest that, during their evolution, those cockroaches conserved several bacterial communities from the microbiota of a common ancestor. The ecological stability of those microbial communities may imply the important functional role for the survival of the host of providing nutrients in appropriate quantities and balance.

Dyadic behavioural interactions in cockroaches (Blaberidae): ecomorphological and evolutionary implications

Few studies have assessed the relative importance of morphological, ecological and phylogenetic factors in the evolution of social behaviour. We examine the role of these factors in social evolution among blaberid cockroaches. We first analyse and compare behavioural interactions in 13 species. We then ask how the nature of these interactions relates to body shape, phylogeny and habitat. We showed that, although these cockroaches display diverse behavioural interactions, a structure in these data exists with some species clustering together. We found that similarity in social interactions was related to species body shape, but not to ecology or phylogenetic relationships. We suggest that body shape plays an important role in the evolution of social behaviour and that this factor should be investigated further in future analyses.

The phylogeny of termites (Dictyoptera: Isoptera) based on mitochondrial and nuclear markers: Implications for the evolution of the worker and pseudergate castes, and foraging behaviors

A phylogenetic hypothesis of termite relationships was inferred from DNA sequence data. Seven gene fragments (12S rDNA, 16S rDNA, 18S rDNA, 28S rDNA, cytochrome oxidase I, cytochrome oxidase II and cytochrome b) were sequenced for 40 termite exemplars, representing all termite families and 14 outgroups. Termites were found to be monophyletic with Mastotermes darwiniensis (Mastotermitidae) as sister group to the remainder of the termites. In this remainder, the family Kalotermitidae was sister group to other families. The families Kalotermitidae, Hodotermitidae and Termitidae were retrieved as monophyletic whereas the Termopsidae and Rhinotermitidae appeared paraphyletic. All of these results were very stable and supported with high bootstrap and Bremer values. The evolution of worker caste and foraging behavior were discussed according to the phylogenetic hypothesis. Our analyses suggested that both true workers and pseudergates ("false workers") were the result of at least two different origins. Our data support a traditional hypothesis of foraging behavior, in which the evolutionary transition from a one-piece type to a separate life type occurred through an intermediate behavioral form.

The conservation-refugium value of small and disturbed Brazilian Atlantic forest fragments for the endemic ovoviviparous cockroach Monastria biguttata (Insecta: Dictyoptera, Blaberidae, Blaberinae)

The Brazilian Atlantic forest is a biodiversity hotspot and harbors many endemic species showing peculiar and unique traits. However, it has been reduced to less than 8% of its original surface and is distributed in scattered fragments, the great majority of which are smaller than 20 hectares and very disturbed, making it worth asking about their value for conservation. In this paper we assess the refugium value of small fragments to the conservation of one of the endemics of the Atlantic forest, the ovoviviparous cockroach Monastria biguttata. Our results showed that this species was ubiquitous in large and small forest fragments, but never present in plantations or pastures. The population age structure and sex ratio were balanced in every fragment, and total population size in the smallest fragments was at least several hundreds of individuals. Colony size, sex ratio, age structure, and density per piece of dead tree trunk indicated that populations from small fragments were not unbalanced or at risk of extinction. According to the analysis of resource availability, small fragments can provide suitable habitat for this species. In this situation, even very small forest fragments have a high refugium value for some endemic insect species. Considering their number in the landscape, these fragments should be considered with more attention in strategies of biodiversity conservation.

Wood-feeding cockroaches as models for termite evolution (Insecta: Dictyoptera): Cryptocercus vs. Parasphaeria boleiriana

Isoptera are highly specialized cockroaches and are one of the few eusocial insect lineages. Cryptocercus cockroaches have appeared to many as ideal models for inference on the early evolution of termites, due to their possible phylogenetic relationship and several shared key attributes in life history. Recently, Pellens, Grandcolas, and colleagues have proposed the blaberid cockroach Parasphaeria boleiriana to be an alternative model for the early evolution in termites. We compare the usefulness of Cryptocercus and P. boleiriana as models for termite evolution. Cryptocercus and lower Isoptera (1) can both feed on comparatively recalcitrant wood, (2) have an obligate, rich and unique hypermastigid and oxymonadid fauna in the hindgut, (3) transfer these flagellates to the next generation by anal trophallaxis, (4) have social systems that involve long-lasting biparental care, and, finally, (5) are strongly suggested to be sister groups, so that the key attributes (1)-(4) appear to be homologous between the two taxa. On the other hand, P. boleiriana (1) feeds on soft, ephemeral wood sources, (2) shows no trace of the oxymonadid and hypermastigid hindgut fauna unique to Cryptocercus and lower Isoptera, nor does it have any other demonstrated obligate relationship with hindgut flagellates, (3) is likely to lack anal trophallaxis, (4) has only a short period of uniparental brood care, and (5) is phylogenetically remote from the Cryptocercus+Isoptera clade. These facts would argue against any reasonable usage of P. boleiriana as a model for the early evolution of Isoptera or even of the clade Cryptocercus+Isoptera. Cryptocercus thus remains an appropriate model-taxon-by-homology for early termite evolution. As compared to P. boleiriana, some other Blaberidae (such as the Panesthiinae Salganea) appear more useful as model-taxa-by-homoplasy for the early evolution of the Cryptocercus+Isoptera clade, as their brooding behavior is more elaborate than in P. boleiriana.

Weakly aggressive behaviour towards nymphs in the cockroach Schultesia nitor (Blattaria: Zetoborinae)

This paper describes aggressive behaviour in the cockroach Schultesia nitor, a tropical forest species living in bird nests. Young S. nitor nymphs are known to show active dispersal while old nymphs and adults are contrastingly strongly gregarious, a combination of features never observed in other cockroach species. Our laboratory experiments using video recording of confrontations between pairs show that aggressive behaviour towards conspecific nymphs is not exhibited towards nymphs of the species Phoetalia pallida, and thus can be considered species specific in S. nitor. But, it is not kin oriented: the mother and all adults of both sexes in different physiological states exhibit this behaviour as well. Six types of aggressive interactions were discriminated, occurring in age-symmetric pairs of nymphs and adults. Even more frequent aggression was exhibited by adults and last instar nymphs towards younger nymphs of all instars. The frequency of aggressive acts and types of aggressive interactions varied according to sex and size of the two interacting individuals. The possible function and evolution of this behaviour is discussed, with emphasis on the difficulty of interpreting obvious but weak and not kin-biased aggression.

The origin of a ‘true’ worker caste in termites: mapping the real world on the phylogenetic tree

The evolution of the 'true' worker caste in termites is not decisively inferred by coding and mapping both this character and the foraging behaviour on a phylogenetic tree. Answering to Thompson et al. (2000, 2003), and with reference to Grandcolas and D'Haese (2002), we show that this indecisive inference depends on the correct consideration paid to the outgroups. These last ones could be non subsocial cockroaches, or some wood-eating subsocial cockroaches often considered misleadingly as living ancestors, or even any hemimetabolous insects, all of them would be unambiguously lacking 'true' worker caste and pseudergate caste and not showing the 'one-piece' life type foraging behaviour. These statements derive from observing, coding and mapping the real world on the tree without making ad hoc assumptions. In that respect, because termites do not exist in isolation, apart from the tree of life, mapping the character of interest on the tree must be applied to the outgroups as well.

Living in Atlantic forest fragments: life habits, behaviour, and colony structure of the cockroach Monastria biguttata (Dictyoptera, Blaberidae, Blaberinae) in Espirito Santo, Brazil

The life habits, behaviour, and colony structure of the cockroach Monastria biguttata were studied during 2 years in remnants of the Brazilian Atlantic forest. Monastria biguttata was abundant, conspicuous, and ubiquitous in the semideciduous forest in the north of Espirito Santo, Brazil. It was found in forest fragments of various sizes and disturbance levels, but never in the surrounding plantations. Adults and nymphs were found in colonies of 2.8–11.6 cockroaches, grasping the bark of the underside of dead trunks that were clumped on the ground, in the forest understory. The cockroaches were very sedentary, moved extremely slowly, and immediately froze when disturbed. Based on their brooding behaviour and aggregated distribution on trunks, they appeared to be gregarious with only a slow spread during nymphal development. Brood birth took place during the rainy period, and nymphs developed into adults in a minimum of 2 years. Brood size and egg number in oothecae were not very high (23.0 ± 1.5 and 31.1 ± 1.7 (mean ± SE), respectively). All these traits were analysed to understand the survival of species in forest fragments.

The evolution of soil-burrowing cockroaches (Blattaria: Blaberidae) from wood-burrowing ancestors following an invasion of the latter from Asia into Australia

Morphologically similar cockroaches in the subfamilies Panesthiinae and Geoscapheinae (Blattaria: Blaberidae) display contrasting feeding habits, behaviour and biogeographical distributions. Panesthiinae, found throughout Asia and Australia, all live in and feed on decaying wood that they burrow into. Geoscapheinae are restricted to Australia and construct and live in burrows in the soil, where they feed on dry leaves taken from the surface. A lack of knowledge about phylogenetic relationships among these cockroaches hinders an understanding of the factors that have shaped the evolution of their diverse lifestyles and biogeography. To address this issue, we sequenced three genes from representatives of nine of the 10 genera in the two subfamilies, and performed phylogenetic analyses. The well-supported topology revealed the Panesthiinae to be paraphyletic with respect to the Geoscapheinae. Soil-burrowing cockroaches appear to have evolved from a lineage of wood burrowers that invaded Australia from the north some time after the merging of the Asian and Australian tectonic plates ca. 20 Myr ago. The main factor promoting the evolution of soil burrowing is likely to have been one of the periods of strong aridity that Australia has experienced since the Miocene period.

Electron microscopic identification of the intestinal protozoan flagellates of the xylophagous cockroach Parasphaeria boleiriana from Brazil

Flagellate protozoa of the hindgut of the xylophagous blattid Parasphaeria boleiriana were examined by light and electron microscopy. This species harbours two oxymonad species of the genera Monocercomonoides and Polymastix, the latter bearing Fusiformis bacteria on its surface. A diplomonad was present and has features of the genus Hexamita rather than Spironucleus. In addition, two trichomonads of the genera Monocercomonas and Tetratrichomastix were identified. A precise comparison with species of blattids and other insects was difficult because most of these flagellates have been described only by light microscopy after cell staining and there are few electron microscope studies and no molecular studies. None of the flagellates contained wood fragments in their food vacuoles and so evidently do not participate in the digestion of wood or cellulose.

Colony composition, social behavior and some ecological characteristics of the Korean wood-feeding cockroach (Cryptocercus kyebangensis)

Korean populations of the genus Cryptocercus occur in forested mountains throughout South Korea. They live in monogamous associations in which parents care for their young in complex woody galleries. Single paired adults (23.2%) and one or both parents with their offspring (28.1%) were found most frequently in the field. Among single-parent families adult females (6.7%) were observed more frequently than adult males (1.4%). In families with single or both parents, the mean brood size was 21.6+/-9.4. Oothecae were observed from mid-June to the late July. Oothecae were found in the galleries of only paired adults and never found in families with nymphs. The mean number of eggs per female was 73.7+/-29.8. Most of neonates grew to the third or fourth instar prior to the winter. During the winter, C. kyebangensis in the field remained almost frozen in their galleries, but ones kept in the laboratory continued to grow during winter. Some characteristics of proctodeal feeding behavior are also described based on laboratory observations. We propose that the cold temperate climate, especially of the winter season, is one of the most important causes for the evolution of unusual life history of Cryptocercus including delayed development of nymphs.