Ultrastructural changes during recovery from anabiosis in the plant parasitic nematode, Ditylenchus

Cold adaptive potential of pine wood nematodes overwintering in plant hosts

The pine wood nematode (PWN; Bursaphelenchus xylophilus) is the causal agent of pine wilt disease, which results in severe ecological and economic losses in coniferous forests. During overwintering, PWNs undergo morphological and physiological changes to adapt to low temperature environments. Here, the physiological changes of the PWN populations sampled in the summer and winter were compared to analyze the role of low temperatures in their response. The PWN overwinters as third-stage dispersal juveniles, which showed significantly greater survival rates than summer populations (propagative forms) at sub-zero temperatures. The major biochemical compounds in the populations were analyzed by gas chromatography. Eight dominant fatty acids, with stearic acid being the most important, were identified from PWN propagative stage and third-stage dispersal stage. Compared with the propagative stage, the dispersal stage showed significant increases in the fatty acid content and the proportion of unsaturated fatty acids. Three carbohydrates, trehalose, glycerol and glucose, were detected in the PWN. Compared with the summer population, the levels of trehalose and glycerol increased significantly, while glucose decreased, in the winter population. The modifications in fatty acid composition and cryoprotectant levels, as elements of its changing physiology, play important roles in the overwintering success of the PWN.

Summary: The modifications in fatty acid composition and cryoprotectant levels, as elements of its changing physiology, play important roles in the overwintering success of the pine wood nematode.

Trehalose metabolism genes of Aphelenchoides besseyi (Nematoda: Aphelenchoididae) in hypertonic osmotic pressure survival

Some organisms can survive extreme desiccation caused by hypertonic osmotic pressure by entering a state of suspended animation known as osmobiosis. The free-living mycophagous nematode Aphelenchoides besseyi can be induced to enter osmobiosis by soaking in osmolytes. It is assumed that sugars (in particular trehalose) are instrumental for survival under environmental stress. In A. besseyi, two putative trehalose-6-phosphate synthase genes (TPS) encoding enzymes catalyzing trehalose synthesis, and a putative trehalase gene (TRE) encoding enzymes that catalyze hydrolysis of trehalose were identified and then characterized based on their transcriptome. RT-qPCR analyses showed that each of these genes is expressed as mRNA when A. besseyi is entering in, during and recovering from osmobiosis, but only for certain periods. The changes of TRE activity were consistent with the transcript level changes of the TRE gene, and the trehalose level declined at certain periods when the nematodes were in, as well as recovering from, osmobiosis; this suggested that the hydrolysis of threhalose is essential. The feeding method of RNA interference (RNAi) was used to temporarily knock down the expression of each of the TPS and TRE genes. No obviously different phenotype was observed from any of the genes silenced individually or simultaneously, but the survival under hypertonic osmotic pressure reduced significantly and the recovery was delayed. These results indicated that trehalose metabolism genes should play a role in osmobiosis regulation and function within a restricted time frame.

Summary: Trehalose metabolism genes should play a role in osmobiosis regulation and also function within a restricted time frame. Silence of any of these genes will cut down the nematode survival under hypertonic osmotic condition.

Desiccation tolerance in the tardigrade Richtersius coronifer relies on muscle mediated structural reorganization

Life unfolds within a framework of constraining abiotic factors, yet some organisms are adapted to handle large fluctuations in physical and chemical parameters. Tardigrades are microscopic ecdysozoans well known for their ability to endure hostile conditions, such as complete desiccation--a phenomenon called anhydrobiosis. During dehydration, anhydrobiotic animals undergo a series of anatomical changes. Whether this reorganization is an essential regulated event mediated by active controlled processes, or merely a passive result of the dehydration process, has not been clearly determined. Here, we investigate parameters pivotal to the formation of the so-called "tun", a state that in tardigrades and rotifers marks the entrance into anhydrobiosis. Estimation of body volume in the eutardigrade Richtersius coronifer reveals an 87 % reduction in volume from the hydrated active state to the dehydrated tun state, underlining the structural stress associated with entering anhydrobiosis. Survival experiments with pharmacological inhibitors of mitochondrial energy production and muscle contractions show that i) mitochondrial energy production is a prerequisite for surviving desiccation, ii) uncoupling the mitochondria abolishes tun formation, and iii) inhibiting the musculature impairs the ability to form viable tuns. We moreover provide a comparative analysis of the structural changes involved in tun formation, using a combination of cytochemistry, confocal laser scanning microscopy and 3D reconstructions as well as scanning electron microscopy. Our data reveal that the musculature mediates a structural reorganization vital for anhydrobiotic survival, and furthermore that maintaining structural integrity is essential for resumption of life following rehydration.

MAPK phosphorylation is implicated in the adaptation to desiccation stress in nematodes

Some nematodes can survive almost complete desiccation by entering an ametabolic state called anhydrobiosis requiring the accumulation of protective molecules such as trehalose and LEA proteins. However, it is not known how anhydrobiotic organisms sense and regulate the response to water loss. Mitogen-activated protein kinases (MAPKs) are highly conserved signalling proteins that regulate adaptation to various stresses. Here, we first compared the anhydrobiotic potential of three nematode species, Caenorhabditis elegans (Maupas, 1900), Aphelenchus avenae (Bastian, 1865) and Panagrolaimus superbus (Fuchs, 1930), and then determined the phosphorylation status of the MAPKs p38, JNK and ERK during desiccation and rehydration. C. elegans was unable to undergo anhydrobiosis even after an initial phase of slow drying (preconditioning), while A. avenae did survive desiccation after preconditioning. In contrast, P. superbus withstood desiccation under rapid drying conditions, although survival rates improved with preconditioning. These results characterise C. elegans as desiccation sensitive, A. avenae as a slow desiccation strategist anhydrobiote, and P. superbus as a fast desiccation strategist anhydrobiote. Both C. elegans and A. avenae showed increased MAPK phosphorylation during drying, consistent with an attempt to mount protection systems against desiccation stress. In P. superbus, however, MAPK phosphorylation was apparent prior to water loss and then decreased on dehydration, suggesting that signal transduction pathways are constitutively active in this nematode. Inhibition of p38 and JNK in P. superbus decreased its desiccation tolerance. This is consistent with the designation of P. superbus as a fast desiccation strategist and its high level of preparedness for anhydrobiosis in the hydrated state. These findings show that MAPKs play an important role in the survival of organisms during anhydrobiosis.

Desiccation survival of parasitic nematodes

The ability of certain species of parasitic nematodes to survive desiccation for considerable periods is a fascinating example of adaptation to the demands of fluctuating environments that occasionally can become extreme and life threatening. Behavioural and morphological adaptations associated with desiccation survival serve primarily to reduce the rate of drying, either to prolong the time taken for the nematode's water content to reach lethal low levels or, in true anhydrobiotes, to enable the structural and biochemical changes required for long-term survival to take place. Examples of these adaptations are reviewed, together with information on the factors involved in rehydration that ensure successful exit from the dormant state. Information on desiccation survival is central to effective management and control options for parasitic nematodes. It is also required to assess the feasibility of enhancing the longevity of commercial formulations of entomopathogenic nematodes, both before and after application; current research and future prospects for enhancing survival of these bio-insecticides are discussed.

Physiological and morphological changes associated with recovery from anabiosis in the dauer larva of the nematode Anguina agrostis

Morphological and physiological changes that occur during the rehydration of 2nd-stage infective larvae (L2) of Anguina agrostis are described. The first signs of motility, after 2 days of dehydration, were observed 0·5 h after rehydration and 90 % of the nematodes were moving within 15 h. Anabiotic A. agrostis were able to withstand repeated dehydration and rehydration. These procedures caused A. agrostis to accumulate large lipid droplets, which showed most clearly in the tail region. By means of a double labelling technique, it was shown that cuticle permeability of anabiotic L2 of A. agrostis initially increased slightly during rehydration. This was followed by a sharp decrease in permeability between 1 h and 8 h from the start of the experiment. A slower decline in permeability occurred between 8 h and 16 h and little further decrease occurred between 16 h and 24 h. The results are discussed in relation to the possible involvement of the epicuticle.

Changes in surface features during desiccation of the anhydrobiotic plant parasitic nematode Ditylenchus dipsaci

The anhydrobiotic nematode Ditylenchus dipsaci is a fast-dehydration strategist, itself generating the slow rate of water loss necessary for survival. A permeability slump occurs during the initial phases of desiccation. This may be produced by changes in the nematode's cuticle. Two scanning electron microscopic techniques were used to follow changes in surface structures during desiccation. Freeze substitution and critical-point drying produced artifacts that obscured changes produced by the desiccation of the nematode. Low-temperature field emission scanning electron microscopy (FESEM) was successful in following changes that reflected those observed by light microscopy (LM). Significant changes in diameter, the lateral alae, and the cuticular annulations were demonstrated using this technique. Two types of annulations were observed: the major annulations, which extended to meet the margins of the lateral alae, and the minor annulations, which did not. With desiccation the prominence of the annulations increased, their spacing decreased, and the minor annulations extended closer to the margins of the lateral alae. These observations are consistent with the permeability slump resulting from a decrease in the width of the annulation groove and an increase in its depth. However, this requires confirmation using techniques that can follow annulation changes in individual nematodes.

Ultrastructural changes during desiccation of the anhydrobiotic nematode Ditylenchus dipsaci

Ultrastructural changes during desiccation of the anhydrobiotic nematode Ditylenchus dipsaci were followed and quantified after preparation of material at different levels of hydration using freeze substitution techniques. Some shrinkage was caused by processing in the more hydrated specimens but the changes observed correspond to those observed in live nematodes by light microscopy, indicating that the technique is useful for following changes during desiccation. The overall pattern of changes was a rapid decrease in the magnitude of the measured parameter during the first 5 min of desiccation, followed by a slower rate of decrease upon further desiccation. This was observed in the cuticle, the lateral hypodermal cords and the muscle cells and is consistent with the pattern of water loss of the nematode. The contractile region of the muscle cells, however, proved an exception and the muscle fibres appear to resist shrinkage and packing until water loss becomes severe. The mitochondria swell and then shrink during desiccation, which may indicate disruption of the permeability of the mitochondrial membrane. A decrease in the thickness of the cortical zone was the most prominent change in the cuticle and this may be related to the permeability slump which occurs during the first 5 min of desiccation.

Anhydrobiosis in the infective juveniles of Trichostrongylus colubriformis (Nematoda: Trichostrongylidae)

The infective juveniles of Trichostrongylus colubriformis can survive exposure to 0% relative humidity after desiccation at higher relative humidities. During rehydration there is a lag phase before recovery. The infective juveniles are thus capable of anhydrobiosis. Removal of the sheath does not affect desiccation survival but does affect the duration of the lag phase. Morphological changes during the lag phase are described. The appearance of birefringence in the muscle cells during the anomalous shrinkage which occurs during the lag phase is considered to reflect physiological changes necessary for the recovery of normal muscle function.

The structure of the cuticle and sheath of the infective juvenile of Trichostrongylus colubriformis

The infective third-stage juvenile of Trichostrongylus colubriformis is surrounded by its own cuticle as well as the incompletely moulted cuticle of the second-stage juvenile, which is referred to as the sheath. The sheath comprises an outer epicuticle, an amorphous cortical zone, a fibrous basal zone and an inner electron-dense layer. The basal zone of the sheath consists of three layers of fibres; the fibres are parallel within each layer, but the fibre direction of the middle layer is at an angle to that of the inner and outer layers. The cuticle comprises a complex outer epicuticle, an amorphous cortical zone and a striated basal zone. The lateral alae of the cuticle and the sheath are aligned and overlie the lateral hypodermal cords. The lateral alae of the sheath consist of two wing-like expansions of the cortical zone with associated specializations of the inner electron-dense layer which form a groove. The cuticular lateral alae consist of two tube-like expansions of the cortical zone. The lateral alar complex of the cuticle and the sheath may maximise locomotory efficiency and prevent rotation of the juvenile within the sheath.