Transcriptomic response to low salinity stress in gills of the Pacific white shrimp, Litopenaeus vannamei

Evidence from transcriptome analysis unravelled the roles of eyestalk in salinity adaptation in Pacific white shrimp (Litopenaeus vannamei)

Eyestalk is considered the main neuroendocrine organ in crustaceans. Eyestalk regulates reproduction, molting, and energy metabolism by secreting several neurohormones. However, the role of eyestalk in salinity adaptation in crustaceans remains unclear. To reveal the role of eyestalk in salinity adaptation in Litopenaeus vannamei, we performed RNA-seq to compare the transcriptomic response of the eyestalk under low salinity (salinity 3) with that of the control group (salinity 25) for 8 weeks. A total of 479 mRNAs, including 150 upregulated and 329 downregulated mRNAs, were differentially expressed between the two salinity groups. The majority of the differentially expressed genes (DEGs) were enriched in biological pathways related to osmoregulation, metabolism and energy production, and oxidative stress. The most important DEGs associated with osmoregulation were CA4, ATP1A, ATP2B, ABCB1, ABCC4, PhoA, PhoB, NOS1, ACE, ANPEP, and the V-type H⁺-ATPase E-subunit. The metabolism-related DEGs were divided into three main categories: carbohydrate and energy metabolism (i.e., G6PC, UGT), protein and amino acid metabolism (i.e., SLC15A1, AhcY, GFAT), and lipid and fatty acid metabolism (i.e., GPAT3_4, CYP2J). The key DEGs related to the oxidative stress response were UGT, NDUFB1, QCR7, QCR8, P5CDh, COX6B, and CES1. These results provide evidence for the existence of an eyestalk-salinity adaptation-stress endocrine axis in L. vannamei. These findings provide a better understanding of the molecular mechanism underlying salinity adaptation in L. vannamei.

Response of microbiota and immune function to different hypotonic stress levels in giant freshwater prawn Macrobrachium rosenbergii post-larvae

This study explored the effects of different hypotonic stress levels on antioxidant capacity, microbial composition, and gene expression of Macrobrachium rosenbergii post-larvae. The salinity of the control group was 15 ‰ (S15), and the hypotonic stress groups included three levels of 10 ‰ (S10), 8 ‰ (S8), and 6 ‰ (S6). Different hypotonic stress levels caused oxidative damage in post-larvae, evidenced by decreased superoxide dismutase (SOD) and anti-superoxide anion free radical (ASAFR). They increased malondialdehyde (MDA), nitric oxide (NO), and inducible nitric oxide synthase (iNOS) levels. Microbiological analysis exhibited that different hypotonic stress levels significantly changed microbial composition and diversity. The microbial composition in the water environment where post-larvae living was different from post-larvae. The pathogenic bacteria, including Vibrio and Flavobacterium, were abundant in S6. Transcriptome analysis showed 2, 7967, 297 DEGs, including 1, 3564, 27 up-regulated genes and 1, 4403, 270 down-regulated genes in S10, S8, and S6 groups, respectively. KEGG enrichment results showed that immune and glucose metabolism-related pathways were enriched significantly. Correlation network analysis demonstrated close interactions among antioxidant parameters, microbes, and differentially-expressed genes. In conclusion, hypotonic stress reduced the antioxidant capacity, caused oxidative damage, and altered microbial composition in M. rosenbergii post-larvae. Moreover, when the salinity is below 8 ‰, hypotonic stress impairs the immune system of M. rosenbergii post-larvae.

Transcriptomic and proteomic analyses provide insights into the adaptive responses to the combined impact of salinity and alkalinity in Gymnocypris przewalskii

Gymnocypris przewalskii is the only high-land endemic teleost living in Qinghai Lake, the largest saline-alkaline lake in China. Its osmoregulatory physiology remains elusive due to a lack of precise identification of the response proteins. In the present study, DIA/SWATH was used to identify differentially expressed proteins (DEPs) under alkaline (pH = 10.1, carbonate buffer), saline (12‰, sodium chloride), and saline-alkaline [carbonate buffer (pH = 10.1) plus 11‰ sodium chloride] stresses. A total of 66,056 unique peptides representing 7,150 proteins and 230 DEPs [the false discovery rate (FDR) ≤ 0.05, fold change (FC) ≥ 1.5] were identified under different stresses. Comparative analyses of the proteome and transcriptome indicated that over 86% of DEPs did not show consistent trends with mRNA. In addition to consistent enrichment results under different stresses, the specific DEPs involved in saline-alkaline adaptation were primarily enriched in functions of homeostasis, hormone synthesis and reactions of defense response, complement activation and reproductive development. Meanwhile, a protein-protein interaction (PPI) network analysis of these specific DEPs indicated that the hub genes were ITGAX, MMP9, C3, F2, CD74, BTK, ANXA1, NCKAP1L, and CASP8. This study accurately isolated the genes that respond to stress, and the results could be helpful for understanding the physiological regulation mechanisms regarding salinity, alkalinity, and salinity-alkalinity interactions.

Transcriptome Analysis Reveals the Molecular Response to Salinity Challenge in Larvae of the Giant Freshwater Prawn Macrobrachium rosenbergii

Salinity is a crucial factor influencing the growth, development, immunity, and reproduction of aquatic organisms; however, little is known about the molecular mechanism of the response to salinity challenge in larvae of the giant freshwater prawn Macrobrachium rosenbergii. Herein, larvae cultured in three treatment groups with salinities of 10, 13, and 16‰ (S10, S13, and S16) were collected, and then transcriptome analysis was conducted by RNA-seq. A total of 6,473, 3,830 and 3,584 differentially expressed genes (DEGs) were identified in the S10 vs. S13 comparison, S10 vs. S16 comparison and S13 vs. S16 comparison, respectively. These genes are involved in osmoregulation, energy metabolism, molting, and the immune response. qPCR analysis was used to detect the expression patterns of 16 DEGs to verify the accuracy of the transcriptome data. Protein–protein interaction (PPI) analysis for DEGs and microsatellite marker screening were also conducted to reveal the molecular mechanism of salinity regulation. Together, our results will provide insight into the molecular genetic basis of adaptation to salinity challenge for larvae of M. rosenbergii.

Analysis of transcriptome difference between rapid-growing and slow-growing in Penaeus vannamei

Penaeus vannamei is the principle cultured shrimp species in China. However, with the increase of culture density, the growth difference between individuals is also expanding. Here, we make use of RNA-seq to study the growth mechanisms of P. vannamei. After 120 days, we examined the transcriptomes of rapid-growing individuals (RG) and slow-growing individuals (SG). A total of 2116 and 176 differentially expressed genes (DEGs) were found in SG and RG, respectively. Moreover, the main DEGs are opsin, heat shock protein (HSP), actin, myosin, superoxide dismutase (SOD), cuticle protein, and chitinase. GO analysis further revealed that the DEGs were enriched in biological processes significantly, such as "sensory perception," "sensory perception of light stimulus," "response to stimulus," and "response to stress." Additionally, KEGG enrichment analysis showed that the DEGs were mainly enriched in "pentose and glucuronate interconversions," "amino sugar and nucleotide sugar metabolism," "glycophospholipid biosynthesis," and "glutathione metabolism." Interestingly, the upstream genes in the ecdysone signaling pathway, including molting inhibition hormone (MIH) and crustacean hyperglycemic hormone (CHH), did not differ significantly between RG and SG, which suggests that the cause for the inconsistent growth performance is due to the stress levels rather than the ecdysone signal pathway. In summary, this work provides data that will be useful for future studies on shrimp growth and development.

Expression analysis of immune-associated genes in hemocytes of mud crab Scylla paramamosain under low salinity challenge

To gain knowledge on the immune response in Scylla paramamosain under low salinity challenge, S. paramamosain we investigated digital gene expression (DEG) in S. paramamosain hemocytes using the deep-sequencing platform Illumina Hiseq XTen. A total of 97,257 high quality unigenes with mean length 786.59 bp were found to be regulated by low salinity challenge, among which 93 unigenes were significantly up regulated, and 71 were significantly down regulated. Functional categorization and pathways analysis of differentially expressed genes revealed that immune signaling pathway including cAMP and cGMP signaling pathway were affected in low salinity stress. Cellular immunity-related genes including low-density lipoprotein receptor-related protein 6 (LRP6) and xanthine dehydrogenase (XDH) were down-regulated, indicating phagocytosis and oxygen dependent mechanism of phagocyte were suppressed in low salinity stress; Humoral immunity-related genes serine proteases and serpins 3 were up- and down-regulated, respectively, suggest that the proPO system was influenced by low salinity significantly; Moreover, processes related to immune response including carbohydrate metabolism, protein synthesis and lipid transport were found differentially regulated, implying the integrity of the immune response in low salinity stress. This study gained comprehensive insights on the immune mechanism of S. paramamosain at low salinity stress at the molecular level. The findings provide a theoretical basis for understanding immune mechanisms of S. paramamosain under low salinity stress, and technical reference for evaluating physiological adaptation in fresh water environment.

Comparative transcriptomics reveals eyestalk ablation induced responses of the neuroendocrine-immune system in the Pacific white shrimp Litopenaeus vannamei

In decapod crustaceans, eyestalk ablation is widely used to expedite ovarian maturation and spawning because of the removal of a gonad inhibiting hormone produced by the X-organ sinus gland. However, eyestalk ablation also results in negative impacts on the immunocompetence of the eyestalk-ablated females. In the current study, we investigated the impact of eyestalk ablation on the transcriptomic responses of three major nervous organs of shrimp, including the eyestalk ganglion, brain and thoracic ganglion, using the Illumina Hiseq™ 4000 platform. A total of 48,249 unigenes with an average length of 1253 bp and a N50 value of 2482 bp were obtained. Following eyestalk ablation treatment, a total of 2,983, 6325 and 6575 unigenes were detected as differentially expressed (log₂Ratio >1 and FDR <0.05) from the eyestalk, brain and thoracic ganglia, respectively. Functional GO and KEGG analysis of these differential expression genes (DEGs) showed that these DEGs were associated with a wide variety of biological processes and pathways. The distribution of DEGs among three comparison groups was similar, and many DEGs were mapped to the phagosome pathway, indicating that eyestalk ablation triggers activation of the neuroendocrine-immune (NEI) system. Interestingly, several important pathways were uniquely enriched in the brain tissue, suggesting that the brain may play a crucial role in the NEI system in response to eyestalk ablation. This is the first report on the transcriptomic regulation of the nervous system in response to eyestalk ablation in L. vannamei. The genes and pathways identified in this study will help to elucidate the molecular mechanisms of neuroendocrine-immune responses to eyestalk ablation in penaeid shrimp.

Fine-scale population structure and evidence for local adaptation in Australian giant black tiger shrimp (Penaeus monodon) using SNP analysis

BACKGROUND

Restrictions to gene flow, genetic drift, and divergent selection associated with different environments are significant drivers of genetic differentiation. The black tiger shrimp (Penaeus monodon), is widely distributed throughout the Indian and Pacific Oceans including along the western, northern and eastern coastline of Australia, where it is an important aquaculture and fishery species. Understanding the genetic structure and the influence of environmental factors leading to adaptive differences among populations of this species is important for farm genetic improvement programs and sustainable fisheries management.

RESULTS

Based on 278 individuals obtained from seven geographically disparate Australian locations, 10,624 high-quality SNP loci were used to characterize genetic diversity, population structure, genetic connectivity, and adaptive divergence. Significant population structure and differentiation were revealed among wild populations (average F_ST = 0.001-0.107; p <  0.05). Eighty-nine putatively outlier SNPs were identified to be potentially associated with environmental variables by using both population differentiation (BayeScan and PCAdapt) and environmental association (redundancy analysis and latent factor mixed model) analysis methods. Clear population structure with similar spatial patterns were observed in both neutral and outlier markers with three genetically distinct groups identified (north Queensland, Northern Territory, and Western Australia). Redundancy, partial redundancy, and multiple regression on distance matrices analyses revealed that both geographical distance and environmental factors interact to generate the structure observed across Australian P. monodon populations.

CONCLUSION

This study provides new insights on genetic population structure of Australian P. monodon in the face of environmental changes, which can be used to advance sustainable fisheries management and aquaculture breeding programs.

A novel modulation of physiological regulation in cultured Chinese mitten crab (Eriocheir japonica sinensis) in response to consistent salinity changes

The life history of the Chinese mitten crab (Eriocheir japonica sinensis) includes two migrations: a feeding migration and a reproductive migration. Ambient salinity is one of the most critical factors during migration. In this study, the salinity adaptation mechanism of Chinese mitten crabs was simulated using continuous salinity changes. The expression of six key genes [Na⁺/K⁺-ATPase α subunit (NAK-α), V-type H⁺-ATPase subunit A (VHA-A), Zinc transporter (ZnT), Cl^- channel protein 2 (CLCN2), ubiquitin/ribosomal S27 fusionprotein (S27), and glutathione S-transferase (GST)] and the activities of three enzymes [Na⁺/K⁺-ATPase (NAK), V-type H⁺-ATPase (VHA), and glutathione S-transferase (GST)] were evaluated in ten groups exposed to a range of salinity changes during mariculture based on the transcriptome data obtained from short term salinity-induced crabs (ES) compared to control group in freshwater crabs (EF). The results revealed that different genes exhibited different roles in physiological regulation. In total, 3,599 unigenes were significantly and differentially expressed in a comparison between the EF and ES treatments. A novel modulation of gene expression and the corresponding enzyme activity of NAK and VHA exhibited similar patterns. As genes related to osmoregulation, NAK and VHA showed similar patterns of both gene expression and enzyme activity in mariculture. During the gradual change in salinity from 0‰ to 25‰ and back to 0‰, the gene expression and enzyme activities of NAK and VHA initially increased (0‰ → 10‰), weakened (10‰ → 20‰) and then increased again (20‰ → 25‰ → 0‰). S27 could serve as a reference gene in the expression analysis of Chinese mitten crabs under salinity stress. ZnT and CLCN2 were involved in osmoregulation as functional proteins. Our findings provide insights into the regulation mechanisms employed during the migration of the Chinese mitten crab.

Transcriptomic analysis of Litopenaeus vannamei hepatopancreas under cold stress in cold-tolerant and cold-sensitive cultivars

Litopenaeus vannamei (L. vannamei) is one of the most widely cultured shrimp species in the world. The species often suffers from cold stress. To understand the molecular mechanism of cold tolerance, we performed transcriptomic analysis on two contrasting cultivars of L. vannamei, namely, cold-tolerant Guihai 2 (GH2) and cold-sensitive Guihai1 (GH1), under a control temperature (28 °C), cold stress (16 °C), and recovery to 28 °C. A total of 84.5 Gb of sequences were generated from 12 L. vannamei hepatopancreas libraries. The de-novo assembly generated a total of 143,029 unigenes with a mean size of 1,052 bp and an N50 of 2,604 bp, of which 34.08% were annotated in the Nr database. We analyzed the differentially expressed genes (DEGs) between nine comparison groups and detected a total of 21,026 DEGs. KEGG pathways, including lysosome, sphingolipid metabolism and nitrogen metabolism, were significantly enriched by DEGs between different temperatures in GH2. Furthermore, eight of the most significantly DEGs under cold stress from the transcriptomic analysis were selected for quantitative real-time PCR (qPCR) validation. Overall, we compared gene expression changes under cold stress in cold-tolerant and cold-sensitive L. vannamei for the first time. The results may further extend our understanding of the cold stress-response mechanism in L. vannamei.

Transcriptomic response to three osmotic stresses in gills of hybrid tilapia (Oreochromis mossambicus female × O. urolepis hornorum male)

Background

Osmotic stress is a widespread phenomenon in aquatic animal. The ability to cope with salinity stress and alkaline stress is quite important for the survival of aquatic species under natural conditions. Tilapia is an important commercial euryhaline fish species. What’s more tilapia is a good experimental material for osmotic stress regulation research, but the molecular regulation mechanism underlying different osmotic pressure of tilapia is still unexplored.

Results

To elucidate the osmoregulation strategy behind its hyper salinity, alkalinity and salinity-alkalinity stress of tilapia, the transcriptomes of gills in hybrid tilapia (Oreochromis mossambicus ♀ × O. urolepis hornorum ♂) under salinity stress (S: 25‰), alkalinity stress(A: 4‰) and salinity-alkalinity stress (SA: S: 15‰, A: 4‰) were sequenced using deep-sequencing platform Illumina/HiSeq-2000 and differential expression genes (DEGs) were identified. A total of 1958, 1472 and 1315 upregulated and 1824, 1940 and 1735 downregulated genes (P-value < 0.05) were identified in the salt stress, alkali stress and saline-alkali stress groups, respectively, compared with those in the control group. Furthermore, Kyoto Encyclopedia of Genes and Genomes pathway analyses were conducted in the significant different expression genes. In all significant DEGs, some of the typical genes involved in osmoregulation, including carbonic anhydrase (CA), calcium/calmodulin-dependent protein kinase (CaM kinase) II (CAMK2), aquaporin-1(AQP1), sodium bicarbonate cotransporter (SLC4A4/NBC1), chloride channel 2(CLCN2), sodium/potassium/chloride transporter (SLC12A2 / NKCC1) and other osmoregulation genes were also identified. RNA-seq results were validated with quantitative real-time PCR (qPCR), the 17 random selected genes showed a consistent direction in both RNA-Seq and qPCR analysis, demonstrated that the results of RNA-seq were reliable.

Conclusions

The present results would be helpful to elucidate the osmoregulation mechanism of aquatic animals adapting to saline-alkali challenge. This study provides a global overview of gene expression patterns and pathways that related to osmoregulation in hybrid tilapia, and could contribute to a better understanding of the molecular regulation mechanism in different osmotic stresses.

Disparate responses to salinity across species and organizational levels in anchialine shrimps

Environmentally induced plasticity in gene expression is one of the underlying mechanisms of adaptation to habitats with variable environments. For example, euryhaline crustaceans show predictable changes in the expression of ion-transporter genes during salinity transfers, although studies have typically been limited to specific genes, taxa and ecosystems of interest. Here, we investigated responses to salinity change at multiple organizational levels in five species of shrimp representing at least three independent invasions of the anchialine ecosystem, defined as habitats with marine and freshwater influences with spatial and temporal fluctuations in salinity. Although all five species were generally strong osmoregulators, salinity-induced changes in gill physiology and gene expression were highly species specific. While some species exhibited patterns similar to those of previously studied euryhaline crustaceans, instances of distinct and atypical patterns were recovered from closely related species. Species-specific patterns were found when examining: (1) numbers and identities of differentially expressed genes, (2) salinity-induced expression of genes predicted a priori to play a role in osmoregulation, and (3) salinity-induced expression of orthologs shared among all species. Notably, ion transport genes were unchanged in the atyid Halocaridina rubra while genes normally associated with vision and light perception were among those most highly upregulated. Potential reasons for species-specific patterns are discussed, including variation among anchialine habitats in salinity regimes and divergent evolution in anchialine taxa. Underexplored mechanisms of osmoregulation in crustaceans revealed here by the application of transcriptomic approaches to ecologically and taxonomically understudied systems are also explored.

Single-molecule long-read sequencing facilitates shrimp transcriptome research

Although shrimp are of great economic importance, few full-length shrimp transcriptomes are available. Here, we used Pacific Biosciences single-molecule real-time (SMRT) long-read sequencing technology to generate transcripts from the Pacific white shrimp (Litopenaeus vannamei). We obtained 322,600 full-length non-chimeric reads, from which we generated 51,367 high-quality unique full-length transcripts. We corrected errors in the SMRT sequences by comparison with Illumina-produced short reads. We successfully annotated 81.72% of all unique SMRT transcripts against the NCBI non-redundant database, 58.63% against Swiss-Prot, 45.38% against Gene Ontology, 32.57% against Clusters of Orthologous Groups of proteins (COG), and 47.83% against Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. Across all transcripts, we identified 3,958 long non-coding RNAs (lncRNAs) and 80,650 simple sequence repeats (SSRs). Our study provides a rich set of full-length cDNA sequences for L. vannamei, which will greatly facilitate shrimp transcriptome research.

Transcriptomic analysis reveal an efficient osmoregulatory system in Siberian sturgeon Acipenser baeri in response to salinity stress

Sturgeons are euryhaline fish species that have developed specific mechanisms of osmotic and ion regulation to adapt to waters of varying salinity. For the aim to elucidate the osmoregulation strategy behind its high salinity tolerance of sturgeons, the transcriptomes of gills in Siberian sturgeon Acipenser baeri under salinity stress (30 ppt) were sequenced using deep-sequencing platform Illumina/HiSeq-2500 and differential expression genes (DEGs) were identified. A total of 167, 501, 278 clean reads were obtained and 280, 238 unigenes were composed of those clean reads with the mean length of 520nt, and the N50 of 630 bp. Unigenes Sequence alignment was implemented via KEGG, KOG, NT, NR, PFAM, Swiss-Prot, and GO databases. 62, 242 unigenes (22.21%) were annoated in at least one database. 11380 significantly differentially expressed unigenes were found, 6969 of which were up-regulated and 4411 were down-regulated by salinity stress. Amongst the top 20 KEGG pathways with the most amount of annotation sequences, some pathways such as glycerophospholipid metabolism, fatty-acid biosynthesis, glycolysis/gluconeogenesis, oxidative phosphorylation have been comprehensively proved to be relevant to osmoregulation. Despite of these, three possible osmoregulation-related signaling pathways as lipid metabolism related pathways, tight junction pathway and thyroid hormone signaling pathway have been widely analyzed in the current study. In all DEGs, some of the typical genes involved in osmoregulation, including calcium-transporting ATPase 4 (ATP2B4), Na⁺/K⁺-ATPase alpha subunit (α-NKA), potassium-transporting ATPase alpha chain 1 (ATP4A) and Ras GTPase-activating protein (RasGAP) etc were also identified. RNA-seq results were validated with quantitative real-time PCR (qPCR), the 12 selected genes showed a consistent direction in both DGE library and qPCR analysis, proving that the RNA-seq results are reliable. The present results would be helpful to elucidate the osmoregulation mechanism of aquatic animals adapting to salinity challenge.

De novo assembly, characterization, functional annotation and expression patterns of the black tiger shrimp (Penaeus monodon) transcriptome

The black tiger shrimp (Penaeus monodon) remains the second most widely cultured shrimp species globally; however, issues with disease and domestication have seen production levels stagnate over the past two decades. To help identify innovative solutions needed to resolve bottlenecks hampering the culture of this species, it is important to generate genetic and genomic resources. Towards this aim, we have produced the most complete publicly available P. monodon transcriptome database to date based on nine adult tissues and eight early life-history stages (BUSCO - Complete: 98.2% [Duplicated: 51.3%], Fragmented: 0.8%, Missing: 1.0%). The assembly resulted in 236,388 contigs, which were then further segregated into 99,203 adult tissue specific and 58,678 early life-history stage specific clusters. While annotation rates were low (approximately 30%), as is typical for a non-model organisms, annotated transcript clusters were successfully mapped to several hundred functional KEGG pathways. Transcripts were clustered into groups within tissues and early life-history stages, providing initial evidence for their roles in specific tissue functions, or developmental transitions. We expect the transcriptome to provide an essential resource to investigate the molecular basis of commercially relevant-significant traits in P. monodon and other shrimp species.

Transcriptomic analysis of the hepatopancreas induced by eyestalk ablation in shrimp, Litopenaeus vannamei

Although eyestalk ablation (ESA) is currently considered the most effective method to facilitate molting and maturation, its physiological responses are still not clearly explained in decapod crustaceans. In this study, we analyzed the hepatopancreatic transcriptomes of Litopenaeus vannamei after ESA using the Illumina Miseq platform. After screening 53,029 contigs with high cutoff values (fold change>|10|; P-value<0.05; RPKM>1), we were able to identify 105 differentially expressed genes (DEGs), of which 100 were up-regulated and five were down-regulated. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that many DEGs were involved in the synthetic pathways for glycerol and trehalose, which are known to function as the major protectants under conditions of low temperature and osmotic stress in arthropods. Additional analysis of the other DEGs enabled us to classify them in four categories: immunity; cellular trafficking; transcriptional regulation; molting and maturation. Many DEGs were involved in immunity and stress responses, in particular the proPO activation system, which is the major immune and wound-healing system in arthropods. In addition to immunity and stress responses, we were also able to identify DEGs involved in molting and maturation processes (e.g., group I chitinase), as well as those involved in hormone metabolism and trafficking. Collectively, based on the transcriptomic analysis, ESA causes not only stress and immune responses, but also molting and maturation in L. vannamei. The DEGs identified in this study could be useful markers to understand the physiological responses that ESA induces in shrimp, such as molting, maturation, and immunity.

Transcriptomic analyses on muscle tissues of Litopenaeus vannamei provide the first profile insight into the response to low temperature stress

The Pacific white shrimp (Litopenaeus vannamei) is an important cultured crustacean species worldwide. However, little is known about the molecular mechanism of this species involved in the response to cold stress. In this study, four separate RNA-Seq libraries of L. vannamei were generated from 13°C stress and control temperature. Total 29,662 of Unigenes and overall of 19,619 annotated genes were obtained. Three comparisons were carried out among the four libraries, in which 72 of the top 20% of differentially-expressed genes were obtained, 15 GO and 5 KEGG temperature-sensitive pathways were fished out. Catalytic activity (GO: 0003824) and Metabolic pathways (ko01100) were the most annotated GO and KEGG pathways in response to cold stress, respectively. In addition, Calcium, MAPK cascade, Transcription factor and Serine/threonine-protein kinase signal pathway were picked out and clustered. Serine/threonine-protein kinase signal pathway might play more important roles in cold adaptation, while other three signal pathway were not widely transcribed. Our results had summarized the differentially-expressed genes and suggested the major important signaling pathways and related genes. These findings provide the first profile insight into the molecular basis of L. vannamei response to cold stress.

Convergent Evolution of the Osmoregulation System in Decapod Shrimps

In adaptating to different aquatic environments, seawater (SW) and freshwater (FW) shrimps have exploited different adaptation strategies, which should generate clusters of genes with different adaptive features. However, little is known about the genetic basis of these physiological adaptations. Thus, in this study, we performed comparative transcriptomics and adaptive evolution analyses on SW and FW shrimps and found that convergent evolution may have happened on osmoregulation system of shrimps. We identified 275 and 234 positively selected genes in SW and FW shrimps, respectively, which enriched in the functions of ion-binding and membrane-bounded organelles. Among them, five (CaCC, BEST2, GPDH, NKA, and Integrin) and four (RasGAP, RhoGDI, CNK3, and ODC) osmoregulation-related genes were detected in SW and FW shrimps, respectively. All five genes in SW shrimps have been reported to have positive effects on ion transportation, whereas RasGAP and RhoGDI in FW shrimps are associated with negative control of ion transportation, and CNK3 and ODC play central roles in cation homeostasis. Besides, the phylogenetic tree reconstructed from the positively selected sites separated the SW and FW shrimps into two groups. Distinct subsets of parallel substitutions also have been found in these osmoregulation-related genes in SW and FW shrimps. Therefore, our results suggest that distinct convergent evolution may have occurred in the osmoregulation systems of SW and FW shrimps. Furthermore, positive selection of osmoregulation-related genes may be beneficial for the regulation of water and salt balance in decapod shrimps.