Pan-evolutionary and regulatory genome architecture delineated by an integrated macro- and microsynteny approach.

The forthcoming massive genome data generated by the Earth BioGenome Project will open up a new era of comparative genomics, for which genome synteny analysis provides an important framework. Profiling genome synteny represents an essential step in elucidating genome architecture, regulatory blocks/elements and their evolutionary history. Here we describe PanSyn, ( https://github.com/yhw320/PanSyn ), the most comprehensive and up-to-date genome synteny pipeline, providing step-by-step instructions and application examples to demonstrate its usage. PanSyn inherits both basic and advanced functions from existing popular tools, offering a user-friendly, highly customized approach for genome macrosynteny analysis and integrated pan-evolutionary and regulatory analysis of genome architecture, which are not yet available in public synteny software or tools. The advantages of PanSyn include: (i) advanced microsynteny analysis by functional profiling of microsynteny genes and associated regulatory elements; (ii) comprehensive macrosynteny analysis, including the inference of karyotype evolution from ancestors to extant species; and (iii) functional integration of microsynteny and macrosynteny for pan-evolutionary profiling of genome architecture and regulatory blocks, as well as integration with external functional genomics datasets from three- or four-dimensional genome and ENCODE projects. PanSyn requires basic knowledge of the Linux environment and Perl programming language and the ability to access a computer cluster, especially for large-scale genomic comparisons. Our protocol can be easily implemented by a competent graduate student or postdoc and takes several days to weeks to execute for dozens to hundreds of genomes. PanSyn provides yet the most comprehensive and powerful tool for integrated evolutionary and functional genomics.

The Effects of Larval Cryopreservation on the Epigenetics of the Pacific Oyster Crassostrea gigas.

High mortalities and highly variable results during the subsequent development of post-thaw larvae have been widely considered as key issues restricting the application of cryopreservation techniques to support genetic improvement programs and hatchery production in farmed marine bivalve species. To date, few studies have been undertaken to investigate the effects of cryodamage at the molecular level in bivalves. This study is the first to evaluate the effect of larval cryopreservation on the epigenetics of the resultant progenies of the Pacific oyster Crassostrea gigas. The results show that the level of DNA methylation was significantly (p < 0.05) higher and lower than that of the control when the trochophore larvae were revived and when they developed to D-stage larvae (day 1 post-fertilization), respectively, but the level returned to the control level from day 8 post-fertilization onwards. The expression of the epigenetic regulator genes DNMT3b, MeCP2, JmjCA, KDM2 and OSA changed significantly (p < 0.05) when the trochophore larvae were thawed, and then they reverted to the control levels at the D- and later larval developmental stages. However, the expression of other epigenetic regulator genes, namely, MBD2, DNMT1, CXXC1 and JmjD6, did not change at any post-thaw larval developmental stage. For the newly thawed trochophore larvae, the amount of methylated H3K4Me1 and H3K27Me1 significantly changed, and the expression of all Jumonji orthologs, except that of Jumonji5, significantly (p < 0.05) decreased. These epigenetic results agree with the data collected on larval performances (e.g., survival rate), suggesting that the effect period of the published cryopreservation technique on post-thaw larvae is short in C. gigas.

Reference genomes of channel catfish and blue catfish reveal multiple pericentric chromosome inversions.

BACKGROUND: Channel catfish and blue catfish are the most important aquacultured species in the USA. The species do not readily intermate naturally but F1 hybrids can be produced through artificial spawning. F1 hybrids produced by mating channel catfish female with blue catfish male exhibit heterosis and provide an ideal system to study reproductive isolation and hybrid vigor. The purpose of the study was to generate high-quality chromosome level reference genome sequences and to determine their genomic similarities and differences. RESULTS: We present high-quality reference genome sequences for both channel catfish and blue catfish, containing only 67 and 139 total gaps, respectively. We also report three pericentric chromosome inversions between the two genomes, as evidenced by long reads across the inversion junctions from distinct individuals, genetic linkage mapping, and PCR amplicons across the inversion junctions. Recombination rates within the inversional segments, detected as double crossovers, are extremely low among backcross progenies (progenies of channel catfish female × F1 hybrid male), suggesting that the pericentric inversions interrupt postzygotic recombination or survival of recombinants. Identification of channel catfish- and blue catfish-specific genes, along with expansions of immunoglobulin genes and centromeric Xba elements, provides insights into genomic hallmarks of these species. CONCLUSIONS: We generated high-quality reference genome sequences for both blue catfish and channel catfish and identified major chromosomal inversions on chromosomes 6, 11, and 24. These perimetric inversions were validated by additional sequencing analysis, genetic linkage mapping, and PCR analysis across the inversion junctions. The reference genome sequences, as well as the contrasted chromosomal architecture should provide guidance for the interspecific breeding programs.

Genomic imprinting-like monoallelic paternal expression determines sex of channel catfish.

The X and Y chromosomes of channel catfish have the same gene contents. Here, we report allelic hypermethylation of the X chromosome within the sex determination region (SDR). Accordingly, the X-borne hydin-1 gene was silenced, whereas the Y-borne hydin-1 gene was expressed, making monoallelic expression of hydin-1 responsible for sex determination, much like genomic imprinting. Treatment with a methylation inhibitor, 5-aza-dC, erased the epigenetic marks within the SDR and caused sex reversal of genetic females into phenotypic males. After the treatment, hydin-1 and six other genes related to cell cycle control and proliferative growth were up-regulated, while three genes related to female sex differentiation were down-regulated in genetic females, providing additional support for epigenetic sex determination in catfish. This mechanism of sex determination provides insights into the plasticity of genetic sex determination in lower vertebrates and its connection with temperature sex determination where DNA methylation is broadly involved.

Ancient homomorphy of molluscan sex chromosomes sustained by reversible sex-biased genes and sex determiner translocation.

Contrary to classic theory prediction, sex-chromosome homomorphy is prevalent in the animal kingdom but it is unclear how ancient homomorphic sex chromosomes avoid chromosome-scale degeneration. Molluscs constitute the second largest, Precambrian-originated animal phylum and have ancient, uncharacterized homomorphic sex chromosomes. Here, we profile eight genomes of the bivalve mollusc family of Pectinidae in a phylogenetic context and show 350 million years sex-chromosome homomorphy, which is the oldest known sex-chromosome homomorphy in the animal kingdom, far exceeding the ages of well-known heteromorphic sex chromosomes such as 130-200 million years in mammals, birds and flies. The long-term undifferentiation of molluscan sex chromosomes is potentially sustained by the unexpected intertwined regulation of reversible sex-biased genes, together with the lack of sexual dimorphism and occasional sex chromosome turnover. The pleiotropic constraint of regulation of reversible sex-biased genes is widely present in ancient homomorphic sex chromosomes and might be resolved in heteromorphic sex chromosomes through gene duplication followed by subfunctionalization. The evolutionary dynamics of sex chromosomes suggest a mechanism for 'inheritance' turnover of sex-determining genes that is mediated by translocation of a sex-determining enhancer. On the basis of these findings, we propose an evolutionary model for the long-term preservation of homomorphic sex chromosomes.

Starfish infers signatures of complex genomic rearrangements across human cancers.

Complex genomic rearrangements (CGRs) are common in cancer and are known to form via two aberrant cellular structures-micronuclei and chromatin bridges. However, which of these mechanisms is more relevant to CGR formation in cancer and whether there are other undiscovered mechanisms remain unknown. Here we developed a computational algorithm, 'Starfish', to analyze 2,014 CGRs from 2,428 whole-genome-sequenced (WGS) tumors and discovered six CGR signatures based on their copy number and breakpoint patterns. Extensive benchmarking showed that our CGR signatures are highly accurate and biologically meaningful. Three signatures can be attributed to known biological processes-micronuclei- and chromatin-bridge-induced chromothripsis and circular extrachromosomal DNA. Over half of the CGRs belong to the remaining three signatures, not reported previously. A unique signature, which we named 'hourglass chromothripsis', with localized breakpoints and a low amount of DNA loss, is abundant in prostate cancer. Hourglass chromothripsis is associated with mutant SPOP, which may induce genome instability.

Decoding the byssus fabrication by spatiotemporal secretome analysis of scallop foot.

Secretome is involved in almost all physiological, developmental, and pathological processes, but to date there is still a lack of highly-efficient research strategy to comprehensively study the secretome of invertebrates. Adhesive secretion is a ubiquitous and essential physiological process in aquatic invertebrates with complicated protein components and unresolved adhesion mechanisms, making it a good subject for secretome profiling studies. Here we proposed a computational pipeline for systematic profiling of byssal secretome based on spatiotemporal transcriptomes of scallop. A total of 186 byssus-related proteins (BRPs) were identified, which represented the first characterized secretome of scallop byssal adhesion. Scallop byssal secretome covered almost all of the known structural elements and functional domains of aquatic adhesives, which suggested this secretome-profiling strategy had both high efficiency and accuracy. We revealed the main components of scallop byssus (including EGF-like domain containing proteins, the Tyr-rich proteins and 4C-repeats containing proteins) and the related modification enzymes primarily contributing to the rapid byssus assembly and adhesion. Spatiotemporal expression and co-expression network analyses of BRPs suggested a simultaneous secretion pattern of scallop byssal proteins across the entire region of foot and revealed their diverse functions on byssus secretion. In contrast to the previously proposed "root-initiated secretion and extension-based assembly" model, our findings supported a novel "foot-wide simultaneous secretion and in situ assembly" model of scallop byssus secretion and adhesion. Systematic analysis of scallop byssal secretome provides important clues for understanding the aquatic adhesive secretion process, as well as a common framework for studying the secretome of non-model invertebrates.

Identification of an Epigenetically Marked Locus within the Sex Determination Region of Channel Catfish.

Channel catfish has an XY sex determination system. However, the X and Y chromosomes harbor an identical gene content of 950 genes each. In this study, we conducted comparative analyses of methylome and transcriptome of genetic males and genetic females before gonadal differentiation to provide insights into the mechanisms of sex determination. Differentially methylated CpG sites (DMCs) were predominantly identified on the sex chromosome, most notably within the sex determination region (SDR), although the overall methylation profiles across the entire genome were similar between genetic males and females. The drastic differences in methylation were located within the SDR at nucleotide position 14.0-20.3 Mb of the sex chromosome, making this region an epigenetically marked locus within the sex determination region. Most of the differentially methylated CpG sites were hypermethylated in females and hypomethylated in males, suggesting potential involvement of methylation modification in sex determination in channel catfish. Along with the differential methylation in the SDR, a number of differentially expressed genes within the SDR were also identified between genetic males and females, making them potential candidate genes for sex determination and differentiation in channel catfish.

High-quality reannotation of the king scallop genome reveals no 'gene-rich' feature and evolution of toxin resistance.

The king scallop, Pecten maximus is a well-known, commercially important scallop species and is featured with remarkable tolerance to potent phytotoxins such as domoic acid. A high-quality genome can shed light on its biology and innovative evolution of toxin resistance. A reference genome has recently been published for P. maximus, however, it is suspicious that over 67,700 genes are annotated in this genome, which is unexpectedly larger than its close relatives of pectinids. Herein, we provide an improved high-quality chromosome-level reference genome assembly and annotation for the king scallop P. maximus. A final set of 26,995 genes is annotated after carefully checking and curation of the predicted gene models, which significantly improves the accuracy of gene structure information. The large number of gene duplicates in the previous genome is mainly distorted by the fragmented annotation. Through integrated genomic, evolutionary and transcriptomic analyses, we reveal that the Phi subfamily of ionotropic glutamate receptors (iGluRs) are well preserved in molluscs, and P. maximus experienced the rapid expansion of the Phi class of iGluR (GluF) gene family. The GluF genes exhibit ubiquitously high expression and altered sequence characteristics for ligand selectivity, which may contribute to the remarkable tolerance to neurotoxins in P. maximus. Taken together, our study disapproves the previous claim of the 'gene-rich' genome of this species and provides a high-quality genome assembly for further understanding of its biology and evolution of toxin resistance.

Whole-genome characterization of lung adenocarcinomas lacking the RTK/RAS/RAF pathway.

RTK/RAS/RAF pathway alterations (RPAs) are a hallmark of lung adenocarcinoma (LUAD). In this study, we use whole-genome sequencing (WGS) of 85 cases found to be RPA(-) by previous studies from The Cancer Genome Atlas (TCGA) to characterize the minority of LUADs lacking apparent alterations in this pathway. We show that WGS analysis uncovers RPA(+) in 28 (33%) of the 85 samples. Among the remaining 57 cases, we observe focal deletions targeting the promoter or transcription start site of STK11 (n = 7) or KEAP1 (n = 3), and promoter mutations associated with the increased expression of ILF2 (n = 6). We also identify complex structural variations associated with high-level copy number amplifications. Moreover, an enrichment of focal deletions is found in TP53 mutant cases. Our results indicate that RPA(-) cases demonstrate tumor suppressor deletions and genome instability, but lack unique or recurrent genetic lesions compensating for the lack of RPAs. Larger WGS studies of RPA(-) cases are required to understand this important LUAD subset.

A chromosome-level genome of black rockfish, Sebastes schlegelii, provides insights into the evolution of live birth.

The black rockfish (Sebastes schlegelii) is a teleost in which eggs are fertilized internally and retained in the maternal reproductive system, where they undergo development until live birth (viviparity). In the present study, we report a chromosome-level black rockfish genome assembly. High-throughput transcriptome analysis (RNA-seq and ATAC-seq) coupled with in situ hybridization (ISH) and immunofluorescence reveal several candidate genes for maternal preparation, sperm storage and release, and hatching. We propose that zona pellucida (ZP) proteins retain sperm at the oocyte envelope, while genes in two distinct astacin metalloproteinase subfamilies serve to release sperm from the ZP and free the embryo from chorion at prehatching stage. We present a model of black rockfish reproduction, and propose that the rockfish ovarian wall has a similar function to the uterus of mammals. Together, these genomic data reveal unprecedented insights into the evolution of an unusual teleost life history strategy, and provide a sound foundation for studying viviparity in nonmammalian vertebrates and an invaluable resource for rockfish ecological and evolutionary research.

The Y chromosome sequence of the channel catfish suggests novel sex determination mechanisms in teleost fish.

BACKGROUND: Sex determination mechanisms in teleost fish broadly differ from mammals and birds, with sex chromosomes that are far less differentiated and recombination often occurring along the length of the X and Y chromosomes, posing major challenges for the identification of specific sex determination genes. Here, we take an innovative approach of comparative genome analysis of the genomic sequences of the X chromosome and newly sequenced Y chromosome in the channel catfish. RESULTS: Using a YY channel catfish as the sequencing template, we generated, assembled, and annotated the Y genome sequence of channel catfish. The genome sequence assembly had a contig N50 size of 2.7 Mb and a scaffold N50 size of 26.7 Mb. Genetic linkage and GWAS analyses placed the sex determination locus within a genetic distance less than 0.5 cM and physical distance of 8.9 Mb. However, comparison of the channel catfish X and Y chromosome sequences showed no sex-specific genes. Instead, comparative RNA-Seq analysis between females and males revealed exclusive sex-specific expression of an isoform of the breast cancer anti-resistance 1 (BCAR1) gene in the male during early sex differentiation. Experimental knockout of BCAR1 gene converted genetic males (XY) to phenotypic females, suggesting BCAR1 as a putative sex determination gene. CONCLUSIONS: We present the first Y chromosome sequence among teleost fish, and one of the few whole Y chromosome sequences among vertebrate species. Comparative analyses suggest that sex-specific isoform expression through alternative splicing may underlie sex determination processes in the channel catfish, and we identify BCAR1 as a potential sex determination gene.

Genome sequence of walking catfish (Clarias batrachus) provides insights into terrestrial adaptation.

BACKGROUND: Walking catfish (Clarias batrachus) is a freshwater fish capable of air-breathing and locomotion on land. It usually inhabits various low-oxygen habitats, burrows inside the mudflat, and sometimes "walks" to search for suitable environments during summer. It has evolved accessory air-breathing organs for respiring air and corresponding mechanisms to survive in such challenging environments. Thereby, it serves as a great model for understanding adaptations to terrestrial life. RESULTS: Comparative genomics with channel catfish (Ictalurus punctatus) revealed specific adaptations of C. batrachus in DNA repair, enzyme activator activity, and small GTPase regulator activity. Comparative analysis with 11 non-air-breathing fish species suggested adaptive evolution in gene expression and nitrogenous waste metabolic processes. Further, myoglobin, olfactory receptor related to class A G protein-coupled receptor 1, and sulfotransferase 6b1 genes were found to be expanded in the air-breathing walking catfish genome, with 15, 15, and 12 copies, respectively, compared to non-air-breathing fishes that possess only 1-2 copies of these genes. Additionally, we sequenced and compared the transcriptomes of the gill and the air-breathing organ to characterize the mechanism of aerial respiration involved in elastic fiber formation, oxygen binding and transport, angiogenesis, ion homeostasis and acid-base balance. The hemoglobin genes were expressed dramatically higher in the air-breathing organ than in the gill of walking catfish. CONCLUSIONS: This study provides an important genomic resource for understanding the adaptive mechanisms of walking catfish to terrestrial environments. It is possible that the coupling of enhanced abilities for oxygen storage and oxygen transport through genomic expansion of myoglobin genes and transcriptomic up-regulation of hemoglobin and angiogenesis-related genes are important components of the molecular basis for adaptation of this aquatic species to terrestrial life.

Increased Alternative Splicing as a Host Response to Edwardsiella ictaluri Infection in Catfish.

Alternative splicing is the process of generating multiple transcripts from a single pre-mRNA used by eukaryotes to regulate gene expression and increase proteomic complexity. Although alternative splicing profiles have been well studied in mammalian species, they have not been well studied in aquatic species, especially after biotic stresses. In the present study, genomic information and RNA-Seq datasets were utilized to characterize alternative splicing profiles and their induced changes after bacterial infection with Edwardsiella ictaluri in channel catfish (Ictalurus punctatus). A total of 27,476 alternative splicing events, derived from 9694 genes, were identified in channel catfish. Exon skipping was the most abundant while mutually exclusive exon was the least abundant type of alternative splicing. Alternative splicing was greatly induced by E. ictaluri infection with 21.9% increase in alternative splicing events. Interestingly, genes involved in RNA binding and RNA splicing themselves were significantly enriched in differentially alternatively spliced genes after infection. Sequence analyses of splice variants of a representative alternatively spliced gene, splicing factor srsf2, revealed that certain spliced transcripts may undergo nonsense-mediated decay (NMD), suggesting functional significance of the induced alternative splicing. Although statistical analysis was not possible with such large datasets, results from quantitative real-time PCR from representative differential alternative splicing events provided general validation of the bacterial infection-induced alternative splicing. This is the first comprehensive study of alternative splicing and its changes in response to bacterial infection in fish species, providing insights into the molecular mechanisms of host responses to biotic stresses.

Genome-wide association analysis of intra-specific QTL associated with the resistance for enteric septicemia of catfish.

Disease resistance is one of the most important traits for aquaculture industry. For catfish industry, enteric septicemia of catfish (ESC), caused by the bacterial pathogen Edwardsiella ictaluri, is the most severe disease, causing enormous economic losses every year. In this study, we used three channel catfish families with 900 individuals (300 fish per family) and the 690K catfish SNP array, and conducted a genome-wide association study to detect the quantitative trait loci (QTL) associated with ESC resistance. Three significant QTL, with two of located on LG1 and one on LG26, and three suggestive QTL located on LG1, LG3, and LG21, respectively, were identified to be associated with ESC resistance. With a well-assembled- and -annotated reference genome sequence, genes around the involved QTL regions were identified. Among these genes, 37 genes had known functions in immunity, which may be involved in ESC resistance. Notably, nlrc3 and nlrp12 identified here were also found in QTL regions of ESC resistance in the channel catfish × blue catfish interspecific hybrid system, suggesting this QTL was operating within both intra-specific channel catfish populations and interspecific hybrid backcross populations. Many of the genes of the Class I MHC pathway, for mediated antigen processing and presentation, were found in the QTL regions. The positional correlation found in this study and the expressional correlation found in previous studies indicated that Class I MHC pathway was significantly associated with ESC resistance. This study validated one QTL previously identified using the second and fourth generation of the interspecific hybrid backcross progenies, and identified five additional QTL among channel catfish families. Taken together, it appears that there are only a few major QTL for ESC disease resistance, making marker-assisted selection an effective approach for genetic improvements of ESC resistance.

Sea cucumber genome provides insights into saponin biosynthesis and aestivation regulation.

Echinoderms exhibit several fascinating evolutionary innovations that are rarely seen in the animal kingdom, but how these animals attained such features is not well understood. Here we report the sequencing and analysis of the genome and extensive transcriptomes of the sea cucumber Apostichopus japonicus, a species from a special echinoderm group with extraordinary potential for saponin synthesis, aestivation and organ regeneration. The sea cucumber does not possess a reorganized Hox cluster as previously assumed for all echinoderms, and the spatial expression of Hox7 and Hox11/13b potentially guides the embryo-to-larva axial transformation. Contrary to the typical production of lanosterol in animal cholesterol synthesis, the oxidosqualene cyclase of sea cucumber produces parkeol for saponin synthesis and has "plant-like" motifs suggestive of convergent evolution. The transcriptional factors Klf2 and Egr1 are identified as key regulators of aestivation, probably exerting their effects through a clock gene-controlled process. Intestinal hypometabolism during aestivation is driven by the DNA hypermethylation of various metabolic gene pathways, whereas the transcriptional network of intestine regeneration involves diverse signaling pathways, including Wnt, Hippo and FGF. Decoding the sea cucumber genome provides a new avenue for an in-depth understanding of the extraordinary features of sea cucumbers and other echinoderms.

Chemokine C-C motif ligand 33 is a key regulator of teleost fish barbel development.

Barbels are important sensory organs in teleosts, reptiles, and amphibians. The majority of ∼4,000 catfish species, such as the channel catfish (Ictalurus punctatus), possess abundant whisker-like barbels. However, barbel-less catfish, such as the bottlenose catfish (Ageneiosus marmoratus), do exist. Barbeled catfish and barbel-less catfish are ideal natural models for determination of the genomic basis for barbel development. In this work, we generated and annotated the genome sequences of the bottlenose catfish, conducted comparative and subtractive analyses using genome and transcriptome datasets, and identified differentially expressed genes during barbel regeneration. Here, we report that chemokine C-C motif ligand 33 (ccl33), as a key regulator of barbel development and regeneration. It is present in barbeled fish but absent in barbel-less fish. The ccl33 genes are differentially expressed during barbel regeneration in a timing concordant with the timing of barbel regeneration. Knockout of ccl33 genes in the zebrafish (Danio rerio) resulted in various phenotypes, including complete loss of barbels, reduced barbel sizes, and curly barbels, suggesting that ccl33 is a key regulator of barbel development. Expression analysis indicated that paralogs of the ccl33 gene have both shared and specific expression patterns, most notably expressed highly in various parts of the head, such as the eye, brain, and mouth areas, supporting its role for barbel development.

The annotation of repetitive elements in the genome of channel catfish (Ictalurus punctatus).

Channel catfish (Ictalurus punctatus) is a highly adaptive species and has been used as a research model for comparative immunology, physiology, and toxicology among ectothermic vertebrates. It is also economically important for aquaculture. As such, its reference genome was generated and annotated with protein coding genes. However, the repetitive elements in the catfish genome are less well understood. In this study, over 417.8 Megabase (MB) of repetitive elements were identified and characterized in the channel catfish genome. Among them, the DNA/TcMar-Tc1 transposons are the most abundant type, making up ~20% of the total repetitive elements, followed by the microsatellites (14%). The prevalence of repetitive elements, especially the mobile elements, may have provided a driving force for the evolution of the catfish genome. A number of catfish-specific repetitive elements were identified including the previously reported Xba elements whose divergence rate was relatively low, slower than that in untranslated regions of genes but faster than the protein coding sequences, suggesting its evolutionary restrictions.

Comparative transcriptome analysis of the swimbladder reveals expression signatures in response to low oxygen stress in channel catfish, Ictalurus punctatus.

Channel catfish is the leading aquaculture species in the US, and one of the reasons for its application in aquaculture is its relatively high tolerance against hypoxia. However, hypoxia can still cause huge economic losses to the catfish industry. Studies on hypoxia tolerance, therefore, are important for aquaculture. Fish swimbladder has been considered as an accessory respiration organ surrounded by a dense capillary countercurrent exchange system. In this regard, we conducted RNA-Seq analysis with swimbladder samples of catfish under hypoxic and normal conditions to determine if swimbladder was responsive to low oxygen treatment and to reveal genes, their expression patterns, and pathways involved in hypoxia responses in catfish. A total of 155 differentially expressed genes (DEGs) were identified from swimbladder of adult catfish, whereas a total of 2,127 DEGs were identified from swimbladder of fingerling catfish under hypoxic condition as compared with untreated controls. Subsequent pathway analysis revealed that many DEGs under hypoxia were involved in HIF signaling pathway ( nos2, eno2, camk2d2, prkcb, cdkn1a, eno1, and tfrc), MAPK signaling pathway (voltage-dependent calcium channel subunit genes), PI3K/Akt/mTOR signaling pathway ( itga6, g6pc, and cdkn1a), Ras signaling pathway ( efna3 and ksr2), and signaling by VEGF ( fn1, wasf3, and hspb1) in catfish swimbladder. This study provided insights into regulation of gene expression and their involved gene pathways in catfish swimbladder in response to low oxygen stresses.