Fine structure of the midgut epithelium in the millipede Telodeinopus aoutii (Myriapoda, Diplopoda) with special emphasis on epithelial regeneration

The venom and telopodal defence systems of the centipede Lithobius forficatus are functionally convergent serial homologues

BACKGROUND

Evolution of novelty is a central theme in evolutionary biology, yet studying the origins of traits with an apparently discontinuous origin remains a major challenge. Venom systems are a well-suited model for the study of this phenomenon because they capture several aspects of novelty across multiple levels of biological complexity. However, while there is some knowledge on the evolution of individual toxins, not much is known about the evolution of venom systems as a whole. One way of shedding light on the evolution of new traits is to investigate less specialised serial homologues, i.e. repeated traits in an organism that share a developmental origin. This approach can be particularly informative in animals with repetitive body segments, such as centipedes.

RESULTS

Here, we investigate morphological and biochemical aspects of the defensive telopodal glandular organs borne on the posterior legs of venomous stone centipedes (Lithobiomorpha), using a multimethod approach, including behavioural observations, comparative morphology, proteomics, comparative transcriptomics and molecular phylogenetics. We show that the anterior venom system and posterior telopodal defence system are functionally convergent serial homologues, where one (telopodal defence) represents a model for the putative early evolutionary state of the other (venom). Venom glands and telopodal glandular organs appear to have evolved from the same type of epidermal gland (four-cell recto-canal type) and while the telopodal defensive secretion shares a great degree of compositional overlap with centipede venoms in general, these similarities arose predominantly through convergent recruitment of distantly related toxin-like components. Both systems are composed of elements predisposed to functional innovation across levels of biological complexity that range from proteins to glands, demonstrating clear parallels between molecular and morphological traits in the properties that facilitate the evolution of novelty.

CONCLUSIONS

The evolution of the lithobiomorph telopodal defence system provides indirect empirical support for the plausibility of the hypothesised evolutionary origin of the centipede venom system, which occurred through functional innovation and gradual specialisation of existing epidermal glands. Our results thus exemplify how continuous transformation and functional innovation can drive the apparent discontinuous emergence of novelties on higher levels of biological complexity.

Histologic lesions of giant African millipedes (Archispirostreptus gigas) from a zoological institution

Histopathologic data of millipedes are scarce. Little is known about health and disease of these invertebrates despite their exhibition at zoological institutions and use in ecotoxicological studies. In a retrospective study of 69 zoo-housed giant African millipedes (Archispirostreptus gigas) submitted between 2018 and 2021, most deaths occurred during midwinter and in 2021. The most common lesion was inflammation (n = 55; 80%). Necrosis was seen concurrently in 31 (45%) millipedes and of these, bacteria (20; 29%) and fungi (7; 10%) were detected in lesions. Inflammation was seen in the head/collum (20; 29%), hemocoel (16; 23%), and appendages (9; 13%), specifically in perivisceral fat body (42; 61%), gut (16; 23%), tracheae (26; 38%), skeletal muscle (24; 35%), and ventral nerve (17; 25%). Inflammatory cell types and patterns included agranular hemocytes (61; 88%), granular hemocytes (39; 57%), and nodulation/encapsulation (47; 68%) often accompanied by melanization. The oral cavity or gut (ingestion), spiracles (inhalation), or cuticular defects were considered plausible routes of bacterial entry. Metazoan parasites (adult nematodes: 2, 3%; trematode ova: 2, 3%; and arthropods: 1, 1%) were associated with gut necrosis and inflammation in 5 millipedes. In addition, adult nematodes were noted in the gut of 4 millipedes without lesions. Neoplasia was not detected in any millipedes. Speculatively, environmental factors may have predisposed to disease, as most deaths occurred during winter months. Disease surveillance of millipedes is critical to optimize husbandry practices in zoo populations and investigate potential impacts of environmental degradation and climate change on wild millipedes.

Effect of gluten on the digestive tract and fat body of Telodeinopus aoutii (Diplopoda)

Adult specimens or larvae of invertebrates used as food for vertebrates are often maintained close to gluten so they might become vectors for cereal proteins. However, the tissues and internal organs can respond differently in animals with different feeding habits. The midgut epithelium might be a first and sufficient barrier preventing uptake and effects of gluten on the whole body, while the fat body is the main organ that accumulates different xenobiotics. Good models for such research are animals that do not feed on gluten-rich products in their natural environment. The project's goal was to investigate alterations in the midgut epithelium and fat body of the herbivorous millipede Telodeinopus aoutii (Diplopoda) and analyze cell death processes activated by gluten. It enabled us to determine whether changes were intensified or reversed by adaptive mechanisms. Adult specimens were divided into control and experimental animals fed with mushrooms supplemented with gluten and analyzed using transmission electron microscopy, flow cytometry, and confocal microscopy. Two organs were isolated for the qualitative and quantitative analysis: the midgut and the fat body. Our study of the herbivorous T. aoutii which does not naturally feed on gluten containing diet showed that continuous and prolonged gluten feeding activates repair processes that inhibit the processes of cell death (apoptosis and necrosis) and induce an increase in cell viability.

De novo metatranscriptomic exploration of gene function in the millipede holobiont

Invertebrate-microbial associations are widespread in the biosphere and are often related to the function of novel genes, fitness advantages, and even speciation events. Despite ~ 13,000 species of millipedes identified across the world, millipedes and their gut microbiota are markedly understudied compared to other arthropods. Exploring the contribution of individual host-associated microbes is often challenging as many are uncultivable. In this study, we conducted metatranscriptomic profiling of different body segments of a millipede at the holobiont level. This is the first reported transcriptome assembly of a tropical millipede Telodeinopus aoutii (Demange, 1971), as well as the first study on any Myriapoda holobiont. High-throughput RNA sequencing revealed that Telodeinopus aoutii contained > 90% of the core Arthropoda genes. Proteobacteria, Bacteroidetes, Firmicutes, and Euryarchaeota represented dominant and functionally active phyla in the millipede gut, among which 97% of Bacteroidetes and 98% of Firmicutes were present exclusively in the hindgut. A total of 37,831 predicted protein-coding genes of millipede holobiont belonged to six enzyme classes. Around 35% of these proteins were produced by microbiota in the hindgut and 21% by the host in the midgut. Our results indicated that although major metabolic pathways operate at the holobiont level, the involvement of some host and microbial genes are mutually exclusive and microbes predominantly contribute to essential amino acid biosynthesis, short-chain fatty acid metabolism, and fermentation.

Ovaries and testes of Lithobius forficatus (Myriapoda, Chilopoda) react differently to the presence of cadmium in the environment

Proper reproduction depends on properly functioning gonads (ovaries and testes). Many xenobiotics, including heavy metals, can cause changes in somatic and germ line cells, thus damaging the reproductive capacity. The aim of this study was to investigate the effect of the heavy metal cadmium on the gonads, including germ line and somatic cells. It is important to determine whether cell death processes are triggered in both types of cells in the gonads, and which gonads are more sensitive to the presence of cadmium in the environment. The research was conducted on the soil-dwelling arthropod Lithobius forficatus (Myriapoda, Chilopoda), which is common for European fauna. Animals were cultured in soil supplemented with Cd for different periods (short- and long-term treatment). Gonads were isolated and prepared for qualitative and quantitative analysis, which enabled us to describe all changes which appeared after both the short- and long-term cadmium treatment. The results of our study showed that cadmium affects the structure and ultrastructure of both gonads in soil-dwelling organisms including the activation of cell death processes. However, the male germ line cells are more sensitive to cadmium than female germ line cells. We also observed that germ line cells are protected by the somatic cells of both gonads.

Methanogenesis in the Digestive Tracts of the Tropical Millipedes Archispirostreptus gigas (Diplopoda, Spirostreptidae) and Epibolus pulchripes (Diplopoda, Pachybolidae)

Methanogens represent the final decomposition step in anaerobic degradation of organic matter, occurring in the digestive tracts of various invertebrates. However, factors determining their community structure and activity in distinct gut sections are still debated. In this study, we focused on the tropical millipede species Archispirostreptus gigas (Diplopoda, Spirostreptidae) and Epibolus pulchripes (Diplopoda, Pachybolidae), which release considerable amounts of methane. We aimed to characterize relationships between physicochemical parameters, methane production rates, and methanogen community structure in the two major gut sections, midgut and hindgut. Microsensor measurements revealed that both sections were strictly anoxic, with reducing conditions prevailing in both millipedes. Hydrogen concentration peaked in the anterior hindgut of E. pulchripes. In both species, the intestinal pH was significantly higher in the hindgut than in the midgut. An accumulation of acetate and formate in the gut indicated bacterial fermentation activities in the digestive tracts of both species. Phylogenetic analysis of 16S rRNA genes showed a prevalence of Methanobrevibacter spp. (Methanobacteriales), accompanied by a small fraction of so-far-unclassified "Methanomethylophilaceae" (Methanomassiliicoccales), in both species, which suggests that methanogenesis is mostly hydrogenotrophic. We conclude that anoxic conditions, negative redox potential, and bacterial production of hydrogen and formate promote gut colonization by methanogens. The higher activities of methanogens in the hindgut are explained by the higher pH of this compartment and their association with ciliates, which are restricted to this compartment and present an additional source of methanogenic substrates. IMPORTANCE Methane (CH₄) is the second most important atmospheric greenhouse gas after CO₂ and is believed to account for 17% of global warming. Methanogens are a diverse group of archaea and can be found in various anoxic habitats, including digestive tracts of plant-feeding animals. Termites, cockroaches, the larvae of scarab beetles, and millipedes are the only arthropods known to host methanogens and emit large amounts of methane. Millipedes are ranked as the third most important detritivores after termites and earthworms, and they are considered keystone species in many terrestrial ecosystems. Both methane-producing and non-methane-emitting species of millipedes have been observed, but what limits their methanogenic potential is not known. In the present study, we show that physicochemical gut conditions and the distribution of symbiotic ciliates are important factors determining CH₄ emission in millipedes. We also found close similarities to other methane-emitting arthropods, which might be associated with their similar plant-feeding habits.

Enzymatic activities in the digestive tract of spirostreptid and spirobolid millipedes (Diplopoda: Spirostreptida and Spirobolida)

Millipedes represent a model for the study of organic matter transformation, animal-microbial interactions, and compartmentalisation of digestion. The activity of saccharidases (amylase, laminarinase, cellulase, xylanase, chitinase, maltase, cellobiase, and trehalase) and protease were measured in the midgut and hindgut contents and walls of the millipedes Archispirostreptus gigas and Epibolus pulchripes. Assays done at pH 4 and 7 confirmed activities of all enzymes except xylanase. Hydrolysing of starch and laminarin prevailed. The hindgut of E. pulchripes was shorter, less differentiated. Micro-apocrine secretion was observed only in the midgut of A. gigas. Merocrine secretion was present in midgut and hindgut of E. pulchripes, and in the pyloric valve and anterior hindgut of A. gigas. Alpha-polysaccharidases were mostly active in the midgut content and walls, with higher activity at pH 4. The low activity of amylase (A. gigas) and laminarinase (E. pulchripes) in midgut tissue may indicate their synthesis in salivary glands. Cellulases were found in midgut. Chitinases, found in midgut content and tissue (E. pulchripes) or concentrated in the midgut wall (A. gigas), were more active at an acidic pH. Polysaccharidases were low in hindguts. Protease shows midgut origin and alkaline activity extending to the hindgut in E. pulchripes, whereas in A. gigas it is of salivary gland origin and acid activity restricted to the midgut. Some disaccharidases, with more alkaline activity, showed less apparent midgut-hindgut differences. It may indicate an axial separating of the primary and secondary digestion along the intestinal pH gradient or the presence of enzymes of hindgut parasites.

Autophagy and Apoptosis in the Midgut Epithelium of Millipedes

The process of autophagy has been detected in the midgut epithelium of four millipede species: Julus scandinavius, Polyxenus lagurus, Archispirostreptus gigas, and Telodeinopus aoutii. It has been examined using transmission electron microscopy (TEM), which enabled differentiation of cells in the midgut epithelium, and some histochemical methods (light microscope and fluorescence microscope). While autophagy appeared in the cytoplasm of digestive, secretory, and regenerative cells in J. scandinavius and A. gigas, in the two other species, T. aoutii and P. lagurus, it was only detected in the digestive cells. Both types of macroautophagy, the selective and nonselective processes, are described using TEM. Phagophore formation appeared as the first step of autophagy. After its blind ends fusion, the autophagosomes were formed. The autophagosomes fused with lysosomes and were transformed into autolysosomes. As the final step of autophagy, the residual bodies were detected. Autophagic structures can be removed from the midgut epithelium via, e.g., atypical exocytosis. Additionally, in P. lagurus and J. scandinavius, it was observed as the neutralization of pathogens such as Rickettsia-like microorganisms. Autophagy and apoptosis ca be analyzed using TEM, while specific histochemical methods may confirm it.

Fine structure of the midgut epithelium of Thulinius ruffoi (Tardigrada, Eutardigrada, Parachela) in relation to oogenesis and simplex stage

Thulinius ruffoi is a small freshwater tardigrade that lives in both non-polluted and polluted freshwater environments. As a result of tardigradan body miniaturization, the digestive system is reduced and simplified. It consists of a short fore- and hindgut, and the midgut in the shape of a short tube is lined with a simple epithelium. The midgut epithelium is formed by the digestive cells and two rings of crescent-shaped cells were also detected. The anterior ring is located at the border between the fore- and midgut, while the posterior ring is situated between the mid- and hindgut. The precise ultrastructure of the digestive and crescent-shaped cells was examined using transmission electron microscopy, serial block face scanning electron microscopy and histochemical methods. We analyzed the changes that occurred in the midgut epithelial cells according to oogenesis (the species is parthenogenetic and there were only females in the laboratory culture). We focused on the accumulation of reserve material and the relationship between this and the intensity of autophagy. We concluded that autophagy supplies energy during a natural period of starvation (the simplex stage) and delivers the energy and probably the substances that are required during oogenesis. Apoptosis was not detected in the midgut epithelium of T. ruffoi.