antarctic terrestrial organisms live permanently on the continent ( unlike penguins and seals that only breed there ) and survive in one of the harshest environments on earth . sites that support life are largely limited to regions that are ice free , for at least part of the year , and which receive meltwater in spring and summer . living at the limits of life , these organisms may be particularly sensitive indicators of climate change and are good models for studying how life survives in extreme environments . antarctic species show high levels of endemicity and recent molecular studies suggest that many terrestrial antarctic organisms have ancient origins , dating from before the break up of gondwana . although controversial , there is increasing interest in bioprospecting amongst antarctic organisms for molecules with practical uses . although the most obvious stress faced by organisms in antarctica is cold and the risk of freezing , there are a variety of other stressors that are significant . the most important factor determining their distribution is the presence of liquid water , to which organisms must have at least occasional access in order to grow and reproduce . anhydrobiosis is a feature of many organisms in habitats where they are exposed to desiccation . among animals many species of nematodes are capable of anhydrobiosis and nematodes have proved to be good models for the study of this phenomenon . the disaccharide trehalose has long been thought to be important for anhydrobiosis ; especially by acting as a replacement for water , preserving the function of membranes and proteins . more recently , other mechanisms have been recognized . in particular , a group of proteins called late embryogenesis abundant ( lea ) proteins , first identified from plant seeds , are associated with anhydrobiosis in a number of animals . however , focusing on specific adaptations , such as trehalose and lea proteins , may result in important mechanisms being overlooked . the construction and screening of cdna libraries and cdna arrays have proved successful in identifying freezing - responsive gene expression in a freezing - tolerant frog , rana sylvatica . similar approaches have been used to study the responses of plants to a variety of stressors , including desiccation , and have been applied to desiccation survival and anhydrobiosis in nematodes . an arctic springtail ( collembola ) , onychiurus arcticus , overwinters by desiccating at low temperatures ( cryoprotective dehydration ) . an expressed sequence tag ( est ) analysis has indicated that a number of biochemical pathways are associated with desiccation and recovery . a recent paper in bmc genomics describes the first est and differential expression analysis of the response of a terrestrial antarctic animal to environmental stress . in this work , adhikari et al . describe changes in gene expression in response to desiccation in the free - living nematode plectus murrayi ( figure 1 ) . this nematode is found in the dry valleys and coastal sites in the mcmurdo sound region ( figure 2 ) , and from several other areas of continental east antarctica , where it is the most widely distributed and abundant free - living terrestrial nematode . despite this abundance lake canopus in the wright valley , dry valleys area of east antarctica is one of the locations where plectus murrayi is found , in the mat of cyanobacteria at the edge of the lake . in the study by adhikari et al .   a total of 2,486 ests were generated comprising 1,387 unique transcripts . the caenorhabditis elegans genome comprises about 20,000 genes and the p. murrayi genome is probably of a similar size . the transcripts reported in this paper are therefore likely to represent only a small proportion of those expressed overall . 38% were considered to have homologs in c. elegans , 7% showed matches with other nematode databases ( typically c. briggsae ) , and a further 11% of transcripts were similar to sequences from other organisms . the breakdown of functions was assessed by gene ontology ( go database ) and by assignment to metabolic pathways using the kyoto encyclopedia of genes and genomes ( kegg ) database . these analyses showed a wide range of functions associated with the est transcripts , representing most of the functions that might be expected of an eukaryotic organism . analysis of abundant transcripts suggested that metabolic genes and those associated with the processing of environmental information were highly expressed . the authors note that ribosomal protein transcripts were abundant ; findings consistent with protein expression . in particular , the most abundant transcript detected in the desiccation cdna library was s28 , a ribosomal small subunit component . this is perhaps not too surprising , as any increase in protein synthesis associated with environmental stress is likely to be associated with an increase in ribosome number , and therefore the synthesis of ribosomal proteins ( and rna ) . in particular , a cathepsin - l - like protease was identified that is implicated in protein turnover during development and differentiation in c. elegans , especially in molting . in addition to sequencing a library of transcripts present in desiccated nematodes , a library of differentially expressed transcripts was also made . two rounds of subtractive hybridization using cdnas from desiccated and hydrated p. murrayi produced 80 sequences specific to the desiccated sample ( in subtractive hybridization , two samples are hybridized to remove cdnas present in equal amounts in both samples ) . about a quarter ( 22 ) of these were considered to be associated with metabolism , 15 were involved in environmental information processing ( including a homolog of type ii antifreeze protein from fish ( genbank accession number fk670242 ) ) , 23 with genetic information processing , 17 were considered to encode novel proteins , and three matched hypothetical proteins of unknown function . fourteen of the genes identified by subtractive hybridization were further examined by real - time pcr . many of these showed a significant increase in mrna abundance after desiccation . among this group were lea , trehalose-6-phosphate synthase ( tps ) , aldehyde dehydrogenase and glycerol kinase . both glycogen synthase and the clone identified as an antifreeze protein homolog showed a decrease in expression ( along with an unidentified protein ) . water stress increases the formation of reactive oxygen species so the production of antioxidants may be a part of an anhydrobiotic response . genes associated with antioxidant production and that are stimulated during desiccation in p. murrayi include superoxide dismutase , ras - related protein and glutathione s - transferase . an aquaporin ( proteins that regulate the flow of water across cell membranes ) is one of the most abundant transcripts in the cdna library , which is consistent with the need to control water flow as osmotic strength changes during desiccation in p. murrrayi . it is possible that some genes are constitutively expressed , rather than induced by stress , but are nonetheless important in desiccation ( and perhaps freezing ) . for example , in notothenioid fish , which are highly represented in the antarctic , hsp70 is not induced by environmental challenge but instead is constitutively expressed at a high level . the finding of adhikari et al .   that hsp70 and hsp90 expression is not increased by desiccation suggests that this might be the case in p. murrayi . those genes that had homologs in other animals suggest the usual range of metabolic activities that might be expected of a nematode . does this mean that desiccation tolerance reflects subtle modifications to the normal suite of metabolic processes ? alternatively , does the as - yet unidentified 44% of the total est library encode components of new pathways that confer resistance to desiccation in p. murrayi ? it is not unusual for organisms newly sequenced to reveal unique reading frames and transcripts , and it is not always clear whether these new transcripts are expressed and play a role . however , it is likely that at least some of these unique sequences play some interesting role in the metabolism of the organism that is their host . about a quarter of the transcripts identified by subtractive hybridization belong in the category of new sequences , and these may provide a fruitful set of candidates to answer this question . identifying the roles of these genes will prove challenging . we will need some integrated molecular , physiological and biochemical studies to see if some of the potential mechanisms identified by est and similar approaches do in fact play an important role in anhydrobiosis . exposed the nematodes to a relatively mild desiccation stress of 87% relative humidity at 23c for 2 days . are they capable of anhydrobiosis , which we may practically define as surviving exposure to 0% relative humidity ? are further mechanisms invoked when nematodes are exposed to more severe desiccation ? what effect does lowered temperature have and is the response to freezing similar to that to desiccation ? as perhaps might be expected , enzymes involved in trehalose synthesis ( such as tps ) and lea proteins are upregulated during desiccation of p. murrayi . however , a note of caution may be that c. elegans has several tps and lea genes and yet is not particularly desiccation tolerant . it has only been shown to survive exposure to 97% relative humidity , which can hardly be considered anhydrobiotic . nematode species vary widely in their ability to survive desiccation . to identify the mechanisms that are important in anhydrobiosis , rather than looking at changes in gene expression in response to desiccation in a particular species , it may be more instructive to compare the responses to desiccation between species that are not capable of anhydrobiosis , those that will survive anhydrobiotically if they are dried slowly , and those that will survive immediate exposure to severe desiccation . nevertheless , this study is an important first step in understanding the survival mechanisms of terrestrial antarctic organisms .
anhydrobiosis , or extreme desiccation tolerance , is one of the strategies that allows terrestrial antarctic organisms to survive in a harsh environment . 
 a new study in bmc genomics analyses gene expression in an antarctic nematode during desiccation , and sheds new light on this phenomenon .