Autophagy as the cell survival in response to a microsporidian infection of the midgut epithelium ofIsohypsibius granulifer granulifer(Eutardigrada: Hypsibiidae)

Pinpointing the microbiota of tardigrades: What is really there?

Microbiota are considered significant in the biology of tardigrades, yet their diversity and distribution remain largely unexplored. This is partly due to the methodological challenges associated with studying the microbiota of small organisms that inhabit microbe-rich environments. In our study, we characterized the microbiota of 31 species of cultured tardigrades using 16S rRNA amplicon sequencing. We employed various sample preparation strategies and multiple types of controls and estimated the number of microbes in samples using synthetic DNA spike-ins. We also reanalysed data from previous tardigrade microbiome studies. Our findings suggest that the microbial communities of cultured tardigrades are predominantly composed of bacterial genotypes originating from food, medium, or reagents. Despite numerous experiments, we found it challenging to identify strains that were enriched in certain tardigrades, which would have indicated likely symbiotic associations. Putative tardigrade-associated microbes rarely constituted more than 20% of the datasets, although some matched symbionts identified in other studies. We also uncovered serious contamination issues in previous tardigrade microbiome studies, casting doubt on some of their conclusions. We concluded that tardigrades are not universally dependent on specialized microbes. Our work underscores the need for rigorous safeguards in studies of the microbiota of microscopic organisms and serves as a cautionary tale for studies involving samples with low microbiome abundance.

Elevated external temperature affects cell ultrastructure and heat shock proteins (HSPs) in Paramacrobiotus experimentalis Kaczmarek, Mioduchowska, Poprawa, & Roszkowska, 2020

Increasing temperature influences the habitats of various organisms, including microscopic invertebrates. To gain insight into temperature-dependent changes in tardigrades, we isolated storage cells exposed to various temperatures and conducted biochemical and ultrastructural analysis in active and tun-state Paramacrobiotus experimentalis Kaczmarek, Mioduchowska, Poprawa, & Roszkowska, 2020. The abundance of heat shock proteins (HSPs) and ultrastructure of the storage cells were examined at different temperatures (20 °C, 30 °C, 35 °C, 37 °C, 40 °C, and 42 °C) in storage cells isolated from active specimens of Pam. experimentalis. In the active animals, upon increase in external temperature, we observed an increase in the levels of HSPs (HSP27, HSP60, and HSP70). Furthermore, the number of ultrastructural changes in storage cells increased with increasing temperature. Cellular organelles, such as mitochondria and the rough endoplasmic reticulum, gradually degenerated. At 42 °C, cell death occurred by necrosis. Apart from the higher electron density of the karyoplasm and the accumulation of electron-dense material in some mitochondria (at 42 °C), almost no changes were observed in the ultrastructure of tun storage cells exposed to different temperatures. We concluded that desiccated (tun-state) are resistant to high temperatures, but not active tardigrades (survival rates of tuns after 24 h of rehydration: 93.3% at 20 °C, 60.0% at 35 °C, 33.3% at 37 °C, 33.3% at 40 °C, and 20.0% at 42 °C).

Size-dependent and sex-specific negative effects of micro- and nano-sized polystyrene particles in the terrestrial invertebrate model Drosophila melanogaster

Microplastic pollution is believed to be one of the most widespread and long-lasting changes on a global scale. Our understanding that microplastics significantly impact terrestrial systems and are a global change stressor continues to grow. In the present study, we investigated the negative effect of long-term (28 days of exposure in food) polystyrene particles of micro (1.0-1.9 µm, 0.4-0.6 µm) and nano (0.04-0.06 µm) scale, in low doses, on the fruit fly - representing a common, globally distributed terrestrial invertebrate, and a model species in many fields. Our observations involved such parameters as ingestion and transfer of particles, survival, reproduction, changes in ultrastructure and tissue and cell responses in midgut epithelium (the place of direct contact with plastic), ovary, and testis in adults, and transgenerational effects in larvae. These observations may indicate possible toxic effects of the tested substances, even in low doses, that can be expected in other taxa, in terrestrial ecosystems. We observed a negative impact of polystyrene particles on the fruit fly survival, midgut, ovary, and testis, involving ultrastructural alterations, such as autophagy and/or ultimately necrosis in the midgut, triggering oxidative stress and activating processes of antioxidative protection. Despite the changes, midgut function and reproduction were not altered - spermatogenesis and oogenesis proceeded normally. The effect was size-dependent - the smaller the polystyrene particles were, the more substantial was the impact they caused. Ultrastructural changes and studied parameters, i.e., generation of ROS (overproduction of which generates oxidative stress), total glutathione concentration (involved in defense against ROS, acting in distinct pathways), and total antioxidant concentration (the oxidative defense system) showed the highest levels after exposure to the smallest nanoparticles, and vice versa. The effect was also sex-dependent, with male flies being more sensitive. Negative effects in males were more substantial and more prominent, even after contact with larger particles, compared to females. The smaller particles (0.4-0.6 µm, 0.04-0.06 µm) were transferred to the ovary and accumulated in the oocytes. In this case, a transgenerational negative effect was detected in larvae. It was characterized by size-dependent alterations, with smaller particles triggering higher levels of ROS and cellular oxidative response. Only the largest particles (1.0-1.9 µm) did not pass into the gonad and did not alter the larvae. These observations together demonstrated that polystyrene particles of micro- and nanoscale, even in a low dose, can induce numerous negative effects on terrestrial invertebrates.

Generation of a Microsporidia Species Attribute Database and Analysis of the Extensive Ecological and Phenotypic Diversity of Microsporidia

Microsporidia are a large group of fungus-related obligate intracellular parasites. Though many microsporidia species have been identified over the past 160 years, depiction of the full diversity of this phylum is lacking. To systematically describe the characteristics of these parasites, we created a database of 1,440 species and their attributes, including the hosts they infect and spore characteristics. We find that microsporidia have been reported to infect 16 metazoan and 4 protozoan phyla, with smaller phyla being underrepresented. Most species are reported to infect only a single host, but those that are generalists are also more likely to infect a broader set of host tissues. Strikingly, polar tubes are threefold longer in species that infect tissues besides the intestine, suggesting that polar tube length is a determinant of tissue specificity. Phylogenetic analysis revealed four clades which each contain microsporidia that infect hosts from all major habitats. Although related species are more likely to infect similar hosts, we observe examples of changes in host specificity and convergent evolution. Taken together, our results show that microsporidia display vast diversity in their morphology and the hosts they infect, illustrating the flexibility of these parasites to evolve new traits.

The fungicide iprodione affects midgut cells of non-target honey bee Apis mellifera workers

The honey bee Apis mellifera is an important pollinator of agricultural crops and natural forests. Honey bee populations have declined over the years, as a result of diseases, pesticides, and management problems. Fungicides are the main pesticides found in pollen grains, which are the major source of protein for bees. The objective of this study was to evaluate the cytotoxic effects of the fungicide iprodione on midgut cells of adult A. mellifera workers. Bees were fed on iprodione (LD₅₀, determined by the manufacturer) for 12 or 24 h, and the midgut was examined using light and transmission electron microscopies. The expression level of the autophagy gene atg1 was assessed in midgut digestive cells. Cells of treated bees had signs of apoptosis: cytoplasmic vacuolization, apical cell protrusions, nuclear fragmentation, and chromatin condensation. Ultrastructural analysis revealed some cells undergoing autophagy and necrosis. Expression of atg1 was similar between treated and control bees, which can be explained by the facts that digestive cells had autolysosomes, whereas ATG-1 is found in the initial phases of autophagy. Iprodione acts by inhibiting the synthesis of glutathione, leading to the generation of reactive oxygen species, which in turn can induce different types of cell death. The results indicate that iprodione must be used with caution because it has side effects on non-target organisms, such as pollinator bees.

Interfaces between microbes and membranes of host epithelial cells in hemipteran midguts

Certain families of plant-feeding insects in the order Hemiptera (infraorder Pentatomomorpha) have established symbiotic relationships with microbes that inhabit specific pouches (caeca) of their midgut epithelium. The placement of these caeca in a well-delineated region at the most posterior end of the midgut bordering the hindgut is conserved in these families; in situ the convoluted midgut is predictably folded so that this caecal region lies adjacent to the anterior-most region of the midgut. Depending on the hemipteran family, caeca vary in their number and configuration at a given anterior-posterior location. At the host-microbe interface, epithelial plasma membranes of midgut epithelial cells interact with nonself antigens of microbial surfaces. In the different hemipteran species examined, a continuum of interactions is observed between microbes and host membranes. Bacteria can exist as free living cells within the midgut lumen without contacting host membranes while other host cells physically interact extensively with microbial surfaces by extending numerous processes that interdigitate with microbes; and, in many instances, processes completely envelope the microbes. The host cells can embrace the foreign microbes, completely enveloping each with a single host membrane or sometimes enveloping each with the two additional host membranes of a phagosome.

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.

Changes in the midgut diverticula epithelial cells of the European cave spider, Meta menardi, under controlled winter starvation

The European cave spider, Meta menardi, is among the most common troglophile species inhabiting the cave entrance zone in Europe, where prey is scarce in winter. Spiders feed only if prey is available; otherwise, they are subjected to long-term winter starvation. We carried out a four-month winter starvation of M. menardi under controlled conditions to analyze ultrastructural changes in the midgut diverticula epithelial cells at the beginning, in the middle and at the end of the starvation period. We used light microscopy, TEM and quantified reserve lipids and glycogen. The midgut diverticula epithelium consisted of secretory cells, digestive cells and adipocytes. During starvation, gradual vacuolization of some digestive cells, and some necrotic digestive cells and adipocytes appeared. Autophagic structures, autophagosomes, autolysosomes and residual bodies were found in all three cell types. Spherites and the energy-reserve compounds were gradually exploited, until in some spherites only the membrane remained. Comparison between spring, autumn and winter starvation reveals that, during the growth period, M. menardi accumulate reserve compounds in spherites and protein granules, and energy-supplying lipids and glycogen, like many epigean, overwintering arthropods. In M. menardi, otherwise active all over the year, this is an adaptive response to the potential absence of prey in winter.

The role of autophagy in the midgut epithelium of Parachela (Tardigrada)

The process of cell death has been detected in the midgut epithelium of four tardigrade species which belong to Parachela: Macrobiotus diversus, Macrobiotus polonicus, Hypsibius dujardini and Xerobiotus pseudohufelandi. They originated from different environments so they have been affected by different stressors: M. polonicus was extracted from a moss sample collected from a railway embankment; M. diversus was extracted from a moss sample collected from a petrol station; X. pseudohufelandi originated from sandy and dry soil samples collected from a pine forest; H. dujardini was obtained commercially but it lives in a freshwater or even in wet terrestrial environment. Autophagy is caused in the digestive cells of the midgut epithelium by different factors. However, a distinct crosstalk between autophagy and necrosis in tardigrades' digestive system has been described at the ultrastructural level. Apoptosis has not been detected in the midgut epithelium of analyzed species. We also determined that necrosis is the major process that is responsible for the degeneration of the midgut epithelium of tardigrades, and "apoptosis-necrosis continuum" which is the relationship between these two processes, is disrupted.

The structure of the desiccated Richtersius coronifer (Richters, 1903)

Tun formation is an essential morphological adaptation for entering the anhydrobiotic state in tardigrades, but its internal structure has rarely been investigated. We present the structure and ultrastructure of organs and cells in desiccated Richtersius coronifer by transmission and scanning electron microscopy, confocal microscopy, and histochemical methods. A 3D reconstruction of the body organization of the tun stage is also presented. The tun formation during anhydrobiosis of tardigrades is a process of anterior-posterior body contraction, which relocates some organs such as the pharyngeal bulb. The cuticle is composed of epicuticle, intracuticle and procuticle; flocculent coat; and trilaminate layer. Moulting does not seem to restrict the tun formation, as evidenced from tardigrade tuns that were in the process of moulting. The storage cells of desiccated specimens filled up the free inner space and surrounded internal organs, such as the ovary and digestive system, which were contracted. All cells (epidermal cells, storage cells, ovary cells, cells of the digestive system) underwent shrinkage, and their cytoplasm was electron dense. Lipids and polysaccharides dominated among reserve material of storage cells, while the amount of protein was small. The basic morphology of specific cell types and organelles did not differ between active and anhydrobiotic R. coronifer.

Ultrastructural analysis of apoptosis and autophagy in the midgut epithelium of Piscicola geometra (Annelida, Hirudinida) after blood feeding

Cell death in the endodermal region of the digestive tract of the blood-feeding leech Piscicola geometra was analyzed using light and transmission electron microscopes and the fluorescence method. Sexually mature specimens of P. geometra were bred under laboratory conditions and fed on Danio rerio. After copulation, the specimens laid cocoons. The material for our studies were non-feeding juveniles collected just after hatching, non-feeding adult specimens, and leeches that had been fed with fish blood (D. rerio) only once ad libitum. The fed leeches were prepared for our studies during feeding and after 1, 3, 7, and 14 days (not sexually mature specimens) and some weeks after feeding (the sexually mature). Autophagy in all regions of the endodermal part of the digestive system, including the esophagus, the crop, the posterior crop caecum (PCC), and the intestine was observed in the adult non-feeding and feeding specimens. In fed specimens, autophagy occurred at very high levels--in 80 to 90 % of epithelial cells in all four regions. In contrast, in adult specimens that did not feed, this process occurred at much lower levels--about 10 % (esophagus and intestine) and about 30 % (crop and PCC) of the midgut epithelial cells. Apoptosis occurred in the feeding adult specimens but only in the crop and PCC. However, it was absent in the non-feeding adult specimens and the specimens that were collected during feeding. Moreover, neither autophagy nor apoptosis were observed in the juvenile, non-feeding specimens. The appearance of autophagy and apoptosis was connected with feeding on toxic blood. We concluded that autophagy played the role of a survival factor and was involved in the protection of the epithelium against the products of blood digestion. Quantitative analysis was prepared to determine the number of autophagic and apoptotic cells.

Germ cell cluster organization and oogenesis in the tardigrade Dactylobiotus parthenogeneticus Bertolani, 1982 (Eutardigrada, Murrayidae)

Germ cell cluster organization and the process of oogenesis in Dactylobiotus parthenogeneticus have been described using transmission electron microscopy and light microscopy. The reproductive system of D. parthenogeneticus is composed of a single, sac-like, meroistic ovary and a single oviduct that opens into the cloaca. Two zones can be distinguished in the ovary: a small germarium that is filled with oogonia and a vitellarium that is filled with germ cell clusters. The germ cell cluster, which has the form of a modified rosette, consists of eight cells that are interconnected by stable cytoplasmic bridges. The cell that has the highest number of stable cytoplasmic bridges (four bridges) finally develops into the oocyte, while the remaining cells become trophocytes. Vitellogenesis of a mixed type occurs in D. parthenogeneticus. One part of the yolk material is produced inside the oocyte (autosynthesis), while the second part is synthesized in the trophocytes and transported to the oocyte through the cytoplasmic bridges. The eggs are covered with two envelopes: a thin vitelline envelope and a three-layered chorion. The surface of the chorion forms small conical processes, the shape of which is characteristic for the species that was examined. In our paper, we present the first report on the rosette type of germ cell clusters in Parachela.

Reduced survival and reproductive success generates selection pressure for the dengue mosquito Aedes aegypti to evolve resistance against infection by the microsporidian parasite Vavraia culicis

The success and sustainability of control measures aimed at reducing the transmission of mosquito-borne diseases will depend on how they influence the fitness of mosquitoes in targeted populations. We investigated the effects of the microsporidian parasite Vavraia culicis on the survival, blood-feeding behaviour and reproductive success of female Aedes aegypti mosquitoes, the main vector of dengue. Infection reduced survival to adulthood and increased adult female mosquito age-dependent mortality relative to uninfected individuals; this additional mortality was closely correlated with the number of parasite spores they harboured when they died. In the first gonotrophic cycle, infected females were less likely to blood-feed, took smaller meals when they did so, and developed fewer eggs than uninfected females. Even though the conditions of this laboratory study favoured minimal developmental times, the costs of infection were already being experienced by the time females reached an age at which they could first reproduce. These results suggest there will be selection pressure for mosquitoes to evolve resistance against this pathogen if it is used as an agent in a control program to reduce the transmission of mosquito-borne human diseases.