Single-Larva RNA Sequencing Identifies Markers of Copper Toxicity and Exposure in Early Mytilus californianus Larvae.

One of the challenges facing efforts to generate molecular biomarkers for toxins is distinguishing between markers that are indicative of exposure and markers that provide evidence of the effects of toxicity. Phenotypic anchoring provides an approach to help segregate markers into these categories based on some phenotypic index of toxicity. Here we leveraged the mussel embryo-larval toxicity assay in which toxicity is estimated by the fraction of larvae that exhibit an abnormal morphology, to isolate subsets of larvae that were abnormal and thus showed evidence of copper-toxicity, versus others that while exposed to copper exhibited normal morphology. Mussel larvae reared under control conditions or in the presence of increasing levels of copper (3-15 µg/L Cu2+) were physically sorted according to whether their morphology was normal or abnormal, and then profiled using RNAseq. Supervised differential expression analysis identified sets of genes whose differential expression was specific to the pools of abnormal larvae versus normal larvae, providing putative markers of copper toxicity versus exposure. Markers of copper exposure and copper-induced abnormality were involved in many of the same pathways, including development, shell formation, cell adhesion, and oxidative stress, yet unique markers were detected in each gene set. Markers of effect appeared to be more resolving between phenotypes at the lower copper concentration, while markers of exposure were informative at both copper concentrations.

Variable gene transcription underlies phenotypic convergence of hypoxia tolerance in sculpins.

BACKGROUND: The degree by which mechanisms underlying phenotypic convergence are similar among taxa depends on the number of evolutionary paths available for selection to act upon. Likelihood of convergence will be influenced by an interplay of factors such as genetic architecture, phylogenetic history and population demography. To determine if there is convergence or divergence in mechanisms underlying phenotypic similarity, we assessed whether gene transcription patterns differed among species with similar levels of hypoxia tolerance. RESULTS: Three species of marine fish from the superfamily Cottoidea (smoothhead sculpin [Artedius lateralis], sailfin sculpin [Nautichthys oculofasciatus] and Pacific staghorn sculpin [Leptocottus armatus]), all of which have previously been shown to share the same level of hypoxia tolerance, were exposed to short-(8 h) and longer-term (72 h) hypoxia and mRNA transcripts were assessed using a custom microarray. We examined hypoxia-induced transcription patterns in metabolic and protein production pathways and found that a high proportion of genes associated with these biological processes showed significant differences among the species. Specifically, the data suggest that the smoothhead sculpin, unlike the sailfin sculpin and the Pacific staghorn sculpin, relied on amino acid degradation rather than glycolysis or fatty acid oxidation to generate ATP during hypoxia exposure. There was also variation across the species in the transcription of genes involved in protein production (e.g. mRNA processing and protein translation), such that it increased in the smoothhead sculpin, decreased in the sailfin sculpin and was variable in the Pacific staghorn sculpin. CONCLUSIONS: Changes in metabolic and protein production pathways are part of the key responses of fishes to exposures to environmental hypoxia. Yet, species with similar overall hypoxia tolerance exhibited different transcriptional responses in these pathways, indicating flexibility and complexity of interactions in the evolution of the mechanisms underlying the hypoxia tolerance phenotype. The variation in the hypoxia-induced transcription of genes across species with similar hypoxia tolerance suggests that similar whole-animal phenotypes can emerge from divergent evolutionary paths that may affect metabolically important functions.

Transcriptional and metabolomic characterization of spontaneous metabolic cycles in Mytilus californianus under subtidal conditions.

Intertidal mussels are often used to understand how life is orchestrated under tidal regimes of submergence and aerial emersion. The California ribbed mussel, Mytilus californianus, occurs predominantly in the rocky intertidal and is generally studied in the context of intertidal adaptation. We monitored M. californianus under conditions of constant submergence and observed spontaneous episodes of normal versus reduced cardiac activity. Complementary transcriptome and metabolome data indicated that these episodes were correlated with large-scale changes in the physiological state of the mussel, with periods of low cardiac activity indicative of a switch to anaerobic metabolism. A comparative analysis of these data revealed that the transcriptional and metabolomic signatures of mussels sampled during periods of normal versus reduced cardiac activity were remarkably similar to the signatures of mussels sampled under intertidal conditions of submerged and emersed conditions respectively. Our finding that submerged mussels voluntarily enter bouts of valve closure and anaerobiosis on a daily basis raises questions regarding the role that anaerobiosis plays in the overall biology of bivalves. For example, the cycling of anaerobic and aerobic phases may serve to temporally compartmentalize physiological processes or to extend life through reductions of oxidative stress.

Modulation of digestive physiology and biochemistry in Mytilus californianus in response to feeding level acclimation and microhabitat.

The intertidal mussel Mytilus californianus is a critical foundation species that is exposed to fluctuations in the environment along tidal- and wave-exposure gradients. We investigated feeding and digestion in mussels under laboratory conditions and across environmental gradients in the field. We assessed whether mussels adopt a rate-maximization (higher ingestion and lower assimilation) or a yield-maximization acquisition (lower ingestion and higher assimilation) strategy under laboratory conditions by measuring feeding physiology and digestive enzyme activities. We used digestive enzyme activity to define resource acquisition strategies in laboratory studies, then measured digestive enzyme activities in three microhabitats at the extreme ends of the tidal- and wave-exposure gradients within a stretch of shore (<20 m) projected sea-ward. Our laboratory results indicated that mussels benefit from a high assimilation efficiency when food concentration is low and have a low assimilation efficiency when food concentration is high. Additionally, enzyme activities of carbohydrases amylase, laminarinase and cellulase were elevated when food concentration was high. The protease trypsin, however, did not increase with increasing food concentration. In field conditions, low-shore mussels surprisingly did not have high enzyme activities. Rather, high-shore mussels exhibited higher cellulase activities than low-shore mussels. Similarly, trypsin activity in the high-shore-wave-sheltered microhabitat was higher than that in high-shore-wave-exposed. As expected, mussels experienced increasing thermal stress as a function of reduced submergence from low to high shore and shelter from wave-splash. Our findings suggest that mussels compensate for limited feeding opportunities and thermal stress by modulating digestive enzyme activities.

Second-Generation Linkage Maps for the Pacific Oyster Crassostrea gigas Reveal Errors in Assembly of Genome Scaffolds.

The Pacific oyster Crassostrea gigas, a widely cultivated marine bivalve mollusc, is becoming a genetically and genomically enabled model for highly fecund marine metazoans with complex life-histories. A genome sequence is available for the Pacific oyster, as are first-generation, low-density, linkage and gene-centromere maps mostly constructed from microsatellite DNA makers. Here, higher density, second-generation, linkage maps are constructed from more than 1100 coding (exonic) single-nucleotide polymorphisms (SNPs), as well as 66 previously mapped microsatellite DNA markers, all typed in five families of Pacific oysters (nearly 172,000 genotypes). The map comprises 10 linkage groups, as expected, has an average total length of 588 cM, an average marker-spacing of 1.0 cM, and covers 86% of a genome estimated to be 616 cM. All but seven of the mapped SNPs map to 618 genome scaffolds; 260 scaffolds contain two or more mapped SNPs, but for 100 of these scaffolds (38.5%), the contained SNPs map to different linkage groups, suggesting widespread errors in scaffold assemblies. The 100 misassembled scaffolds are significantly longer than those that map to a single linkage group. On the genetic maps, marker orders and intermarker distances vary across families and mapping methods, owing to an abundance of markers segregating from only one parent, to widespread distortions of segregation ratios caused by early mortality, as previously observed for oysters, and to genotyping errors. Maps made from framework markers provide stronger support for marker orders and reasonable map lengths and are used to produce a consensus high-density linkage map containing 656 markers.

The environmentally tuned transcriptomes of Mytilus mussels.

Transcriptomics is a powerful tool for elucidating the molecular mechanisms that underlie the ability of organisms to survive and thrive in dynamic and changing environments. Here, we review the major contributions in this field, and we focus on studies of mussels in the genus Mytilus, which are well-established models for the study of ecological physiology in fluctuating environments. Our review is organized into four main sections. First, we illustrate how the abiotic forces of the intertidal environment drive the rhythmic coupling of gene expression to diel and tidal cycles in Mytilus californianus. Second, we discuss the challenges and pitfalls of conducting transcriptomic studies in field-acclimatized animals. Third, we examine the link between transcriptomic responses to environmental stress and biogeographic distributions in blue mussels, Mytilus trossulus and Mytilus galloprovincialis. Fourth, we present a comparison of transcriptomic datasets and identify 175 genes that share common responses to heat stress across Mytilus species. Taken together, these studies demonstrate that transcriptomics can provide an informative snapshot of the physiological state of an organism within an environmental context. In a comparative framework, transcriptomics can reveal how natural selection has shaped patterns of transcriptional regulation that may ultimately influence biogeography.

Divergent transcriptional patterns are related to differences in hypoxia tolerance between the intertidal and the subtidal sculpins.

Transcriptionally mediated phenotypic plasticity as a mechanism of modifying traits in response to an environmental challenge remains an important area of study. We compared the transcriptional responses to low oxygen (hypoxia) of the hypoxia-tolerant intertidal fish, the tidepool sculpin (Oligocottus maculosus) with the closely related hypoxia-intolerant subtidal fish, the silverspotted sculpin (Blepsias cirrhosus) to determine whether these species use different mechanisms to cope with hypoxia. Individuals from each species were exposed to environmental O(2) tensions chosen to yield a similar level of tissue hypoxia, and gene transcription was assessed in the liver over time. There was an effect of time in hypoxia, where the greatest transcriptional change in the silverspotted sculpin occurred between 3 and 24 h in contrast to the tidepool sculpin where the largest transcriptional change occurred between 24 and 72 h of hypoxia. A number of genes showed similar hypoxia-induced transcription patterns in both species (e.g. genes associated with glycolysis and apoptosis) suggesting they are involved in a conserved hypoxia response. A large set of genes showed divergent transcriptional patterns in the two species, including fatty acid oxidation and oxidative phosphorylation, suggesting that these biological processes may contribute to explaining variation in hypoxia tolerance in these species. When both species were exposed to a single environmental O(2) tension, large transcriptional responses were seen in the hypoxia-intolerant silverspotted sculpin while almost no response was observed in the hypoxia-tolerant tidepool sculpin. Overall, divergent transcription patterns in response to both magnitude and duration of hypoxia provide insights into the processes that may determine an animal's capacity to tolerate frequent bouts of hypoxia in the wild.

The apoptotic initiator caspase-8: its functional ubiquity and genetic diversity during animal evolution.

The caspases, a family of cysteine proteases, play multiple roles in apoptosis, inflammation, and cellular differentiation. Caspase-8 (Casp8), which was first identified in humans, functions as an initiator caspase in the apoptotic signaling mediated by cell-surface death receptors. To understand the evolution of function in the Casp8 protein family, casp8 orthologs were identified from a comprehensive range of vertebrates and invertebrates, including sponges and cnidarians, and characterized at both the gene and protein levels. Some introns have been conserved from cnidarians to mammals, but both losses and gains have also occurred; a new intron arose during teleost evolution, whereas in the ascidian Ciona intestinalis, the casp8 gene is intronless and is organized in an operon with a neighboring gene. Casp8 activities are near ubiquitous throughout the animal kingdom. Exogenous expression of a representative range of nonmammalian Casp8 proteins in cultured mammalian cells induced cell death, implying that these proteins possess proapoptotic activity. The cnidarian Casp8 proteins differ considerably from their bilaterian counterparts in terms of amino acid residues in the catalytic pocket, but display the same substrate specificity as human CASP8, highlighting the complexity of spatial structural interactions involved in enzymatic activity. Finally, it was confirmed that the interaction with an adaptor molecule, Fas-associated death domain protein, is also evolutionarily ancient. Thus, despite structural diversity and cooption to a variety of new functions, the ancient origins and near ubiquitous distribution of this activity across the animal kingdom emphasize the importance and utility of Casp8 as a central component of the metazoan molecular toolkit.

The candidate histocompatibility locus of a Basal chordate encodes two highly polymorphic proteins.

The basal chordate Botryllus schlosseri undergoes a natural transplantation reaction governed by a single, highly polymorphic locus called the fuhc. Our initial characterization of this locus suggested it encoded a single gene alternatively spliced into two transcripts: a 555 amino acid-secreted form containing the first half of the gene, and a full-length, 1008 amino acid transmembrane form, with polymorphisms throughout the ectodomain determining outcome. We have now found that the locus encodes two highly polymorphic genes which are separated by a 227 bp intergenic region: first, the secreted form as previously described, and a second gene encoding a 531 amino acid membrane-bound gene containing three extracellular immunoglobulin domains. While northern blotting revealed only these two mRNAs, both PCR and mRNA-seq detect a single capped and polyadenylated transcript that encodes processed forms of both genes linked by the intergenic region, as well as other transcripts in which exons of the two genes are spliced together. These results might suggest that the two genes are expressed as an operon, during which both genes are co-transcribed and then trans-spliced into two separate messages. This type of transcriptional regulation has been described in tunicates previously; however, the membrane-bound gene does not encode a typical Splice Leader (SL) sequence at the 5' terminus that usually accompanies trans-splicing. Thus, the presence of stable transcripts encoding both genes may suggest a novel mechanism of regulation, or conversely may be rare but stable transcripts in which the two mRNAs are linked due to a small amount of read-through by RNA polymerase. Both genes are highly polymorphic and co-expressed on tissues involved in histocompatibility. In addition, polymorphisms on both genes correlate with outcome, although we have found a case in which it appears that the secreted form may be major allorecognition determinant.

High-resolution analysis of metabolic cycles in the intertidal mussel Mytilus californianus.

Inhabitants of the marine rocky intertidal live in an environment that alternates between aquatic and terrestrial due to the rise and fall of the tide. The tide creates a cyclical availability of oxygen with animals having access to oxygenated water during episodes of submergence, while access to oxygen is restricted during aerial emergence. Here we performed liquid chromatography and gas chromatography-mass spectrometry enabled metabolomic profiling of gill samples isolated from the California ribbed mussel, Mytilus californianus, to investigate how metabolism is orchestrated in this variable environment. We created a simulated intertidal environment in which mussels were acclimated to alternating high and low tides of 6 h duration, and samples were taken every 2 h for 72 h to capture reproducible changes in metabolite levels over six high and six low tides. We quantified 169 named metabolites of which 24 metabolites cycled significantly with a 12-h period that was linked to the tidal cycle. These data confirmed the presence of alternating phases of fermentation and aerobic metabolism and highlight a role for carnitine-conjugated metabolites during the anaerobic phase of this cycle. Mussels at low tide accumulated eight carnitine-conjugated metabolites, arising from the degradation of fatty acids, branched-chain amino acids, and mitochondrial ß-oxidation end products. The data also implicate sphingosine as a potential signaling molecule during aerial emergence. These findings identify new levels of metabolic control whose role in intertidal adaptation remains to be elucidated.

Circadian cycles are the dominant transcriptional rhythm in the intertidal mussel Mytilus californianus.

Residents in the marine intertidal, the zone where terrestrial and marine habitats converge, inhabit an environment that is subject to both the 24-h day and night daily rhythm of the terrestrial earth and also the 12.4-h ebb and flow of the tidal cycle. Here, we investigate the relative contribution of the daily and tidal cycle on the physiology of intertidal mussels, Mytilus californianus, by monitoring rhythms of gene expression in both simulated and natural tidal environments. We report that >40% of the transcriptome exhibits rhythmic gene expression, and that depending on the specific tidal conditions, between 80% and 90% of the rhythmic transcripts follow a circadian expression pattern with a period of 24 to 26 h. Consistent with the dominant effect of the circadian cycle we show that the expression of clock genes oscillates with a 24-h period. Our data indicate that the circadian 24-h cycle is the dominant driver of rhythmic gene expression in this intertidal inhabitant despite the profound environmental and physiological changes associated with aerial exposure during tidal emergence.

Hypoxia-induced mobilization of stored triglycerides in the euryoxic goby Gillichthys mirabilis.

Environmental hypoxia is a common challenge that many aquatic organisms experience in their habitat. Responding to hypoxia requires metabolic reprogramming so that energy-demanding processes are regulated to match available energy reserves. In this study we explored the transcriptional control of metabolic reorganization in the liver of a hypoxia-tolerant burrow-dwelling goby, Gillichthys mirabilis. Gene expression data revealed that pathways associated with triglyceride hydrolysis were upregulated by hypoxia whereas pathways associated with triglyceride synthesis were downregulated. This finding was supported by tissue histology, which showed that the size of hepatic lipid droplets declined visibly during exposure to hypoxia. Proton nuclear magnetic resonance analysis confirmed the mobilization of hepatic triglycerides, which declined 2.7-fold after 5 days of hypoxia. The enzyme, adipose triglyceride lipase, was implicated in the mobilization of triglycerides because its expression increased at the level of both transcript and protein. This observation raises questions regarding the regulation of fat metabolism during hypoxia and the role played by the hypoxia-responsive gene leptin.

Mass mortality in Pacific oysters is associated with a specific gene expression signature.

Mass mortality events occur in natural and cultured communities of bivalve molluscs. The Pacific oyster, Crassostrea gigas, is a dominant species in many intertidal locations as well as an important aquacultured bivalve species, and for the last 50 years, adult oysters have suffered frequent and extreme mass mortality events during summer months. To investigate the molecular changes that precede these mortality events, we employed a novel nonlethal sampling approach to collect haemolymph samples from individual oysters during the period that preceded a mortality event. Microarray-based gene expression screening of the collected haemolymph was used to identify a mortality gene expression signature that distinguished oysters that survived the mortality event from those individuals that died during the event. The signature was cross-validated by comparing two separate episodes of mortality. Here, we report that near-mortality oysters can be distinguished from longer-lived oysters by the elevated expression of genes associated with cell death, lysosomal proteolysis, and cellular assembly and organization. These results show the potential utility of nonlethal sampling approaches for investigating the environmental causes of mortality in natural populations in the field, and for predicting when such events could occur and which individuals will be affected.

ExprAlign--the identification of ESTs in non-model species by alignment of cDNA microarray expression profiles.

BACKGROUND: Sequence identification of ESTs from non-model species offers distinct challenges particularly when these species have duplicated genomes and when they are phylogenetically distant from sequenced model organisms. For the common carp, an environmental model of aquacultural interest, large numbers of ESTs remained unidentified using BLAST sequence alignment. We have used the expression profiles from large-scale microarray experiments to suggest gene identities. RESULTS: Expression profiles from approximation 700 cDNA microarrays describing responses of 7 major tissues to multiple environmental stressors were used to define a co-expression landscape. This was based on the Pearsons correlation coefficient relating each gene with all other genes, from which a network description provided clusters of highly correlated genes as 'mountains'. We show that these contain genes with known identities and genes with unknown identities, and that the correlation constitutes evidence of identity in the latter. This procedure has suggested identities to 522 of 2701 unknown carp ESTs sequences. We also discriminate several common carp genes and gene isoforms that were not discriminated by BLAST sequence alignment alone. Precision in identification was substantially improved by use of data from multiple tissues and treatments. CONCLUSION: The detailed analysis of co-expression landscapes is a sensitive technique for suggesting an identity for the large number of BLAST unidentified cDNAs generated in EST projects. It is capable of detecting even subtle changes in expression profiles, and thereby of distinguishing genes with a common BLAST identity into different identities. It benefits from the use of multiple treatments or contrasts, and from the large-scale microarray data.

Elevated expression of protein biosynthesis genes in liver and muscle of hibernating black bears (Ursus americanus).

We conducted a large-scale gene expression screen using the 3,200 cDNA probe microarray developed specifically for Ursus americanus to detect expression differences in liver and skeletal muscle that occur during winter hibernation compared with animals sampled during summer. The expression of 12 genes, including RNA binding protein motif 3 (Rbm3), that are mostly involved in protein biosynthesis, was induced during hibernation in both liver and muscle. The Gene Ontology and Gene Set Enrichment analysis consistently showed a highly significant enrichment of the protein biosynthesis category by overexpressed genes in both liver and skeletal muscle during hibernation. Coordinated induction in transcriptional level of genes involved in protein biosynthesis is a distinctive feature of the transcriptome in hibernating black bears. This finding implies induction of translation and suggests an adaptive mechanism that contributes to a unique ability to reduce muscle atrophy over prolonged periods of immobility during hibernation. Comparing expression profiles in bears to small mammalian hibernators shows a general trend during hibernation of transcriptional changes that include induction of genes involved in lipid metabolism and carbohydrate synthesis as well as depression of genes involved in the urea cycle and detoxification function in liver.

Rhythms of gene expression in a fluctuating intertidal environment.

The physiological strategies that enable organisms to thrive in habitats where environmental factors vary dramatically on a daily basis are poorly understood. One of the most variable and unpredictable habitats on earth is the marine rocky intertidal zone located at the boundary between the terrestrial and marine environments. Mussels dominate rocky intertidal habitats throughout the world and, being sessile, endure wide variations in temperature, salinity, oxygen, and food availability due to diurnal, tidal, and climatic cycles. Analysis of gene-expression changes in the California ribbed mussel (Mytilus californianus) at different phases in the tidal cycle reveals that intertidal mussels exist in at least four distinct physiological states, corresponding to a metabolism and respiration phase, a cell-division phase, and two stress-response signatures linked to moderate and severe heat-stress events. The metabolism and cell-division phases appear to be functionally linked and are anticorrelated in time. The magnitudes and timings of these states varied by vertical position on the shore and appear to be driven by microhabitat conditions. The results provide new insights into the strategies that allow life to flourish in fluctuating environments and demonstrate the importance of time course data collected from field animals in situ in understanding organism-environment interactions.

Ancient and modern duplication events and the evolution of stearoyl-CoA desaturases in teleost fishes.

Stearoyl-CoA desaturases (SCDs) are key enzymes of fatty acid biosynthesis whose regulation underpins responses to dietary, thermal, and hormonal treatment. Although two isoforms are known to exist in the common carp and human and four in mouse, there is no coherent view on how this gene family evolved to generate functionally diverse members. Here we identify numerous new SCD homologs in teleost fishes, using sequence data from expressed sequence tag (EST) and cDNA collections and genomic model species. Phylogenetic analyses of the deduced coding sequences produced only partially resolved molecular trees. The multiple SCD isoforms were, however, consistent with having arisen by an ancient gene duplication event in teleost fishes together with a more recent duplication in the tetraploid carp and possibly also salmonid lineages. Critical support for this interpretation comes from comparison across all vertebrate groups of the gene order in the genomic environments of the SCD isoforms. Using syntenically aligned chromosomal fragments from large-insert clones of common carp and grass carp together with those from genomically sequenced model species, we show that the ancient and modern SCD duplication events in the carp lineage were each associated with large chromosomal segment duplications, both possibly linked to whole genome duplications. By contrast, the four mouse isoforms likely arose by tandem duplications. Each duplication in the carp lineage gave rise to differentially expressed SCD isoforms, either induced by cold or diet as previously shown for the recent duplicated carp isoforms or tissue specific as demonstrated here for the ancient duplicate zebrafish isoforms.

Global cooling: cold acclimation and the expression of soluble proteins in carp skeletal muscle.

The common carp (Cyprinus carpio) has a well-developed capacity to modify muscle properties in response to changes in temperature. Understanding the mechanisms underpinning this phenotypic response at the protein level may provide fundamental insights into the molecular basis of adaptive processes in skeletal muscle. In this study, common carp were subjected to a cooling regimen and soluble extracts of muscle homogenates were separated by 1-D SDS-PAGE and 2-DE. Proteins were identified using MALDI-TOF-MS and de novo peptide sequencing using LC-MS/MS. The 2-D gel was populated with numerous protein spots that were fragments of all three muscle isoforms (M1, M2 and M3) of carp creatine kinase (CK). The accumulation of the CK fragments was enhanced when the carp were cooled to 10 degrees C. The protein changes observed in the skeletal muscle of carp subjected to cold acclimation were compared to changes described in a previous transcript analysis study. Genes encoding CK isoforms were downregulated and the genes encoding key proteins of the ubiquitin-proteasome pathway were upregulated. These findings are consistent with a specific cold-induced enhancement of proteolysis of CK.

Interpreting physiological responses to environmental change through gene expression profiling.

Identification of differentially expressed genes in response to environmental change offers insights into the roles of the transcriptome in the regulation of physiological responses. A variety of methods are now available to implement large-scale gene expression screens, and each method has specific advantages and disadvantages. Construction of custom cDNA microarrays remains the most popular route to implement expression screens in the non-model organisms favored by comparative physiologists, and we highlight some factors that should be considered when embarking along this path. Using a carp cDNA microarray, we have undertaken a broad, system-wide gene expression screen to investigate the physiological mechanisms underlying cold and hypoxia acclimation. This dataset provides a starting point from which to explore a range of specific mechanistic hypotheses at all levels of organization, from individual biochemical pathways to the level of the whole organism. We demonstrate the utility of two data analysis methods, Gene Ontology profiling and rank-based statistical methods, to summarize the probable physiological function of acclimation-induced gene expression changes, and to prioritize specific genes as candidates for further study.

An explicit test of the phospholipid saturation hypothesis of acquired cold tolerance in Caenorhabditis elegans.

Protection of poikilothermic animals from seasonal cold is widely regarded as being causally linked to changes in the unsaturation of membrane phospholipids, yet in animals this proposition remains formally untested. We have now achieved this by the genetic manipulation of lipid biosynthesis of Caenorhabditis elegans independent of temperature. Worms transferred from 25 degrees C to 10 degrees C develop over several days a much-increased tolerance of lethal cold (0 degrees C) and also an increased phospholipid unsaturation, as in higher animal models. Of the three C. elegans Delta9-desaturases, transcript levels of fat-7 only were up-regulated by cold transfer. RNAi suppression of fat-7 caused the induction of fat-5 desaturase, so to control desaturase expression we combined RNAi of fat-7 with a fat-5 knockout. These fat-5/fat-7 manipulated worms displayed the expected negative linear relationship between lipid saturation and cold tolerance at 0 degrees C, an outcome confirmed by dietary rescue. However, this change in lipid saturation explains just 16% of the observed difference between cold tolerance of animals held at 25 degrees C and 10 degrees C. Thus, although the manipulated lipid saturation affects the tolerable thermal window, and altered Delta9-desaturase expression accounts for cold-induced lipid adjustments, the effect is relatively small and none of the lipid manipulations were sufficient to convert worms between fully cold-sensitive and fully cold-tolerant states. Critically, transfer of 10 degrees C-acclimated worms back to 25 degrees C led to them restoring the usual cold-sensitive phenotype within 24 h despite retaining a lipid profile characteristic of 10 degrees C worms. Other nonlipid mechanisms of acquired cold protection clearly dominate inducible cold tolerance.