David A. Wharton

Nematode egg-shells

The transmission of parasites often involves a high mortality of free-living stages in the environment outside the host. This may be offset by a high biotic potential. In addition, adaptations of nematode eggs and larvae that ensure their survival or increase their chances of infecting a host will reduce the potential wastage rate. Increasing transmission will have an effect equivalent to increasing the fecundity of the parasite and, energetically, may be the more favourable strategy.

A functional biology of nematodes

1. Introduction.- 2. Functional Organisation.- 3. Movement and Co-ordination.- 4. Reproductive Biology.- 5. Parasitism.- 6. Life Cycle.- 7. Environmental Physiology.- References.

The environmental physiology of Antarctic terrestrial nematodes: a review

The environmental physiology of terrestrial Antarctic nematodes is reviewed with an emphasis on their cold-tolerance strategies. These nematodes are living in one of the most extreme environments on Earth and face a variety of stresses, including low temperatures and desiccation. Their diversity is low and declines with latitude. They show resistance adaptation, surviving freezing and desiccation in a dormant state but reproducing when conditions are favourable. At high freezing rates in the surrounding medium the Antarctic nematode Panagrolaimus davidi freezes by inoculative freezing but can survive intracellular freezing. At slow freezing rates this nematode does not freeze but undergoes cryoprotective dehydration. Cold tolerance may be aided by rapid freezing, the production of trehalose and by an ice-active protein that inhibits recrystallisation. P. davidi relies on slow rates of water loss from its habitat, and can survive in a state of anhydrobiosis, perhaps aided by the ability to synthesise trehalose. Teratocephalus tilbrooki and Ditylenchus parcevivens are fast-dehydration strategists. Little is known of the osmoregulatory mechanisms of Antarctic nematodes. Freezing rates are likely to vary with water content in Antarctic soils. Saturated soils may produce slow freezing rates and favour cryoprotective dehydration. As the soil dries freezing rates may become faster, favouring freezing tolerance. When the soil dries completely the nematodes survive anhydrobiotically. Terrestrial Antarctic nematodes thus have a variety of strategies that ensure their survival in a harsh and variable environment. We need to more fully understand the conditions to which they are exposed in Antarctic soils and to apply more natural rates of freezing and desiccation to our studies.

Freezing survival and cryoprotective dehydration as cold tolerance mechanisms in the Antarctic nematode Panagrolaimus davidi.

SUMMARY The relative importance of freezing tolerance and cryoprotective dehydration in the Antarctic nematode Panagrolaimus davidi has been investigated. If nucleation of the medium is initiated at a high subzero temperature (-1°C), the nematodes do not freeze but dehydrate. This effect occurs in deionised water, indicating that the loss of water is driven by the difference in vapour pressure of ice and supercooled water at the same temperature. If the nematodes are held above their nucleation temperature for a sufficient time, or are cooled slowly, enough water is lost to prevent freezing (cryoprotective dehydration). However, if the medium is nucleated at lower temperatures or if the sample is cooled at a faster cooling rate, the nematodes freeze and can survive intracellular ice formation. P. davidi thus has a variety of mechanisms that ensure its survival in its harsh terrestrial Antarctic habitat.

Cold acclimation and cryoprotectants in a freeze-tolerant Antarctic nematode, Panagrolaimus davidi.

Panagrolaimus davidi is a freeze-tolerant Antarctic nematode which survives extensive intracellular freezing. This paper describes the development of culture techniques which provide clean samples, with a high degree of freeze tolerance and in sufficient quantities for the analysis of potential cryoprotectants. Cultures grown at 20 °C survived a short-term freezing stress but survival declined with the time spent frozen. Acclimation of cultures at 5 °C enhanced the long-term survival of freezing. Starvation, however, reduced the nematodes ability to survive short-term freezing. The principal cryoprotectants detected by gas chromatography were trehalose and glycerol. The levels of trehalose, but not those of glycerol, increased significantly after acclimation. Trehalose may stabilise membranes and protect them against the dehydrating effects of the osmotic stresses resulting from freeze concentration effects but other factors, such as recrystallisation inhibition, may be involved in long-term survival.

Molecular and physiological basis of nematode survival.

Nematodes are renowned for their ability to survive severe environmental fluctuations. Their mechanisms to withstand temperature extremes, desiccation, and osmotic and ionic stress are presented here together with information on the underlying biochemical basis contributing to survival. Highlighting parallels and contrasts between parasitic and free-living nematode groups, this book integrates strategies that enable nematodes to persist in the absence of food with tactics used by parasitic forms to survive the defence responses of a plant or animal host. This functional study is an essential resource for researchers in nematology, parasitology and zoology.

A survey of terrestrial nematodes from the McMurdo Sound region, Antarctica

Abstract A survey was undertaken of the nematode fauna associated with moss and algal growth from terrestrial sites on Ross Island and the Dry Valleys of the McMurdo Sound region, Antarctica. Six species, in decreasing order of abundance, were identified: Plectus frigophilus, Eudorylaimus antarcticus, Scottnema lindsayae, Plectus antarcticus, Panagrolaimus davidi, and Monhystera villosa. Maps are given showing the distribution of these species. Cultures of Panagrolaimus davidi, Plectus antarcticus, and Plectus frigophilus were establishedonnutrient agar plates, feeding on bacteria.

The production and functional morphology of helminth egg-shells

The high energy costs of egg-shell production in many helminths suggests that this structure plays an important role in their biology. The mechanical and chemical resistance of the egg-shell and the barrier it provides to the entry and loss of material are important in the survival of the free-living stages within the egg. The presence of egg filaments may increase the availability of infective stages and the operculum may be important in the infective process.

Cold tolerance of an Antarctic nematode that survives intracellular freezing: comparisons with other nematode species

Panagrolaimus davidi is an Antarctic nematode with very high levels of cold tolerance. Its survival was compared with that of some other nematodes (P. rigidus, Rhabditophanes sp., Steinernema carpocapsae, Panagrellus redivivus and Ditylenchus dipsaci) in both unacclimated samples and those acclimated at 5°C. Levels of recrystallization inhibition in homogenates were also compared, using the splat-cooling assay. The survival of P. davidi after the freezing of samples was notably higher than that of the other species tested, suggesting that its survival ability is atypical compared to other nematodes. In general, acclimation improved survival. Levels of recrystallization inhibition were not associated with survival but such a relationship may exist for those species that are freezing tolerant.

Ice nucleation and freezing tolerance in New Zealand alpine and lowland weta, Hemideina spp. (Orthoptera; Stenopelmatidae)

Abstract.The alpine tree weta Hemidiena maori Pictet et Saussure (Orthoptera: Stenopelmatidae) is a large, flightless insect found above the treeline on many of the mountain ranges of the South Island of New Zealand. The population found on the Rock and Pillar Range, Central Otago has been identified as freezing tolerant with a haemolymph ice nucleating agent. The ability of H. maori to survive freezing is compared to the lowland weta Hemideina thoracica Walker and H. crassidens Blanchard, both of which are able to survive the formation of some ice in their bodies. Mortality is associated with time spent frozen in H. thoracica, and it is hypothesized that this species is killed when a critical proportion of its body water is frozen. All five subalpine and alpine populations of H. maori surveyed were found to be freezing tolerant.