A chromosome-level genome assembly of the darkbarbel catfish Pelteobagrus vachelli.

The darkbarbel catfish (Pelteobagrus vachelli), an economically important aquaculture species in China, is extensively employed in hybrid yellow catfish production due to its superior growth rate. However, information on its genome has been limited, constraining further genetic studies and breeding programs. Leveraging the power of PacBio long-read sequencing and Hi-C technologies, we present a high-quality, chromosome-level genome assembly for the darkbarbel catfish. The resulting assembly spans 692.10 Mb, with an impressive 99.9% distribution over 26 chromosomes. The contig N50 and scaffold N50 are 13.30 Mb and 27.55 Mb, respectively. The genome is predicted to contain 22,109 protein-coding genes, with 96.1% having functional annotations. Repeat elements account for approximately 35.79% of the genomic landscape. The completeness of darkbarbel catfish genome assembly is highlighted by a BUSCO score of 99.07%. This high-quality genome assembly provides a critical resource for future hybrid catfish breeding, comparative genomics, and evolutionary studies in catfish and other related species.

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 FST 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.

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.

Sexually Dimorphic Gene Expression in X and Y Sperms Instructs Sexual Dimorphism of Embryonic Genome Activation in Yellow Catfish (Pelteobagrus fulvidraco).

Paternal factors play an important role in embryonic morphogenesis and contribute to sexual dimorphism in development. To assess the effect of paternal DNA on sexual dimorphism of embryonic genome activation, we compared X and Y sperm and different sexes of embryos before sex determination. Through transcriptome sequencing (RNA-seq) and whole-genome bisulfite sequencing (WGBS) of X and Y sperm, we found a big proportion of upregulated genes in Y sperm, supported by the observation that genome-wide DNA methylation level is slightly lower than in X sperm. Cytokine-cytokine receptor interaction, TGF-beta, and toll-like receptor pathways play important roles in spermatogenesis. Through whole-genome re-sequencing (WGRS) of parental fish and RNA-seq of five early embryonic stages, we found the low-blastocyst time point is a key to maternal transcriptome degradation and zygotic genome activation. Generally, sexual differences emerged from the bud stage. Moreover, through integrated analysis of paternal SNPs and gene expression, we evaluated the influence of paternal inheritance on sexual dimorphism of genome activation. Besides, we screened out gata6 and ddx5 as potential instructors for early sex determination and gonad development in yellow catfish. This work is meaningful for revealing the molecular mechanisms of sex determination and sexual dimorphism of fish species.

Genome-wide association study reveals the genetic basis of growth trait in yellow catfish with sexual size dimorphism.

Sexual size dimorphism has been widely observed in a large number of animals including fish species. Genome-wide association study (GWAS) is a powerful tool to dissect the genetic basis of complex traits, whereas the sex-differences in the genomics of animal complex traits have been ignored in the GWAS analysis. Yellow catfish (Pelteobagrus fulvidraco) is an important aquaculture fish in China with significant sexual size dimorphism. In this study, GWAS was conducted to identify candidate SNPs and genes related to body length (BL) and body weight (BW) in 125 female yellow catfish from a breeding population. In total, one BL-related SNP and three BW-related SNPs were identified to be significantly associated with the traits. Besides, one of these SNPs (Chr15:19195072) was shared in both the BW and BL traits in female yellow catfish, which was further validated in 185 male individuals and located on the exon of stat5b gene. Transgenic yellow catfish and zebrafish that expressed yellow catfish stat5b showed increased growth rate and reduction of sexual size dimorphism. These results not only reveal the genetic basis of growth trait and sexual size dimorphism in fish species, but also provide useful information for the marker-assisted breeding in yellow catfish.

Identifying difference in primordial germ cells between XX female and XY male yellow catfish embryos.

Primordial germ cells (PGCs) are singled out from somatic cells very early during embryogenesis, then they migrate towards the genital ridge and differentiate into gametes through oogenesis or spermatogenesis. Labeling PGCs with Localized RNAexpression (LRE) technique by fluorescent proteins has been widely applied among teleost species to study the germ cell development and gonad differentiation. In this study, we first cloned and characterized the 3' untranslated regions (3'UTRs) of nanos homolog 1-like (nos1l), dead end (dnd), and vasa in yellow catfish (Pelteobagrus fulvidraco), and then synthesized the GFP-nos1l/dnd/vasa 3'UTR mRNAs. Each of these three 3'UTRs could label PGCs in yellow catfish embryos, of which, vasa 3'UTR exhibited the highest labeling efficiency. To identify the differences in PGCs at embryonic stage, XX all-female and XY all-male yellow catfish embryos were produced and injected with GFP-vasa 3'UTR mRNA. We observed the PGC migration route in these two monosex embryos from 24 hpf to 7 dpf, and found there was no difference between them. Besides, the PGC number was counted at 48 hpf, and the result showed that the average PGC number in XX females (11.3) was significantly larger than that in XY males (8.1).These findings provide an insight into the development of PGCs in yellow catfish embryos and the relationship between embryonicPGCnumberand thelatergonaddifferentiation.

Function and association analysis of Cyclophilin A gene with resistance to Edwardsiella ictaluri in yellow catfish.

Edwardsiella ictaluri (E. ictaluri) is one of the main bacterial pathogens in catfish which has caused serious economic loss to yellow catfish (Pelteobagrus fulvidraco) in China. In our previous work, we demonstrated that CypA was up-regulated at the early stage of E. ictaluri infection in yellow catfish and displayed strong chemotactic activity for leukocytes in vitro. However, the effect of CypA on E. ictaluri is unknown in vivo. Therefore, two homozygous transgenic zebrafish lines expressing yellow catfish CypA (TG-CypA-1 and TG-CypA-2) were generated. After challenged with E. ictaluri at a dose of 1.0 × 104 CFU per adult fish, both two transgenic lines exhibited a higher resistance to bacterial infection than the wildtype zebrafish. Herein, CypA gene in E. ictaluri-challenged yellow catfish was screened for presence of polymorphisms by sequencing and six single nucleotide polymorphisms (SNPs) were identified. SNP association analysis revealed that 528T/C SNP in the first intron was significantly different in disease-susceptible and -resistant groups, which was confirmed in two independent populations of yellow catfish. Moreover, the relative expression of CypA in the resistant group (CC genotype in 528T/C SNP) was significantly higher than that in the susceptible group (TT genotype in 528T/C SNP) in different immune organs of yellow catfish including spleen, head kidney, body kidney and liver. Our results reveal the potential function of CypA in host defense to bacterial infection and suggest the SNP marker in CypA gene associated with the resistance to E. ictaluri may facilitate the selective breeding of disease-resistant yellow catfish in the future.

Igf2bp3 maintains maternal RNA stability and ensures early embryo development in zebrafish.

Early embryogenesis relies on maternally inherited mRNAs. Although the mechanism of maternal mRNA degradation during maternal-to-zygotic transition (MZT) has been extensively studied in vertebrates, how the embryos maintain maternal mRNA stability remains unclear. Here, we identify Igf2bp3 as an important regulator of maternal mRNA stability in zebrafish. Depletion of maternal igf2bp3 destabilizes maternal mRNAs prior to MZT and leads to severe developmental defects, including abnormal cytoskeleton organization and cell division. However, the process of oogenesis and the expression levels of maternal mRNAs in unfertilized eggs are normal in maternal igf2bp3 mutants. Gene ontology analysis revealed that these functions are largely mediated by Igf2bp3-bound mRNAs. Indeed, Igf2bp3 depletion destabilizes while its overexpression enhances its targeting maternal mRNAs. Interestingly, igf2bp3 overexpression in wild-type embryos also causes a developmental delay. Altogether, these findings highlight an important function of Igf2bp3 in controlling early zebrafish embryogenesis by binding and regulating the stability of maternal mRNAs.

Chromosomal-level assembly of yellow catfish genome using third-generation DNA sequencing and Hi-C analysis.

Background: The yellow catfish, Pelteobagrus fulvidraco, belonging to the Siluriformes order, is an economically important freshwater aquaculture fish species in Asia, especially in Southern China. The aquaculture industry has recently been facing tremendous challenges in germplasm degeneration and poor disease resistance. As the yellow catfish exhibits notable sex dimorphism in growth, with adult males about two- to three-fold bigger than females, the way in which the aquaculture industry takes advantage of such sex dimorphism is another challenge. To address these issues, a high-quality reference genome of the yellow catfish would be a very useful resource. Findings: To construct a high-quality reference genome for the yellow catfish, we generated 51.2 Gb short reads and 38.9 Gb long reads using Illumina and Pacific Biosciences (PacBio) sequencing platforms, respectively. The sequencing data were assembled into a 732.8 Mb genome assembly with a contig N50 length of 1.1 Mb. Additionally, we applied Hi-C technology to identify contacts among contigs, which were then used to assemble contigs into scaffolds, resulting in a genome assembly with 26 chromosomes and a scaffold N50 length of 25.8 Mb. Using 24,552 protein-coding genes annotated in the yellow catfish genome, the phylogenetic relationships of the yellow catfish with other teleosts showed that yellow catfish separated from the common ancestor of channel catfish ∼81.9 million years ago. We identified 1,717 gene families to be expanded in the yellow catfish, and those gene families are mainly enriched in the immune system, signal transduction, glycosphingolipid biosynthesis, and fatty acid biosynthesis. Conclusions: Taking advantage of Illumina, PacBio, and Hi-C technologies, we constructed the first high-quality chromosome-level genome assembly for the yellow catfish P. fulvidraco. The genomic resources generated in this work not only offer a valuable reference genome for functional genomics studies of yellow catfish to decipher the economic traits and sex determination but also provide important chromosome information for genome comparisons in the wider evolutionary research community.

A novel PDZ domain-containing gene is essential for male sex differentiation and maintenance in yellow catfish (Pelteobagrus fulvidraco).

The sex-determining genes are found to be variable among different fish species. Yellow catfish (Pelteobagrus fulvidraco) is an important aquaculture fish species in China with XX/XY sex-determining type. Recently, YY super-male yellow catfish has been successfully produced by combining hormonal-induced sex reversal method with sex chromosome-linked markers. Here, we identified a novel PDZ domain-containing gene in yellow catfish designated as pfpdz1, in whose intron the sex-linked marker was located. The coding sequence of pfpdz1 in Y chromosome was identical to that in X chromosome except a missense SNP (A/T) that changes an amino acid (E8V) in the N-terminal region. Pfpdz1 displayed male-specific expression during sex differentiation. Overexpression of pfpdz1 using additive transgenesis induces XX ovary to differentiate into testis-like tissue, while the targeted inactivation of pfpdz1 in Y chromosome using CRISPR/Cas9-mediated mutagenesis triggers ovarian differentiation. Furthermore, we demonstrated that pfpdz1 initiates testicular differentiation through upregulating expression of amh, dmrt1 and sox9a1, as well as downregulating expression of cyp19a1, foxl2 and wnt4. Our data provide functional evidence that pfpdz1 is significant for male differentiation and maintenance in yellow catfish.