Identification of sex chromosome and sex-determining gene of southern catfish (Silurus meridionalis) based on XX, XY and YY genome sequencing

The Cause-Effect Model of Master Sex Determination Gene Acquisition and the Evolution of Sex Chromosomes

The canonical model of vertebrate sex chromosome evolution predicts a one-way trend toward degradation. However, most sex chromosomes in lower vertebrates are homomorphic. Recent progress in studies of sex determination has resulted in the discovery of more than 30 master sex determination (MSD) genes, most of which are from teleost fish. An analysis of MSD gene acquisition, recombination suppression, and sex chromosome-specific sequences revealed correlations in the modes of MSD gene acquisition and the evolution of sex chromosomes. Sex chromosomes remain homomorphic with MSD genes acquired by simple mutations, gene duplications, allelic variations, or neofunctionalization; in contrast, they become heteromorphic with MSD genes acquired by chromosomal inversion, fusion, and fission. There is no recombination suppression with sex chromosomes carrying MSD genes gained through simple mutations. In contrast, there is extensive recombination suppression with sex chromosomes carrying MSD genes gained through chromosome inversion. There is limited recombination suppression with sex chromosomes carrying MSD genes gained through transposition or translocation. We propose a cause-effect model that predicts sex chromosome evolution as a consequence of the acquisition modes of MSD genes, which explains the evolution of sex chromosomes in various vertebrates. A key factor determining the trend of sex chromosome evolution is whether non-homologous regions are created during the acquisition of MSD genes. Chromosome inversion creates inversely homologous but directly non-homologous sequences, which lead to recombination suppression but retain recombination potential. Over time, recurrent recombination in the inverted regions leads to the formation of strata and may cause the degradation of sex chromosomes. Depending on the nature of deletions in the inverted regions, sex chromosomes may evolve with dosage compensation, or the selective retention of haplo-insufficient genes may be used as an alternative strategy.

The origin, evolution, and translocation of sex chromosomes in Silurus catfish mediated by transposons

BACKGROUND

Sex chromosome (SC) evolution is a longstanding topic of focus in evolutionary biology. Teleosts often exhibit rapid turnover of SCs and sex-determining (SD) genes, alongside a diverse range of SC differentiation mechanisms.

RESULTS

On the basis of new chromosome-scale assemblies of three Silurus species (S. microdorsalis, S. glanis, and S. lanzhouensis) and two outgroup species (Pterocryptis cochinchinensis and Kryptopterus bicirrhis), along with our previous assemblies of S. meridionalis and S. asotus, we traced the evolution of SC in the Silurus genus (Siluriformes), following the fate of the known SD gene amhr2y. Phylogenetic analysis showed that amhr2y occurred at least before the divergence of Pterocryptis, Kryptopterus, and Silurus and lost in P. cochinchinensis and K. bicirrhis. Chr24 has become the SC in the ancestor of five Silurus species due to the duplication-and-translocation of amhr2 mediated by LTR transposon. Then, a proto Y was formed and maintained with a shared 60 kb male-specific region of the Y chromosome (MSY) by transposable elements (TEs) expansion and gene gathering. Due to the continuous TEs accumulation, genes other than amhr2y in MSYs have degenerated or been lost, while non-recombinant regions continue to expend, forming MSYs of different sizes in different Silurus species (from 320 to 550 kb). Two turnover events, one homologous (from the left arm to the right arm of Chr24) and one nonhomologous (from Chr24 to Chr5), occurring among five Silurus species were possibly mediated by hAT and Helitron transposons.

CONCLUSIONS

Our results on the dynamic evolutionary trajectory of SD gene amhr2y, MSYs, and SCs in Silurus catfish indicated the variability and diversity of fish SCs and confirmed that frequent turnover is an important way to maintain the homology and low differentiation of fish SCs.

An improved chromosome-level genome assembly and annotation of Hong Kong catfish (Clarias fuscus)

Clarias fuscus, renowned for its resilience and nutritional value, is a significant aquaculture species in China. To facilitate further genetic research and breeding programs in this species, we generated an improved high-quality chromosome-level genome assembly of a female C. fuscus using MGI, PacBio, and Hi-C sequencing technologies. The final genome assembly spans 982.84 Mb, with contig and scaffold N50 values of 36.16 Mb and 37.66 Mb, respectively, and successfully anchors 99.60% of the sequences to 28 pseudochromosomes. We also predicted 24,849 protein-coding genes, with 97.3% of them functionally annotated. BUSCO analysis indicates a completeness of 97.03% for the assembly and 96.6% for the annotation. This study significantly advances the genomic resources available for C. fuscus and supports future molecular breeding and functional genomics research.

Dose-dependent role of AMH and AMHR2 signaling in male differentiation and regulation of sex determination in Spotted knifejaw (Oplegnathus punctatus) with X1X1X2X2/X1X2Y chromosome system

Sex determination mechanisms vary significantly across different chromosomal systems and evolutionary contexts. Nonetheless, the regulatory framework governing the multi-sex chromosome system (X1X1X2X2/X1X2Y) remains enigmatic. Through an examination of sex-related genes (dmrt1, hsd11b2, amh, sox9a, sox9b, foxl2, cyp19a), hormonal influences (E2, 11-KT), and histological analyses of gonadal development, we demonstrate that the critical period for sexual differentiation occurs between 35 to 60 days post-hatching (dph). Our multi-omics analysis identified amhr2 as a candidate sex-determining gene, revealing that the males possess three distinct amhr2 transcripts (amhr2ay, amhr2by, amhr2cy), whereas females express only one (amhr2a). In situ hybridization assays demonstrated that amhr2 is predominantly localized to primary spermatocyte and Sertoli cells of male testes. Notably, the specific mRNA expression of amhr2 is significantly enriched in amhr2cy, whose extracellular domain exhibits the highest binding affinity for Amh protein, with sexual expression differences manifesting as early as 5 dph. The outcomes of amhr2 interference (RNAi) experiments indicate that amhr2 knockdown leads to a reduction in the expression of male-related gene (dmrt1, amh, sox9a, sox9b), androgen synthesis genes (hsd11b2, cyp11a), and female-related genes (wnt4, foxl2, cyp19a, cyp19b). Conversely, overexpression of amhr2 yielded contrasting results. Our research supports the role of amhr2 as a pivotal candidate sex-determining gene. Furthermore, the dosage effect of amhr2, reflected in transcript abundance, mRNA expression levels, and binding efficacy, serves as a fundamental mechanism driving male differentiation and regulatory processes in Spotted knifejaw.

Identification of a candidate sex determination region and sex-specific molecular markers based on whole-genome re‑sequencing in the sea star Asterias amurensis

Sex determination systems are diverse in echinoderms, however, our understanding is still very limited in this research field, especially for Asteroidea species. The northern Pacific seastar, Asterias amurensis, has attracted widespread concern due to its population outbreaks and high-risk invasions. Using whole-genome re-sequencing data from 40 females and 40 males, we identified a candidate sex determination region in A. amurensis. Based on the distribution characteristics of 525 sex-associated single nucleotide polymorphisms, identified by GWAS analysis, 119 sex-specific loci were isolated combining a custom Perl script, PCA analysis, and the selection signatures of fixation index FST, suggesting that a 7-12 Mb region on chromosome 10 is a candidate sex-determining region. The existence of female-specific sequences and the genotypes of sex-specific loci indicated that A. amurensis might utilize a ZZ/ZW sex-determination system. We also developed two pairs of sex-specific primers that could distinguish the genetic sex of this starfish with 100% accuracy. As the first study on sex determination in Asteroidea, it will provide novel insights into diverse sex determination systems in echinoderms and allow for in-depth studies on sex-related eco-physiological issues in A. amurensis.

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.

Transforming growth factor-β (TGF-β): A master signal pathway in teleost sex determination

Sex determination and differentiation in fish has always been a hot topic in genetic breeding of aquatic animals. With the advances in next-generation sequencing (NGS) in recent years, sex chromosomes and sex determining genes can be efficiently identified in teleosts. To date, master sex determination genes have been elucidated in 114 species, of which 72 species have sex determination genes belonging to TGF-β superfamily. TGF-β is the only signaling pathway that the largest proportion of components, which including ligands (amhy, gsdfy, gdf6), receptors (amhr, bmpr), and regulator (id2bby), have opportunity recognized as a sex determination gene. In this review, we focus on the recent studies about teleost sex-determination genes within TGF-β superfamily and propose several hypotheses on how these genes regulate sex determination process. Differing from other reviews, our review specifically devotes significant attention to all members of the TGF-β signal pathway, not solely the sex determination genes within the TGF-β superfamily. However, the functions of the paralogous genes of TGF superfamily are still needed ongoing research. Further studies are required to more accurately interpret the molecular mechanism of TGF-β superfamily sex determination genes.

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.

Role of Mn-LIPA in Sex Hormone Regulation and Gonadal Development in the Oriental River Prawn, Macrobrachium nipponense

This study investigates the role of lysosomal acid lipase (LIPA) in sex hormone regulation and gonadal development in Macrobrachium nipponense. The full-length Mn-LIPA cDNA was cloned, and its expression patterns were analyzed using quantitative real-time PCR (qPCR) in various tissues and developmental stages. Higher expression levels were observed in the hepatopancreas, cerebral ganglion, and testes, indicating the potential involvement of Mn-LIPA in sex differentiation and gonadal development. In situ hybridization experiments revealed strong Mn-LIPA signaling in the spermatheca and hepatopancreas, suggesting their potential role in steroid synthesis (such as cholesterol, fatty acids, cholesteryl ester, and triglycerides) and sperm maturation. Increased expression levels of male-specific genes, such as insulin-like androgenic gland hormone (IAG), sperm gelatinase (SG), and mab-3-related transcription factor (Dmrt11E), were observed after dsMn-LIPA (double-stranded LIPA) injection, and significant inhibition of sperm development and maturation was observed histologically. Additionally, the relationship between Mn-LIPA and sex-related genes (IAG, SG, and Dmrt11E) and hormones (17β-estradiol and 17α-methyltestosterone) was explored by administering sex hormones to male prawns, indicating that Mn-LIPA does not directly control the production of sex hormones but rather utilizes the property of hydrolyzing triglycerides and cholesterol to provide energy while influencing the synthesis and secretion of self-sex hormones. These findings provide valuable insights into the function of Mn-LIPA in M. nipponense and its potential implications for understanding sex differentiation and gonadal development in crustaceans. It provides an important theoretical basis for the realization of a monosex culture of M. nipponense.

Transcriptome Analyses Reveal Expression Profiles of Morphologically Undifferentiated and Differentiated Gonads of Yangtze Sturgeon Acipenser dabryanus

Sturgeon is known as a primitive fish with the ZZ/ZW sex determination system and is highly prized for its valuable caviar. Exploring the molecular mechanisms underlying gonadal differentiation would contribute to broadening our knowledge on the genetic regulation of sex differentiation of fish, enabling improved artificial breeding and management of sturgeons. However, the mechanisms are still poorly understood in sturgeons. This study aimed to profile expression patterns between female and male gonads at morphologically undifferentiated and early differentiated stages and identify vital genes involved in gonadal sex differentiation of sturgeons. The sexes of Yangtze sturgeon (Acipenser dabryanus) juveniles were identified via the sex-specific DNA marker and histological observation. Transcriptome analyses were carried out on female and male gonads at 30, 80 and 180 days post-hatching. The results showed that there was a total of 17 overlapped DEGs in the comparison groups of between female and male gonads at the three developmental stages, in which there were three DEGs related to ovarian steroidogenesis, including hsd17b1, foxl2 and cyp19a1. The three DEGs were highly expressed in the female gonads, of which the expression levels were gradually increased with the number of days after hatching. No well-known testis-related genes were found in the overlapped DEGs. Additionally, the expression levels of hsd17b1 and cyp19a1 mRNA were decreased with the knockdown of foxl2 mRNA via siRNA. The results further suggested that foxl2 should play a crucial role in the ovarian differentiation of sturgeons. In conclusion, this study showed that more genes involved in ovarian development than testis development emerged with sexually dimorphic expression during early gonadal sex differentiation, and it provided a preliminary understanding of the molecular regulation on gonadal differentiation of sturgeons.

Gonadal transcriptomes reveal sex-biased expression genes associated with sex determination and differentiation in red-tail catfish (Hemibagrus wyckioides)

BACKGROUND

Red-tail catfish (Hemibagrus wyckioides) is an important commercially farmed catfish in southern China. Males of red-tail catfish grow faster than females, suggesting that all-male catfish will produce more significant economic benefits in aquaculture practice. However, little research has been reported on sex determination and gonadal development in red-tail catfish.

RESULTS

In this study, we performed the first transcriptomic analysis of male and female gonads at four developmental stages at 10, 18, 30, and 48 days post hatching (dph) using RNA-seq technology. A total of 23,588 genes were screened in 24 sequenced samples, of which 28, 213, 636, and 1381 differentially expressed genes (DEGs) were detected at four developmental stages, respectively. Seven candidate genes of sex determination and differentiation were further identified. Real-time quantitative PCR (RT-qPCR) further confirmed that anti-Mullerian hormone (amh), growth differentiation factor 6a (gdf6a), testis-specific gene antigen 10 (tsga10), and cytochrome P450 family 17 subfamily A (cyp17a) were highly expressed mainly in the male, while cytochrome P450 family 19 subfamily A polypeptide 1b (cyp19a1b), forkhead box L2 (foxl2), and hydroxysteroid 17-beta dehydrogenase 1 (hsd17b1) were highly expressed in the female. The KEGG pathway enrichment data showed that these identified DEGs were mainly involved in steroid hormone biosynthesis and TGF-β signaling pathways.

CONCLUSIONS

Based on RNA-seq data of gonads at the early developmental stages, seven DEGs shared by the four developmental stages were identified, among which amh and gdf6a may be the male-biased expression genes, while foxl2, cyp19a1b and hsd17b1 may be the female-biased expression genes in red-tail catfish. Our study will provide crucial genetic information for the research on sex control in red-tail catfish, as well as for exploring the evolutionary processes of sex determination mechanisms in fish.

Mechanisms of sex differentiation and sex reversal in hermaphrodite fish as revealed by the Epinephelus coioides genome

Most grouper species are functional protogynous hermaphrodites, but the genetic basis and the molecular mechanisms underlying the regulation of this unique reproductive strategy remain enigmatic. In this study, we report a high-quality chromosome-level genome assembly of the representative orange-spotted grouper (Epinephelus coioides). No duplication or deletion of sex differentiation-related genes was found in the genome, suggesting that sex development in this grouper may be related to changes in regulatory sequences or environmental factors. Transcriptomic analyses showed that aromatase and retinoic acid are probably critical to promoting ovarian fate determination, and follicle-stimulating hormone triggers the female-to-male sex change. Socially controlled sex-change studies revealed that, in sex-changing fish, the brain's response to the social environment may be mediated by activation of the phototransduction cascade and the melatonin synthesis pathway. In summary, our genomic and experimental results provide novel insights into the molecular mechanisms of sex differentiation and sex change in the protogynous groupers.

A chromosome-level genome assembly enables the identification of the follicule stimulating hormone receptor as the master sex-determining gene in the flatfish Solea senegalensis

Sex determination (SD) shows huge variation among fish and a high evolutionary rate, as illustrated by the Pleuronectiformes (flatfishes). This order is characterized by its adaptation to demersal life, compact genomes and diversity of SD mechanisms. Here, we assembled the Solea senegalensis genome, a flatfish of great commercial value, into 82 contigs (614 Mb) combining long- and short-read sequencing, which were next scaffolded using a highly dense genetic map (28,838 markers, 21 linkage groups), representing 98.9% of the assembly. Further, we established the correspondence between the assembly and the 21 chromosomes by using BAC-FISH. Whole genome resequencing of six males and six females enabled the identification of 41 single nucleotide polymorphism variants in the follicle stimulating hormone receptor (fshr) consistent with an XX/XY SD system. The observed sex association was validated in a broader independent sample, providing a novel molecular sexing tool. The fshr gene displayed differential expression between male and female gonads from 86 days post-fertilization, when the gonad is still an undifferentiated primordium, concomitant with the activation of amh and cyp19a1a, testis and ovary marker genes, respectively, in males and females. The Y-linked fshr allele, which included 24 nonsynonymous variants and showed a highly divergent 3D protein structure, was overexpressed in males compared to the X-linked allele at all stages of gonadal differentiation. We hypothesize a mechanism hampering the action of the follicle stimulating hormone driving the undifferentiated gonad toward testis.

Identification of sex chromosome and sex-determining gene of southern catfish (Silurus meridionalis) based on XX, XY and YY genome sequencing

Teleosts are important models to study sex chromosomes and sex-determining (SD) genes because they present a variety of sex determination systems. Here, we used Nanopore and Hi-C technologies to generate a high-contiguity chromosome-level genome assembly of a YY southern catfish (Silurus meridionalis). The assembly is 750.0 Mb long, with contig N50 of 15.96 Mb and scaffold N50 of 27.22 Mb. We also sequenced and assembled an XY male genome with a size of 727.2 Mb and contig N50 of 13.69 Mb. We identified a candidate SD gene through comparisons to our previous assembly of an XX individual. By resequencing male and female pools, we characterized a 2.38 Mb sex-determining region (SDR) on Chr24. Analysis of read coverage and comparison of the X and Y chromosome sequences showed a Y specific insertion (approx. 500 kb) in the SDR which contained a male-specific duplicate of amhr2 (named amhr2y). amhr2y and amhr2 shared high-nucleotide identity (81.0%) in the coding region but extremely low identity in the promotor and intron regions. The exclusive expression in the male gonadal primordium and loss-of-function inducing male to female sex reversal confirmed the role of amhr2y in male sex determination. Our study provides a new example of amhr2 as the SD gene in fish and sheds light on the convergent evolution of the duplication of AMH/AMHR2 pathway members underlying the evolution of sex determination in different fish lineages.