Abstract
BackgroundThe phenomenon of desiccation tolerance, also called anhydrobiosis, involves the ability of an organism to survive the loss of almost all cellular water without sustaining irreversible damage. Although there are several physiological, morphological and ecological studies on tardigrades, only limited DNA sequence information is available. Therefore, we explored the transcriptome in the active and anhydrobiotic state of the tardigrade Milnesium tardigradum which has extraordinary tolerance to desiccation and freezing. In this study, we present the first overview of the transcriptome of M. tardigradum and its response to desiccation and discuss potential parallels to stress responses in other organisms.ResultsWe sequenced a total of 9984 expressed sequence tags (ESTs) from two cDNA libraries from the eutardigrade M. tardigradum in its active and inactive, anhydrobiotic (tun) stage. Assembly of these ESTs resulted in 3283 putative unique transcripts, whereof ~50% showed significant sequence similarity to known genes. The resulting unigenes were functionally annotated using the Gene Ontology (GO) vocabulary. A GO term enrichment analysis revealed several GOs that were significantly underrepresented in the inactive stage. Furthermore we compared the putative unigenes of M. tardigradum with ESTs from two other eutardigrade species that are available from public sequence databases, namely Richtersius coronifer and Hypsibius dujardini. The processed sequences of the three tardigrade species revealed similar functional content and the M. tardigradum dataset contained additional sequences from tardigrades not present in the other two.ConclusionsThis study describes novel sequence data from the tardigrade M. tardigradum, which significantly contributes to the available tardigrade sequence data and will help to establish this extraordinary tardigrade as a model for studying anhydrobiosis. Functional comparison of active and anhydrobiotic tardigrades revealed a differential distribution of Gene Ontology terms associated with chromatin structure and the translation machinery, which are underrepresented in the inactive animals. These findings imply a widespread metabolic response of the animals on dehydration. The collective tardigrade transcriptome data will serve as a reference for further studies and support the identification and characterization of genes involved in the anhydrobiotic response.
Highlights
The phenomenon of desiccation tolerance, called anhydrobiosis, involves the ability of an organism to survive the loss of almost all cellular water without sustaining irreversible damage
The datasets used in this study consisted of expressed sequence tags (ESTs) sequences from, M. tardigradum active and inactive stages, H. dujardini [Daub et al Unpublished data 2003] and R. coronifer [35] of which the latter two were retrieved from NCBI (National Center for Biotechnology Information) dbEST and the NCBI Trace Archive
Analysis of the M. tardigradum cDNA library As summarized in Table 1, a total of 9979 clones were sequenced from the M. tardigradum library generated from two stages, active and inactive, in order to obtain various transcripts and to extract putative anhydrobiotic candidate genes
Summary
The phenomenon of desiccation tolerance, called anhydrobiosis, involves the ability of an organism to survive the loss of almost all cellular water without sustaining irreversible damage. The tardigrade phylum currently includes more than 1000 species living in the sea, in fresh water and on land These last, needing at least a film of water to be active, are called limno-terrestrial and include most of the anhydrobiotic species [25]. They have been studied for their fascinating ability to perform anhydrobiosis and serve as a potential model for studying tolerance and survival of multicellular organisms to a variety of extreme environmental conditions. A cross-species comparison between M. tardigradum, H. dujardini and R. coronifer has been performed
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