Chromosome-level genome assembly of the Chinese longsnout catfish Leiocassis longirostris.

The Chinese longsnout catfish ( Leiocassis longirostris Günther) is one of the most economically important freshwater fish in China. As wild populations have declined sharply in recent years, it is also a valuable model for research on sexual dimorphism, comparative biology, and conservation. However, the current lack of high-quality chromosome-level genome information for the species hinders the advancement of comparative genomic analysis and evolutionary studies. Therefore, we constructed the first high-quality chromosome-level reference genome for L. longirostris. The total genome was 703.19 Mb, with 389 contigs and contig N50 length of 4.29 Mb. Using high-throughput chromosome conformation capture (Hi-C) data, the genome sequences (685.53 Mb) were scaffolded into 26 chromosomes ranging from 17.36 to 43.97 Mb, resulting in a chromosomal anchoring rate for the genome of 97.44%. In total, 23 708 protein-coding genes were identified in the genome. Phylogenetic analysis indicated that L. longirostris and its closest related species P. fulvidraco diverged approximately 26.6 million years ago. This high-quality reference genome of L. longirostris should pave the way for future genomic comparisons and evolutionary research.

Full-length transcript sequencing accelerates the transcriptome research of Gymnocypris namensis, an iconic fish of the Tibetan Plateau.

Gymnocypris namensis, the only commercial fish in Namtso Lake of Tibet in China, is rated as nearly threatened species in the Red List of China's Vertebrates. As one of the highest-altitude schizothorax fish in China, G. namensis has strong adaptability to the plateau harsh environment. Although being an indigenous economic fish with high value in research, the biological characterization, genetic diversity, and plateau adaptability of G. namensis are still unclear. Here, we used Pacific Biosciences single molecular real time long read sequencing technology to generate full-length transcripts of G. namensis. Sequences clustering analysis and error correction with Illumina-produced short reads to obtain 319,044 polished isoforms. After removing redundant reads, 125,396 non-redundant isoforms were obtained. Among all transcripts, 103,286 were annotated to public databases. Natural selection has acted on 42 genes for G. namensis, which were enriched on the functions of mismatch repair and Glutathione metabolism. Total 89,736 open reading frames, 95,947 microsatellites, and 21,360 long non-coding RNAs were identified across all transcripts. This is the first study of transcriptome in G. namensis by using PacBio Iso-seq. The acquisition of full-length transcript isoforms might accelerate the transcriptome research of G. namensis and provide basis for further research.

De novo assembly of Schizothorax waltoni transcriptome to identify immune-related genes and microsatellite markers.

Schizothorax waltoni (S. waltoni) is one kind of the subfamily Schizothoracinae and an indigenous economic tetraploid fish to Tibet in China. It is rated as a vulnerable species in the Red List of China's Vertebrates, owing to overexploitation and biological invasion. S. waltoni plays an important role in ecology and local fishery economy, but little information is known about genetic diversity, local adaptation, immune system and so on. Functional gene identification and molecular marker development are the first and essential step for the following biological function and genetics studies. For this purpose, the transcriptome from pooled tissues of three adult S. waltoni was sequenced and analyzed. Using paired-end reads from the Illumina Hiseq4000 platform, 83 103 transcripts with an N50 length of 2337 bp were assembled, which could be further clustered into 66 975 unigenes with an N50 length of 2087 bp. The majority of the unigenes (58 934, 87.99%) were successfully annotated by 7 public databases, and 15 KEGG pathways of immune-related genes were identified for the following functional research. Furthermore, 19 497 putative simple sequence repeats (SSRs) of 1-6 bp unit length were detected from 14 690 unigenes (21.93%) with an average distribution density of 1 : 3.28 kb. We identified 3590 unigenes (5.36%) containing more than one SSR, providing abundant potential polymorphic markers in functional genes. This is the first reported high-throughput transcriptome analysis of S. waltoni, and it would provide valuable genetic resources for the functional genes involved in multiple biological processes, including the immune system, genetic conservation, and molecular marker-assisted breeding of S. waltoni.