Comparative genome anatomy reveals evolutionary insights into a unique amphitriploid fish

The Observation of Meiotic Union Behavior of Gametophytes Provides a New Basis for Ploidy of Carassius auratus gibelio

As an important aquaculture fish, the genus Carassius exhibits different ploidy, including tetraploids and hexaploids [...].

Morpho-histological and Transcriptome Analysis Reveal the Unreduced Sperm Formation Mechanism in cdk1-Depletion Zebrafish

Cyclin-dependent kinases (Cdks) are major molecules related to cell cycle regulation. Polyploidy can be caused by the production of unreduced gametes, which is often related to the abnormal cell cycle of germ cells. Here, we successfully constructed a cdk1 mutation line (cdk1+/-) in zebrafish, a commonly used model organism. It showed that cdk1 depletion resulted in the generation of both polyploid and aneuploid embryos of WT♀ × cdk1+/-♂ zebrafish. In addition to normal sperms (1N), the depletion of cdk1 in zebrafish also led to the production of some large-head 2N sperms and higher ploidy sperms. Results of bivalent analysis of testis and ultrastructure analysis of spermatogonia suggested that the production of these large-head sperms was due to spermatogonia chromosome doubling in cdk1+/- zebrafish. Transcriptome analysis revealed aberrant expressions of some cell cycle and DNA replication-related genes in the early testis of cdk1+/- zebrafish relative to WT zebrafish. Through STRING correlation analysis, we further proved that cdk1 depletion affected the mitosis process and endoduplication initiation of spermatogonia by regulating expressions of some proteins related to cell cycle (i.e., Espl1 and Pp1) and DNA replication (i.e., Orc1 and Rnaseh2b), thereby leading to the formation of unreduced sperms. This study provides important information on revealing the molecular mechanisms of unreduced gamete formation caused by cdk1 mutation. Meanwhile, it also provides an important reference for the creation of fish polyploid germplasm.

Transition from sexuality to androgenesis through a meiotic modification during spermatogenesis in freshwater Corbicula clams

Asexual taxa are often considered as rare and vowed to long-term extinction, notably because of their reduced ability for rapid genetic changes and potential adaptation. The rate at which they derive from sexual ancestors and their developmental mode however influence genetic variation in asexual populations. Understanding the transition from sexuality to asexuality is therefore important to infer the evolutionary outcome of asexual taxa. The present work explored the transition from sexuality to androgenesis, a reproductive mode in which the males use female resources to clone themselves, in the freshwater Corbicula clams. Since androgenetic lineages are distinguishable from sexual clams by the production of unreduced sperm, this study investigated the cytological mechanisms underlying spermatogenesis in Corbicula by following the DNA content variation of male germ cells. The widespread androgenetic C. sp. form A/R lineage was compared to the sexual species C. japonica and C. sandai. While in C. japonica, the last stages of spermatogenesis are reduced through a canonical meiosis process, no reduced or duplicated stages were observed in C. sp. form A/R, suggesting a meiosis modification in this lineage. However, 45% of C. sandai spermatozoa were unreduced. The production of unreduced sperm may condition or provide the potential for the emergence of androgenesis in this sexual species. Being closely related to androgenetic lineages and found in sympatry with them in Lake Biwa (Japan), C. sandai might be an origin of androgenetic lineage emergence, or even an origin of the androgenetic reproductive mode in Corbicula.

An improved transcriptome annotation reveals asymmetric expression and distinct regulation patterns in allotetraploid common carp

In allotetraploid common carp, protein-coding homoeologs presented divergent expression levels between the two subgenomes. However, whether subgenome dominance occurs in other transcriptional and post-transcriptional events remains unknown. Using Illumina RNA sequencing and PacBio full-length sequencing, we refined the common carp transcriptome annotation and explored differences in four transcriptional and post-transcriptional events between the two subgenomes. The results revealed that the B subgenome presented more alternative splicing events, as did lncRNAs and circRNAs. However, the expression levels, tissue specificity, sequence features, and functions of lncRNAs and circRNAs did not significantly differ between the two subgenomes, suggesting a common regulatory mechanism shared by the two subgenomes. Furthermore, both the number and base substitution frequency of RNA editing events were greater in the B subgenome. Functional analyses of these transcriptional events also revealed subgenome bias. Genes that undergo alternative splicing in the A subgenome participate in more biological processes, and lncRNA targets show a preference between subgenomes. CircRNA host genes in the B subgenome were associated with more biological functions, and RNA editing preferentially occurred in noncoding regions or led to nonsynonymous mutations in the B subgenome. Taken together, the refined transcriptome annotation revealed complicated and imbalanced expression strategies in allotetraploid common carp.

Chromosome-level genome assembly of the mud carp (Cirrhinus molitorella) using PacBio HiFi and Hi-C sequencing

The mud carp (Cirrhinus molitorella) is an important economic farmed fish, mainly distributed in South China and Southeast Asia due to its strong adaptability and high yield. Despite its economic importance, the paucity of genomic information has constrained detailed genetic research and breeding efforts. In this study, we utilized PacBio HiFi long-read sequencing and Hi-C technologies to generate a meticulously assembled chromosome-level genome of the mud carp. This assembly spans 1,033.41 Mb, with an impressive 99.82% distributed across 25 chromosomes. The contig N50 and scaffold N50 are 33.29 Mb and 39.86 Mb, respectively. The completeness of the mud carp genome assembly is highlighted by a BUSCO score of 98.05%. We predict 25,865 protein-coding genes, with a BUSCO score of 96.54%, and functional annotations for 91.83% of these genes. Approximately 52.21% of the genome consists of repeat elements. This high-fidelity genome assembly is a vital resource for advancing molecular breeding, comparative genomics, and evolutionary studies of the mud carp and related species.

Decoding the fish genome opens a new era in important trait research and molecular breeding in China

Aquaculture represents the fastest-growing global food production sector, as it has become an essential component of the global food supply. China has the world's largest aquaculture industry in terms of production volume. However, the sustainable development of fish culture is hindered by several concerns, including germplasm degradation and disease outbreaks. The practice of genomic breeding, which relies heavily on genome information and genotypephenotype relationships, has significant potential for increasing the efficiency of aquaculture production. In 2014, the completion of the genome sequencing and annotation of the Chinese tongue sole signified the beginning of the fish genomics era in China. Since then, domestic researchers have made dramatic progress in functional genomic studies. To date, the genomes of more than 60 species of fish in China have been assembled and annotated. Based on these reference genomes, evolutionary, comparative, and functional genomic studies have revolutionized our understanding of a wide range of biologically and economically important traits of fishes, including growth and development, sex determination, disease resistance, metamorphosis, and pigmentation. Furthermore, genomic tools and breeding techniques such as SNP arrays, genomic selection, and genome editing have greatly accelerated genetic improvement through the incorporation of functional genomic information into breeding activities. This review aims to summarize the current status, advances, and perspectives of the genome resources, genomic study of important traits, and genomic breeding techniques of fish in China. The review will provide aquaculture researchers, fish breeders, and farmers with updated information concerning fish genomic research and breeding technology. The summary will help to promote the genetic improvement of production traits and thus will support the sustainable development of fish aquaculture.

Evolution of Key Oxygen-Sensing Genes Is Associated with Hypoxia Tolerance in Fishes

Low dissolved oxygen (hypoxia) is recognized as a major threat to aquatic ecosystems worldwide. Because oxygen is paramount for the energy metabolism of animals, understanding the functional and genetic drivers of whole-animal hypoxia tolerance is critical to predicting the impacts of aquatic hypoxia. In this study, we investigate the molecular evolution of key genes involved in the detection of and response to hypoxia in ray-finned fishes: the prolyl hydroxylase domain (PHD)-hypoxia-inducible factor (HIF) oxygen-sensing system, also known as the EGLN (egg-laying nine)-HIF oxygen-sensing system. We searched fish genomes for HIFA and EGLN genes, discovered new paralogs from both gene families, and analyzed protein-coding sites under positive selection. The physicochemical properties of these positively selected amino acid sites were summarized using linear discriminants for each gene. We employed phylogenetic generalized least squares to assess the relationship between these linear discriminants for each HIFA and EGLN and hypoxia tolerance as reflected by the critical oxygen tension (Pcrit) of the corresponding species. Our results demonstrate that Pcrit in ray-finned fishes correlates with the physicochemical variation of positively selected sites in specific HIFA and EGLN genes. For HIF2A, two linear discriminants captured more than 90% of the physicochemical variation of these sites and explained between 20% and 39% of the variation in Pcrit. Thus, variation in HIF2A among fishes may contribute to their capacity to cope with aquatic hypoxia, similar to its proposed role in conferring tolerance to high-altitude hypoxia in certain lineages of terrestrial vertebrates.

Genomic Anatomy of Homozygous XX Females and YY Males Reveals Early Evolutionary Trajectory of Sex-determining Gene and Sex Chromosomes in Silurus Fishes

Sex chromosomes display remarkable diversity and variability among vertebrates. Compared with research on the X/Y and Z/W chromosomes, which have long evolutionary histories in mammals and birds, studies on the sex chromosomes at early evolutionary stages are limited. Here, we precisely assembled the genomes of homozygous XX female and YY male Lanzhou catfish (Silurus lanzhouensis) derived from an artificial gynogenetic family and a self-fertilized family, respectively. Chromosome 24 (Chr24) was identified as the sex chromosome based on resequencing data. Comparative analysis of the X and Y chromosomes showed an approximate 320 kb Y-specific region with a Y-specific duplicate of anti-Mullerian hormone type II receptor (amhr2y), which is consistent with findings in 2 other Silurus species but on different chromosomes (Chr24 of Silurus meridionalis and Chr5 of Silurus asotus). Deficiency of amhr2y resulted in male-to-female sex reversal, indicating that amhr2y plays a male-determining role in S. lanzhouensis. Phylogenetic analysis and comparative genomics revealed that the common sex-determining gene amhr2y was initially translocated to Chr24 of the Silurus ancestor along with the expansion of transposable elements. Chr24 was maintained as the sex chromosome in S. meridionalis and S. lanzhouensis, whereas a sex-determining region transition triggered sex chromosome turnover from Chr24 to Chr5 in S. asotus. Additionally, gene duplication, translocation, and degeneration were observed in the Y-specific regions of Silurus species. These findings present a clear case for the early evolutionary trajectory of sex chromosomes, including sex-determining gene origin, repeat sequence expansion, gene gathering and degeneration in sex-determining region, and sex chromosome turnover.

Haplotype-resolved genomes of octoploid species in Phyllanthaceae family reveal a critical role for polyploidization and hybridization in speciation

The Phyllanthaceae family comprises a diverse range of plants with medicinal, edible, and ornamental value, extensively cultivated worldwide. Polyploid species commonly occur in Phyllanthaceae. Due to the rather complex genomes and evolutionary histories, their speciation process has been still lacking in research. In this study, we generated chromosome-scale haplotype-resolved genomes of two octoploid species (Phyllanthus emblica and Sauropus spatulifolius) in Phyllanthaceae family. Combined with our previously reported one tetraploid (Sauropus androgynus) and one diploid species (Phyllanthus cochinchinensis) from the same family, we explored their speciation history. The three polyploid species were all identified as allopolyploids with subgenome A/B. Each of their two distinct subgenome groups from various species was uncovered to independently share a common diploid ancestor (Ancestor-AA and Ancestor-BB). Via different evolutionary routes, comprising various scenarios of bifurcating divergence, allopolyploidization (hybrid polyploidization), and autopolyploidization, they finally evolved to the current tetraploid S. androgynus, and octoploid S. spatulifolius and P. emblica, respectively. We further discuss the variations in copy number of alleles and the potential impacts within the two octoploids. In addition, we also investigated the fluctuation of metabolites with medical values and identified the key factor in its biosynthesis process in octoploids species. Our study reconstructed the evolutionary history of these Phyllanthaceae species, highlighting the critical roles of polyploidization and hybridization in their speciation processes. The high-quality genomes of the two octoploid species provide valuable genomic resources for further research of evolution and functional genomics.

Reproduction-associated pathways in females of gibel carp (Carassius gibelio) shed light on the molecular mechanisms of the coexistence of asexual and sexual reproduction

Gibel carp (Carassius gibelio) is a cyprinid fish that originated in eastern Eurasia and is considered as invasive in European freshwater ecosystems. The populations of gibel carp in Europe are mostly composed of asexually reproducing triploid females (i.e., reproducing by gynogenesis) and sexually reproducing diploid females and males. Although some cases of coexisting sexual and asexual reproductive forms are known in vertebrates, the molecular mechanisms maintaining such coexistence are still in question. Both reproduction modes are supposed to exhibit evolutionary and ecological advantages and disadvantages. To better understand the coexistence of these two reproduction strategies, we performed transcriptome profile analysis of gonad tissues (ovaries) and studied the differentially expressed reproduction-associated genes in sexual and asexual females. We used high-throughput RNA sequencing to generate transcriptomic profiles of gonadal tissues of triploid asexual females and males, diploid sexual males and females of gibel carp, as well as diploid individuals from two closely-related species, C. auratus and Cyprinus carpio. Using SNP clustering, we showed the close similarity of C. gibelio and C. auratus with a basal position of C. carpio to both Carassius species. Using transcriptome profile analyses, we showed that many genes and pathways are involved in both gynogenetic and sexual reproduction in C. gibelio; however, we also found that 1500 genes, including 100 genes involved in cell cycle control, meiosis, oogenesis, embryogenesis, fertilization, steroid hormone signaling, and biosynthesis were differently expressed in the ovaries of asexual and sexual females. We suggest that the overall downregulation of reproduction-associated pathways in asexual females, and their maintenance in sexual ones, allows the populations of C. gibelio to combine the evolutionary and ecological advantages of the two reproductive strategies. However, we showed that many sexual-reproduction-related genes are maintained and expressed in asexual females, suggesting that gynogenetic gibel carp retains the genetic toolkits for meiosis and sexual reproduction. These findings shed new light on the evolution of this asexual and sexual complex.

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.

Deciphering male influence in gynogenetic Pengze crucian carp (Carassius auratus var. pengsenensis): insights from Nanopore sequencing of structural variations

In this study, we investigate gynogenetic reproduction in Pengze Crucian Carp (Carassius auratus var. pengsenensis) using third-generation Nanopore sequencing to uncover structural variations (SVs) in offspring. Our objective was to understand the role of male genetic material in gynogenesis by examining the genomes of both parents and their offspring. We discovered a notable number of male-specific structural variations (MSSVs): 1,195 to 1,709 MSSVs in homologous offspring, accounting for approximately 0.52%-0.60% of their detected SVs, and 236 to 350 MSSVs in heterologous offspring, making up about 0.10%-0.13%. These results highlight the significant influence of male genetic material on the genetic composition of offspring, particularly in homologous pairs, challenging the traditional view of asexual reproduction. The gene annotation of MSSVs revealed their presence in critical gene regions, indicating potential functional impacts. Specifically, we found 5 MSSVs in the exonic regions of protein-coding genes in homologous offspring, suggesting possible direct effects on protein structure and function. Validation of an MSSV in the exonic region of the polyunsaturated fatty acid 5-lipoxygenase gene confirmed male genetic material transmission in some offspring. This study underscores the importance of further research on the genetic diversity and gynogenesis mechanisms, providing valuable insights for reproductive biology, aquaculture, and fostering innovation in biological research and aquaculture practices.

Whole-genome Sequencing Reveals Autooctoploidy in Chinese Sturgeon and Its Evolutionary Trajectories

The order Acipenseriformes, which includes sturgeons and paddlefishes, represents "living fossils" with complex genomes that are good models for understanding whole-genome duplication (WGD) and ploidy evolution in fishes. Here, we sequenced and assembled the first high-quality chromosome-level genome for the complex octoploid Acipenser sinensis (Chinese sturgeon), a critically endangered species that also represents a poorly understood ploidy group in Acipenseriformes. Our results show that A. sinensis is a complex autooctoploid species containing four kinds of octovalents (8n), a hexavalent (6n), two tetravalents (4n), and a divalent (2n). An analysis taking into account delayed rediploidization reveals that the octoploid genome composition of Chinese sturgeon results from two rounds of homologous WGDs, and further provides insights into the timing of its ploidy evolution. This study provides the first octoploid genome resource of Acipenseriformes for understanding ploidy compositions and evolutionary trajectories of polyploid fishes.

GCN2-eIF2α signaling pathway negatively regulates the growth of triploid crucian carp

GCN2-eIF2α signaling pathway plays crucial roles in cell growth,development, and protein synthesis. However, in polyploid fish, the function of this pathway is rarely understood. In this study, genes associated with the GCN2-eIF2α pathway (pkr, pek, gcn2, eif2α) are founded lower expression levels in the triploid crucian carp (3nCC) muscle compared to that of the red crucian carp (RCC). In muscle effect stage embryos of the 3nCC, the mRNA levels of this pathway genes are generally lower than those of RCC, excluding hri and fgf21. Inhibiting gcn2 in 3nCC embryos downregulates downstream gene expression (eif2α, atf4, fgf21), accelerating embryonic development. In contrast, overexpressing of eif2α can alter the expression levels of downstream genes (atf4 and fgf21), and decelerates the embryonic development. These results demonstrate the GCN2-eIF2α pathway's regulatory impact on 3nCC growth, advancing understanding of fish rapid growth genetics and offering useful molecular markers for breeding of excellent strains.

Transcriptomic atlas for hypoxia and following re-oxygenation in Ancherythroculter nigrocauda heart and brain tissues: insights into gene expression, alternative splicing, and signaling pathways

Hypoxia is a mounting problem that affects the world's freshwaters, with severe consequence for many species, including death and large economical loss. The hypoxia problem has increased recently due to the combined effects of water eutrophication and global warming. In this study, we investigated the transcriptome atlas for the bony fish Ancherythroculter nigrocauda under hypoxia for 1.5, 3, and 4.5 h and its recovery to normal oxygen levels in heart and brain tissues. We sequenced 21 samples for brain and heart tissues (a total of 42 samples) plus three control samples and obtained an average of 32.40 million raw reads per sample, and 95.24% mapping rate of the filtered clean reads. This robust transcriptome dataset facilitated the discovery of 52,428 new transcripts and 6,609 novel genes. In the heart tissue, the KEGG enrichment analysis showed that genes linked to the Vascular smooth muscle contraction and MAPK and VEGF signaling pathways were notably altered under hypoxia. Re-oxygenation introduced changes in genes associated with abiotic stimulus response and stress regulation. In the heart tissue, weighted gene co-expression network analysis pinpointed a module enriched in insulin receptor pathways that was correlated with hypoxia. Conversely, in the brain tissue, the response to hypoxia was characterized by alterations in the PPAR signaling pathway, and re-oxygenation influenced the mTOR and FoxO signaling pathways. Alternative splicing analysis identified an average of 27,226 and 28,290 events in the heart and brain tissues, respectively, with differential events between control and hypoxia-stressed groups. This study offers a holistic view of transcriptomic adaptations in A. nigrocauda heart and brain tissues under oxygen stress and emphasizes the role of gene expression and alternative splicing in the response mechanisms.

Cytogenetic Analysis of the Fish Genus Carassius Indicates Divergence, Fission, and Segmental Duplication as Drivers of Tandem Repeat and Microchromosome Evolution

Fishes of the genus Carassius are useful experimental vertebrate models for the study of evolutionary biology and cytogenetics. Carassius demonstrates diverse biological characteristics, such as variation in ploidy levels and chromosome numbers, and presence of microchromosomes. Those Carassius polyploids with ≥150 chromosomes have microchromosomes, but the origin of microchromosomes, especially in European populations, is unknown. We used cytogenetics to study evolution of tandem repeats (U1 and U2 small nuclear DNAs and H3 histone) and microchromosomes in Carassius from the Czech Republic. We tested the hypotheses whether the number of tandem repeats was affected by polyploidization or divergence between species and what mechanism drives evolution of microchromosomes. Tandem repeats were found in tetraploid and hexaploid Carassius gibelio, and tetraploid Carassius auratus and Carassius carassius in conserved numbers, with the exception of U1 small nuclear DNA in C. auratus. This conservation indicates reduction and/or loss in the number of copies per locus in hexaploids and may have occurred by divergence rather than polyploidization. To study the evolution of microchromosomes, we used the whole microchromosome painting probe from hexaploid C. gibelio and hybridized it to tetraploid and hexaploid C. gibelio, and tetraploid C. auratus and C. carassius. Our results revealed variation in the number of microchromosomes in hexaploids and indicated that the evolution of the Carassius karyotype is governed by macrochromosome fissions followed by segmental duplication in pericentromeric areas. These are potential mechanisms responsible for the presence of microchromosomes in Carassius hexaploids. Differential efficacy of one or both of these mechanisms in different tetraploids could ensure variability in chromosome number in polyploids in general.

Evolutionary mechanisms and practical significance of reproductive success and clonal diversity in unisexual vertebrate polyploids

Unisexual reproduction is generally relevant to polyploidy, and unisexual vertebrates are often considered an evolutionary "dead end" due to the accumulation of deleterious mutations and absence of genetic diversity. However, some unisexual polyploids have developed strategies to avoid genomic decay, and thus provide ideal models to unveil unexplored evolutionary mechanisms, from the reproductive success to clonal diversity creation. This article reviews the evolutionary mechanisms for overcoming meiotic barrier and generating genetic diversity in unisexual vertebrates, and summarizes recent research advancements in the polyploid Carassius complex. Gynogenetic gibel carp (Carassius gibelio) is a unique amphitriploid that has undergone a recurrent autotriploidy and has overcome the bottleneck of triploid sterility via gynogenesis. Recently, an efficient strategy in which ploidy changes, including from amphitriploid to amphitetraploid, then from amphitetraploid to novel amphitriploid, drive unisexual-sexual-unisexual reproduction transition and clonal diversity has been revealed. Based on this new discovery, multigenomic reconstruction biotechnology has been used to breed a novel strain with superior growth and stronger disease resistance. Moreover, a unique reproduction mode that combines both abilities of ameiotic oogenesis and sperm-egg fusion, termed as ameio-fusiongensis, has been discovered, and it provides an efficient approach to synthesize sterile allopolyploids. In order to avoid ecological risks upon escape and protect the sustainable property rights of the aquaculture seed industry, a controllable fertility biotechnology approach for precise breeding is being developed by integrating sterile allopolyploid synthesis and gene-editing techniques. This review provides novel insights into the origin and evolution of unisexual vertebrates and into the attempts being made to exploit new breeding biotechnologies in aquaculture.

The mechanisms of microcystin-LR-induced genotoxicity and neurotoxicity in fish and mammals: Bibliometric analysis and meta-analysis

Microcystin-leucine arginine (MC-LR) is a typical cyanobacterial toxin, and the threat of this toxin is increasing among organisms. Despite extensive toxicological studies on MC-LR, there is no comprehensive analysis based on previously published data. Therefore, we conducted bibliometric analysis and meta-analysis to identify research hotspots and to elucidate the key mechanism of the relationship between MC-LR and genotoxicity and neurotoxicity among fish and mammals. One of the hotspots is toxic mechanisms (indicated by the frequent appearance of oxidative stress, DNA damage, apoptosis, neurotoxicity, genotoxicity, ROS, comet assay, signalling pathway, and gene expression indicate as keywords). The density visualization shows a high frequency of "microcystin-LR" and "toxicology," and the overlay visualization emphasizes the prominence of "neurotoxicity" in recent years. These findings confirm the importance of studying MC-LR toxicity. Meta-analysis indicated that in both fish and mammals, MC-LR exposure increased ROS levels by 294 % and increased DNA damage biomarkers by 174 % but decreased neurotoxicity biomarkers by 9 %. Intergroup comparisons revealed that the exposure concentration of MC-LR was significantly correlated with genotoxicity and neurotoxicity levels in both fish and mammals (p < 0.05). Furthermore, the random forest (RF) model revealed that exposure concentration was the primary determinant associated with the induction of ROS, genotoxicity, and neurotoxicity induced by MC-LR. This is likely the dominant mechanism by which excessive ROS production induced by MC-LR causes oxidative stress, ultimately leading to genotoxicity and neurotoxicity in both fish and mammals.

Maternal dominance contributes to subgenome differentiation in allopolyploid fishes

Teleost fishes, which are the largest and most diverse group of living vertebrates, have a rich history of ancient and recent polyploidy. Previous studies of allotetraploid common carp and goldfish (cyprinids) reported a dominant subgenome, which is more expressed and exhibits biased gene retention. However, the underlying mechanisms contributing to observed 'subgenome dominance' remains poorly understood. Here we report high-quality genomes of twenty-one cyprinids to investigate the origin and subsequent subgenome evolution patterns following three independent allopolyploidy events. We identify the closest extant relatives of the diploid progenitor species, investigate genetic and epigenetic differences among subgenomes, and conclude that observed subgenome dominance patterns are likely due to a combination of maternal dominance and transposable element densities in each polyploid. These findings provide an important foundation to understanding subgenome dominance patterns observed in teleost fishes, and ultimately the role of polyploidy in contributing to evolutionary innovations.

Subgenome phasing for complex allopolyploidy: case-based benchmarking and recommendations

Accurate subgenome phasing is crucial for understanding the origin, evolution and adaptive potential of polyploid genomes. SubPhaser and WGDI software are two common methodologies for subgenome phasing in allopolyploids, particularly in scenarios lacking known diploid progenitors. Triggered by a recent debate over the subgenomic origins of the cultivated octoploid strawberry, we examined four well-documented complex allopolyploidy cases as benchmarks, to evaluate and compare the accuracy of the two software. Our analysis demonstrates that the subgenomic structure phased by both software is in line with prior research, effectively tracing complex allopolyploid evolutionary trajectories despite the limitations of each software. Furthermore, using these validated methodologies, we revisited the controversial issue regarding the progenitors of the octoploid strawberry. The results of both methodologies reaffirm Fragaria vesca and Fragaria iinumae as progenitors of the octoploid strawberry. Finally, we propose recommendations for enhancing the accuracy of subgenome phasing in future studies, recognizing the potential of integrated tools for advanced complex allopolyploidy research and offering a new roadmap for robust subgenome-based phylogenetic analysis.

Advances on genomes studies of large DNA viruses in aquaculture: A minireview

Genomic studies of viral diseases in aquaculture have received more and more attention with the growth of the aquaculture industry, especially the emerging and re-emerging viruses whose genome could contain recombination, mutation, insertion, and so on, and may lead to more severe diseases and more widespread infections in aquaculture animals. The present review is focused on aquaculture viruses, which is belonged to two clades, Varidnaviria and Duplodnaviria, and one class Naldaviricetes, and respectively three families: Iridoviridae (ranaviruses), Alloherpesviridae (fish herpesviruses), and Nimaviridae (whispoviruses). The viruses possessed DNA genomes nearly or larger than 100 kbp with gene numbers more than 100 and were considered large DNA viruses. Genome analysis and experimental investigation have identified several genes involved in genome replication, transcription, and virus-host interactions. In addition, some genes involved in virus genetic variation or specificity were also discussed. A summary of these advances would provide reference to future discovery and research on emerging or re-emerging aquaculture viruses.

Mitotic defects lead to unreduced sperm formation in cdk1-/- mutants

Unreduced gametes, that are important for species evolution and agricultural development, are generally believed to be formed by meiotic defects. However, we found that male diploid loach (Misgurnus anguillicaudatus) could produce not only haploid sperms, but also unreduced sperms, after cyclin-dependent kinase 1 gene (cdk1, one of the most important kinases in regulating cell mitosis) deletion. Observations on synaptonemal complexes of spermatocyte in prophase of meiosis and spermatogonia suggested that the number of chromosomes in some spermatogonia of cdk1^-/- loach doubled, leading to unreduced diploid sperm production. Then, transcriptome analysis revealed aberrant expressions of some cell cycle-related genes (such as ppp1c and gadd45) in spermatogonia of cdk1^-/- loach relative to wild-type loach. An in vitro and in vivo experiment further validated that Cdk1 deletion in diploid loach resulted in mitotic defects, leading to unreduced diploid sperm formation. In addition, we found that cdk1^-/- zebrafish could also produce unreduced diploid sperms. This study provides important information on revealing the molecular mechanisms behind unreduced gamete formation through mitotic defects, and lays the foundation for a novel strategy for fish polyploidy creation by using cdk1 mutants to produce unreduced sperms, which can then be used to obtain polyploidy, proposed to benefit aquaculture.

An efficient approach to synthesize sterile allopolyploids through the combined reproduction mode of ameiotic oogenesis and sperm-egg fusion in the polyploid Carassius complex

The association between polyploidy and reproduction transition, which is an intriguing issue in evolutionary genetics, can also be exploited as an approach for genetic improvement in agriculture. Recently, we generated novel amphitriploids (NA3n) by integrating the genomes of the gynogenetic Carassius gibelio and sexual C. auratus, and found gynogenesis was recovered in most NA3n females (NA3n♀I). Here, we discovered a unique reproduction mode, termed ameio-fusiongenesis, which combines the abilities of both ameiotic oogenesis and sperm-egg fusion, in a few NA3n females (NA3n♀II). These females inherited ameiotic oogenesis to produce unreduced eggs from gynogenetic C. gibelio and sperm-egg fusion from sexual C. auratus. Subsequently, we utilized this unique reproduction mode to generate a group of synthetic alloheptaploids by crossing NA3n♀II with Megalobrama amblycephala. They contained all chromosomes of maternal NA3n♀II and a chromosomal set of paternal M. amblycephala. Intergenomic chromosome translocations between NA3n♀II and M. amblycephala were also observed in a few somatic cells. Primary oocytes of the alloheptaploid underwent severe apoptosis owing to incomplete double-strand break repair at prophase I. Although spermatocytes displayed similar chromosome behavior at prophase I, they underwent apoptosis due to chromosome separation failure at metaphase I. Therefore, the alloheptaploid females and males were all sterile. Finally, we established a sustainable clone for the large-scale production of NA3n♀II and developed an efficient approach to synthesize diverse allopolyploids containing genomes of different cyprinid species. These findings not only broaden our understanding of reproduction transition but also offer a practical strategy for polyploidy breeding and heterosis fixing.

Genomic and Transcriptional Profiles of Kelch-like (klhl) Gene Family in Polyploid Carassius Complex

Genome duplication supplies raw genetic materials and has been thought to be essential for evolutionary innovation and ecological adaptation. Here, we select Kelch-like (klhl) genes to study the evolution of the duplicated genes in the polyploid Carassius complex, including amphidiploid C. auratus and amphitriploid C. gibelio. Phylogenetic, chromosomal location and read coverage analyses indicate that most of Carassius klhl genes exhibit a 2:1 relationship with zebrafish orthologs and confirm two rounds of polyploidy, an allotetraploidy followed by an autotriploidy, occurred during Carassius evolution. The lineage-specific expansion and biased retention/loss of klhl genes are also found in Carassius. Transcriptome analyses across eight adult tissues and seven embryogenesis stages reveal varied expression dominance and divergence between the two species. The expression of klhls in response to Carassius herpesvirus 2 infection shows different expression changes corresponding to distinct herpesvirus resistances in three C. gibelio gynogenetic clones. Finally, we find that most C. gibelio klhl genes possess three alleles except eight genes that have lost one or two alleles due to genome rearrangement. The allele expression bias is prosperous for Cgklhl genes and varies during embryogenesis owning to the sequential expression manner of the alleles. The current study provides global insights into the genomic and transcriptional evolution of duplicated genes in a given superfamily resulting from multiple rounds of polyploidization.

Yellow catfish RIO kinases (RIOKs) negatively regulate fish interferon-mediated antiviral response

In mammals, right open reading frame kinases (RIOKs) are initially reported to participate in cancer cell proliferation, apoptosis, migration and invasion, and recently they have been related to host immune response. Little is known about the homologs of RIOKs in fish. In the current study, we cloned three homologous genes of RIOK family in yellow catfish (Pelteobagrus fulvidraco), termed Pfriok1, Pfriok2 and Pfriok3. Pfriok1, Pfriok2 and Pfriok3 were constitutively expressed at relatively high levels in yellow catfish tissues, and their mRNA levels were not changed under viral infection. Individual overexpression of PfRIOK1, PfRIOK2 and PfRIOK3 attenuated fish interferon (IFN) response, thereby promoting viral replication in fish cells. Mechanistically, yellow catfish RIOK proteins downregulated fish IFN response through attenuating TBK1 protein levels in cytoplasm. Our findings suggest that yellow catfish RIOK1, RIOK2 and RIOK3 are involved in downregulating fish IFN antiviral response.

Barbel regeneration and function divergence in red-tail catfish (Hemibagrus wyckioides) based on the chromosome-level genomes and comparative transcriptomes

Catfish (Siluriformes) are one of the most diverse vertebrate orders and are characterized by whisker-like barbels, which are important sensory organs in most of teleosts. However, their specific biological functions are still unclear. Red-tail catfish (Hemibagrus wyckioides) is well-known catfish species with four pairs of barbels, of which the maxillary barbels reach two-thirds of the body length. In this study, a 776.58 Mb high-quality chromosome-level genome was assembled into 29 chromosomes. Comparative genome data indicated that the barbeled regeneration gene ccl33 has expanded into 11 tandemly duplicated copies. Transcriptome data revealed the functional differentiation of different barbels and suggested that the maxillary barbel might be necessary for water temperature perception. Taste receptor genes were also characterized in teleosts with different food habits. Selection pressures were revealed to affect the sugar-based solute transport domain of the sweet taste receptor gene t1r2 in carnivorous fishes. In addition, the bitter taste receptor gene t2r200 was found to be lost from the genomes of four catfish species. Therefore, our study provides a genomic foundation for understanding the regeneration and functional differentiation of barbels in red-tail catfish and also reveals novel insights into the feeding evolution of fish species with different feeding habits.

Zebrafish HERC7c Acts as an Inhibitor of Fish IFN Response

In humans, four small HERCs (HERC3-6) exhibit differential degrees of antiviral activity toward HIV-1. Recently we revealed a novel member HERC7 of small HERCs exclusively in non-mammalian vertebrates and varied copies of herc7 genes in distinct fish species, raising a question of what is the exact role for a certain fish herc7 gene. Here, a total of four herc7 genes (named HERC7a-d sequentially) are identified in the zebrafish genome. They are transcriptionally induced by a viral infection, and detailed promoter analyses indicate that zebrafish herc7c is a typical interferon (IFN)-stimulated gene. Overexpression of zebrafish HERC7c promotes SVCV (spring viremia of carp virus) replication in fish cells and concomitantly downregulates cellular IFN response. Mechanistically, zebrafish HERC7c targets STING, MAVS, and IRF7 for protein degradation, thus impairing cellular IFN response. Whereas the recently-identified crucian carp HERC7 has an E3 ligase activity for the conjugation of both ubiquitin and ISG15, zebrafish HERC7c only displays the potential to transfer ubiquitin. Considering the necessity for timely regulation of IFN expression during viral infection, these results together suggest that zebrafish HERC7c is a negative regulator of fish IFN antiviral response.

Adaptive Savitzky–Golay Filters for Analysis of Copy Number Variation Peaks from Whole-Exome Sequencing Data

Copy number variation (CNV) is a form of structural variation in the human genome that provides medical insight into complex human diseases; while whole-genome sequencing is becoming more affordable, whole-exome sequencing (WES) remains an important tool in clinical diagnostics. Because of its discontinuous nature and unique characteristics of sparse target-enrichment-based WES data, the analysis and detection of CNV peaks remain difficult tasks. The Savitzky–Golay (SG) smoothing is well known as a fast and efficient smoothing method. However, no study has documented the use of this technique for CNV peak detection. It is well known that the effectiveness of the classical SG filter depends on the proper selection of the window length and polynomial degree, which should correspond with the scale of the peak because, in the case of peaks with a high rate of change, the effectiveness of the filter could be restricted. Based on the Savitzky–Golay algorithm, this paper introduces a novel adaptive method to smooth irregular peak distributions. The proposed method ensures high-precision noise reduction by dynamically modifying the results of the prior smoothing to automatically adjust parameters. Our method offers an additional feature extraction technique based on density and Euclidean distance. In comparison to classical Savitzky–Golay filtering and other peer filtering methods, the performance evaluation demonstrates that adaptive Savitzky–Golay filtering performs better. According to experimental results, our method effectively detects CNV peaks across all genomic segments for both short and long tags, with minimal peak height fidelity values (i.e., low estimation bias). As a result, we clearly demonstrate how well the adaptive Savitzky–Golay filtering method works and how its use in the detection of CNV peaks can complement the existing techniques used in CNV peak analysis.

The genome of a hadal sea cucumber reveals novel adaptive strategies to deep-sea environments

How organisms cope with coldness and high pressure in the hadal zone remains poorly understood. Here, we sequenced and assembled the genome of hadal sea cucumber Paelopatides sp. Yap with high quality and explored its potential mechanisms for deep-sea adaptation. First, the expansion of ACOX1 for rate-limiting enzyme in the DHA synthesis pathway, increased DHA content in the phospholipid bilayer, and positive selection of EPT1 may maintain cell membrane fluidity. Second, three genes for translation initiation factors and two for ribosomal proteins underwent expansion, and three ribosomal protein genes were positively selected, which may ameliorate the protein synthesis inhibition or ribosome dissociation in the hadal zone. Third, expansion and positive selection of genes associated with stalled replication fork recovery and DNA repair suggest improvements in DNA protection. This is the first genome sequence of a hadal invertebrate. Our results provide insights into the genetic adaptations used by invertebrate in deep oceans.

Origin and chromatin remodeling of young X/Y sex chromosomes in catfish with sexual plasticity

Assembly of a complete Y chromosome is a significant challenge in animals with an XX/XY sex-determination system. Recently, we created YY-supermale yellow catfish by crossing XY males with sex-reversed XY females, providing a valuable model for Y-chromosome assembly and evolution. Here, we assembled highly homomorphic Y and X chromosomes by sequencing genomes of the YY supermale and XX female in yellow catfish, revealing their nucleotide divergences with only less than 1% and with the same gene compositions. The sex-determining region (SDR) was identified to locate within a physical distance of 0.3 Mb by F_ST scanning. Strikingly, the incipient sex chromosomes were revealed to originate via autosome-autosome fusion and were characterized by a highly rearranged region with an SDR downstream of the fusion site. We found that the Y chromosome was at a very early stage of differentiation, as no clear evidence of evolutionary strata and classical structure features of recombination suppression for a rather late stage of Y-chromosome evolution were observed. Significantly, a number of sex-antagonistic mutations and the accumulation of repetitive elements were discovered in the SDR, which might be the main driver of the initial establishment of recombination suppression between young X and Y chromosomes. Moreover, distinct three-dimensional chromatin organizations of the Y and X chromosomes were identified in the YY supermales and XX females, as the X chromosome exhibited denser chromatin structure than the Y chromosome, while they respectively have significantly spatial interactions with female- and male-related genes compared with other autosomes. The chromatin configuration of the sex chromosomes as well as the nucleus spatial organization of the XX neomale were remodeled after sex reversal and similar to those in YY supermales, and a male-specific loop containing the SDR was found in the open chromatin region. Our results elucidate the origin of young sex chromosomes and the chromatin remodeling configuration in the catfish sexual plasticity.

Chromosome-Level Assembly of Male Opsariichthys bidens Genome Provides Insights into the Regulation of the GnRH Signaling Pathway and Genome Evolution

The hook snout carp Opsariichthys bidens is an important farmed fish in East Asia that shows sexual dimorphism in growth, with males growing faster and larger than females. To understand these complex traits and improve molecular breeding, chromosome-level genome assembly of male O. bidens was performed using Illumina, Nanopore, and Hi-C sequencing. The 992.9 Mb genome sequences with a contig N50 of 5.2 Mb were anchored to 38 chromosomes corresponding to male karyotypes. Of 30,922 functionally annotated genes, 97.5% of BUSCO genes were completely detected. Genome evolution analysis showed that the expanded and contracted gene families in the male O. bidens genome were enriched in 76 KEGG pathways, and 78 expanded genes were involved in the GnRH signaling pathway that regulates the synthesis and secretion of luteinizing hormone and glycoprotein hormones, further acting on male growth by inducing growth hormone. Compared to the released female O. bidens genome, the number of annotated genes in males was much higher (23,992). The male chromosome LG06 exhibited over 97% identity with the female GH14/GH38. Male-specific genes were identified for LG06, where structural variation, including deletions and insertions, occurred at a lower rate, suggesting a centric fusion of acrocentric chromosomes GH14 and GH38. The genome-synteny analysis uncovered significant inter-chromosome conservation between male O. bidens and grass carp, the former originating from ancestral chromosome breakage to increase the chromosome number. Our results provide a valuable genetic resource for studying the regulation of sexual dimorphism, sex-determining mechanisms, and molecular-guided breeding of O. bidens.

Changes in Ploidy Drive Reproduction Transition and Genomic Diversity in a Polyploid Fish Complex

Unisexual animals are commonly found in some polyploid species complexes, and most of these species have had a long evolutionary history. However, their method for avoiding genomic decay remains unclear. The polyploid Carassius complex naturally comprises the sexual amphidiploid C. auratus (crucian carp or goldfish) (AABB) and the gynogenetic amphitriploid C. gibelio (gibel carp) (AAABBB). Recently, we developed a fertile synthetic amphitetraploid (AAAABBBB) male from C. gibelio by incorporating a C. auratus genome. In this study, we generated novel amphitriploids (AAABBB) by backcrossing the amphitetraploid male with the amphidiploid C. auratus. Whole-genome resequencing revealed the genomic changes, including recombination and independent assortment between homologs of C. gibelio and C. auratus. The fertility, sex determination system, oocyte development, and fertilization behaviors of the novel amphitriploids were investigated. Approximately 80% of the novel amphitriploid females recovered the unisexual gynogenesis ability. Intriguingly, two types of primary oocyte (with and without homolog synapsis) were discovered, and their distinct development fates were observed. Type I oocytes entered apoptosis due to improper synaptonemal complex assembly and incomplete double-strand break repair, whereas subsequent type II oocytes bypassed meiosis through an alternative ameiotic pathway to develop into mature eggs. Moreover, gynogenesis was stabilized in their offspring, and a new array of diverse gynogenetic amphitriploid clones was produced. These revealed genomic changes and detailed cytological data provide comprehensive evidence that changes in ploidy drive unisexual and sexual reproduction transition, thereby resulting in genomic diversity and allowing C. gibelio avoid genomic decay.

A Fish of Multiple Faces, Which Show Us Enigmatic and Incredible Phenomena in Nature: Biology and Cytogenetics of the Genus Carassius

Sexual vs. asexual reproduction-unisexual vs. bisexual populations-diploid vs. polyploid biotypes-genetic vs. environmental sex determination: all these natural phenomena are associated with the genus of teleost fish, Carassius. This review places emphasis on two Carassius entities with completely different biological characteristics: one globally widespread and invasive Carassius gibelio, and the other C. carassius with a decreasing trend of natural occurrence. Comprehensive biological and cytogenetic knowledge of both entities, including the physical interactions between them, can help to balance the advantages of highly invasive and disadvantages of threatened species. For example, the benefits of a wide-ranged colonization can lead to the extinction of native species or be compensated by parasitic enemies and lead to equilibrium. This review emphasizes the comprehensive biology and cytogenetic knowledge and the importance of the Carassius genus as one of the most useful experimental vertebrate models for evolutionary biology and genetics. Secondly, the review points out that effective molecular cytogenetics should be used for the identification of various species, ploidy levels, and hybrids. The proposed investigation of these hallmark characteristics in Carassius may be applied in conservation efforts to sustain threatened populations in their native ranges. Furthermore, the review focuses on the consequences of the co-occurrence of native and non-native species and outlines future perspectives of Carassius research.