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.

Chromosome-Level Genome Assembly and Transcriptome Comparison Analysis of Cephalopholis sonnerati and Its Related Grouper Species.

The tomato hind, Cephalopholis sonnerati, is a bottom-dwelling coral reef fish, which is widely distributed in the Indo-Pacific and Red Sea. C. sonnerati also features complex social structures and behaviour mechanisms. Here, we present a high-quality, chromosome-level genome assembly for C. sonnerati that was derived using PacBio sequencing and Hi-C technologies. A 1043.66 Mb genome with an N50 length of 2.49 Mb was assembled, produced containing 795 contigs assembled into 24 chromosomes. Overall, 97.2% of the complete BUSCOs were identified in the genome. A total of 26,130 protein-coding genes were predicted, of which 94.26% were functionally annotated. Evolutionary analysis revealed that C. sonnerati diverged from its common ancestor with E. lanceolatus and E. akaara approximately 41.7 million years ago. In addition, comparative genome analyses indicated that the expanded gene families were highly enriched in the sensory system. Finally, we found the tissue-specific expression of 8108 genes. We found that these tissue-specific genes were highly enriched in the brain. In brief, the high-quality, chromosome-level reference genome will provide a valuable genome resource for studies of the genetic conservation, resistance breeding, and evolution of C. sonnerati.

Developing Single Nucleotide Polymorphisms for Identification of Cod Products by RAD-Seq.

The increase in the rate of seafood fraud, particularly in the expensive fishes, forces us to verify the identity of marine products. Meanwhile, the definition of cod lacks consistency at the international level, as few standards and effective application methods are capable of accurately detecting cod species. Genetic fingerprinting is important for both certifying authenticity and traceability of fish species. In this study, we developed a method that combines DNA barcoding and the restriction-site associated DNA sequencing (RAD-Seq) approach for the identification of cod products. We first obtained 6941 high-quality single nucleotide polymorphism (SNP)s from 65.6 gigabases (Gb) of RAD-Seq raw data, and two sequences that contain SNPs were finally used to successfully identify three different cod product species, which are Atlantic cod (Gadus morhua), Greenland turbot (Reinhardtius hippoglossoides), and Patagonian toothfish (Dissostichus eleginoides). This SNP-based method will help us to identify the products, which are sold under the name of "Xue Yu" (Cod) in China, and works in parallel with existing fish identification techniques to establish an efficient framework to detect and prevent fraud at all points of the seafood supply chain.

A Comparative Genomic and Transcriptional Survey Providing Novel Insights into Bone Morphogenetic Protein 2 (bmp2) in Fishes.

Intermuscular bones (IBs) are only found in the muscles of fish. Bone morphogenetic protein 2 (bmp2) is considered to be the most active single osteogenesis factor. It promotes cell proliferation and differentiation during bone repair, as well as inducing the formation of bones and cartilages in vivo. However, detailed investigations of this family in fish are incredibly limited. Here, we have used a variety of published and unpublished bmp2 sequences for teleosts and cartilage fish in order to explore and expand our understanding of bmp2 genes in fish. Our results confirmed that teleost genomes contain two or more bmp2 genes, and the diversity of bmp2 genes in vertebrates appears to be as a result of a combination of whole genome duplication (WGD) and gene loss. Differences were also observed in tissue distribution and relative transcription abundance of the bmp2s through a transcriptomic analysis. Our data also indicated that bmp2b may play an important role in the formation of IBs in teleosts. In addition, protein sequence alignments and 3D structural predictions of bmp2a and bmp2b supported their similar roles in fishes. To summarize, our existing work provided novel insights into the bmp2 family genes in fishes through a mixture of comparative genomic and transcriptomic analysis.

An SNP-Based Genetic Map and QTL Mapping for Growth Traits in the Red-Spotted Grouper (Epinephelus akaara).

The red-spotted grouper (Epinephelus akaara) is one of the most commercially important aquatic species in China. However, its seedstock has low larval survival rates, and its stability is confronted with the danger of overexploitation. In this study, a high-density genetic map was constructed using 3435 single nucleotide polymorphisms (SNPs) from 142 first generation (F1) full-sib offspring and two parents of a red-spotted grouper population. The total genetic length of the map was 2300.12 cM with an average intermarker distance of 0.67 cM. Seventeen genome-wide significant quantitative trait loci (QTLs) for growth-related traits were detected on 24 linkage groups, including 5 QTLs for full length, 7 QTLs for body length, and 5 QTLs for body weight. The contribution values of explained phenotypic variance ranged from 10.7% to 12.9%. Moreover, 13 potential candidate genes for growth-related traits were identified. Collectively, these findings will be useful for conducting marker-assisted selection of the red-spotted grouper in future studies.

A genome-wide association study on growth traits in orange-spotted grouper (Epinephelus coioides) with RAD-seq genotyping.

The orange-spotted grouper, Epinephelus coioides, is one of the most popular fish in China and Southeast Asian countries because of its important economic value. However, molecular mechanism underlying the growth of orange-spotted grouper has never been fully understood. Herein, we performed a genome-wide association study (GWAS) on a natural population of 198 individuals aiming to screen the whole genome of orange-spotted grouper for identification of growth-related loci by restrictionsite associated DNA sequencing. In this research, 261,366 single nucleotide polymorphisms (SNPs) were developed, in which 110 SNPs were identified to be correlated with growth and 20 SNPs were further confirmed to be associated with both body weight and total length. From these identified SNPs, we annotated a total of 34 genes, including adgrb2, csnkza1, cers5, col22a1, creb5, dnd1, dzank1, dnai1, npy2r, fat3, lrrk2, lrp5, map3k9, and so on. Among these candidate genes, npy2r (neuropeptide Y receptor Y2) was reported to play a critical role in growth of the orange-spotted grouper. In addition, population structure, principal component analysis, kinship matrix and linkage disequilibrium were examined to verify the accuracy and reliability of our GWAS results. Our data will also provide a valuable genetic resource for further marker-assisted selection program to improve growth quality in groupers.

The complete mitochondrial genome of Indonesian snakehead, Channa micropeltes (Channiformes, Channidae).

Here we have characterized the complete mitochondrial genome of Indonesian snakehead, Channa micropeltes, and described its organization in this paper. The complete mitochondrial genome, 16,567 bp, is composed of 32.54% A, 14.05% G, 25.82% T and 27.59% C. It includes 13 protein-coding genes, 22 tRNA genes, 2 rRNA genes and a 921-bp D-loop control region. Phylogenetic analysis with five more fish species demonstrated that the Indonesian snakehead is most closely related to the Great snakehead (Channa marulius). Our mitochondrial genomic data will also be valuable for the study on the mitochondrial evolution of fishes.