Abstract
The invertebrates that inhabit Antarctica’s terrestrial ice-free areas are a unique resource for the study of adaptations to extreme conditions and illustrate the limits to which physiology can be stretched by evolution to allow life to continue.
Perhaps the most exciting result of past studies of these adaptations in the Antarctic’s terrestrial fauna was the discovery by Wharton and Ferns in 1995 of the survival of extensive intracellular freezing in the nematode Panagrolaimus davidi. Intracellular ice formation is generally considered fatal, and as the first and, so far, unique example of this adaptation in a multicellular organism, it has remained contentious. Understanding this adaptation could have important applications for cryopreservation, but research has currently been limited to this one species and based on a laboratory culture that have been isolated from its environment for over 20 years. There has been little research into the thermal tolerances of other Antarctic nematode species.
This thesis explores three broad research themes in order to contribute to the understanding of adaptation in Antarctic nematodes generally, and the survival of intracellular freezing specifically.
Firstly, to place their adaptations in an environmental context, the distribution of the nematode species found in two ice-free areas, around Cape Hallett in Northern Victoria Land and Gondwana station in Terra Nova Bay, is described and factors underlying their distribution patterns are investigated using regression models. Four nematode species were found: Panagrolaimus davidi, Scottnema lindsayae, Plectus murrayi and Eudorylaimus antarcticus. Their distribution patterns are correlated to both broad-scale habitat descriptors and to soil geochemistry, revealing species-specific differences in distribution patterns that are broadly consistent with other studies of these species in soils from the Dry Valleys and penguin rookeries on Ross Island. The importance of soil conductivity (as a measure of salinity) in predicting the distribution patterns of all four species supports the hypothesis that salinity plays a primary role in determining invertebrate distribution in the terrestrial Antarctic. The current environmental challenges faced by these Antarctic nematodes in their habitats are then described, providing context for laboratory investigations of their adaptations.
Secondly, the survival strategies employed by these nematodes are investigated. For P. davidi the survival of intracellular freezing was found to be strongly correlated with its nutritional status, explaining some of the variation in survival rates reported in previous studies. Plectus murrayi was also found to survive intracellular ice formation, providing a second example of an organism able to employ this survival strategy. The appearance of internal ice differed markedly between these two nematode species and that in a temperate species, Panagrellus redivivus, suggesting that adaptations enabling the survival of intracellular freezing involve the control of internal ice. Further evidence was also provided for the use of a cryoprotective dehydration survival strategy in response to slow freezing rates in these Antarctic nematodes.
Thirdly, to provide a historical context for their adaptations, and in order to distinguish between the Antarctic species and show their relation to nematode species worldwide, genetic techniques and phylogenetic analyses were employed. Genetic sequencing of the 18S and D3 expansion ribosomal RNA regions and comparisons with published phylogenies for the phylum Nematoda revealed that the Antarctic species do not form a discrete Antarctic clade, but rather are spread over the phylum. This supports current hypotheses of endemism and a long Antarctic history for these species. However, the laboratory culture of P. davidi (referred to now as P. sp. nov.) was found to be a distinct species from the wild populations of P. davidi that were sequenced in this study. The surprising lack of sequence divergence between P. sp. nov. and some Californian Panagrolaimus species brings its origin into question, and suggests that it may represent a relatively recent invasive species to the Ross Sea Region. This finding calls for further research to address the possibility that it is the first example of a successful animal invasion of Continental Antarctica and to investigate the origins of its remarkable adaptations to freezing.