As a protandrous hermaphroditic fish species with natural sex change from male to female, Asian seabass (Lates calcarifer) represents an attractive model for studying sequential hermaphroditism. In this study, we constructed the first telomere-to-telomere (T2T) gap-free genome assembly of Asian seabass, by integration of MGI short-read, PacBio HiFi long-read, ONT ultra-long and Hi-C sequencing technologies. The haplotypic 614.19 Mb genome sequences were successfully anchored onto 24 chromosomes, demonstrating exceptional contiguity with a contig N50 of 26.57 Mb. Comprehensive annotation revealed precise localization of telomeric repeats and centromeric regions across various chromosomes. Good results from Merqury (QV: 57.8), CRAQ (99.45%) and BUSCO (100%) indicate a high level of accuracy for the assembled genome. ONT ultra-long and PacBio HiFi sequencing data were aligned with the assembly using minimap2, resulting in a mapping rate over 98%. Repetitive elements accounted for 18.18% (111.64 Mb) of the entire genome, and a total of 25,093 protein-coding genes were annotated. This high-quality T2T genome assembly provides a valuable genetic resource for in-depth comparative genomics, population genetics, molecular breeding, and functional studies of this economically important marine species. This reference assembly also facilitates investigations into the detailed molecular mechanisms underlying its unique reproductive strategy of the protandrous hermaphrodite Asian seabass.

The peanut worm (Sipunculus nudus) is an important intertidal species worldwide. Species living in the same aquaculture area might suffer different environmental impacts. To increase knowledge of the molecular mechanisms underlying the response to environmental fluctuations, we performed a transcriptome analysis of S. nudus from different intertidal zones using a combination of the SMRT platform and the Illumina sequencing platform. (1) A total of 105,259 unigenes were assembled, and 23,063 unigenes were perfectly annotated. The results of the PacBio Iso-Seq and IIIumina RNA-Seq enriched the genetic database of S. nudus. (2) A total of 830 DEGs were detected in S. nudus from the different groups. In particular, 33 DEGs had differential expression in the top nine KEGG pathways related to pathogens, protein synthesis, and cellular immune response and signaling. The results indicate that S. nudus from different zones experience different environmental stresses. (3) Several DEGs (HSPA1, NFKBIA, eEF1A, etc.) in pathways related to pathogens (influenza A, legionellosis, measles, and toxoplasmosis) had higher expression in groups M and L. HSPA1 was clearly enriched in most of the pathways, followed by NFKBIA. The results show that the peanut worms from the M and L tidal flats might have suffered more severe environmental conditions. (4) Some DEGs (MKP, MRAS, and HSPB1) were upregulated in peanut worms from the H tidal flat, and these DEGs were mainly involved in the MAPK signaling pathway. These results indicate that the MAPK pathway may play a vital role in the immune response of the peanut worm to the effects of different intertidal flats. This study provides a valuable starting point for further studies to elucidate the molecular basis of the response to different environmental stresses in S. nudus.

Background: Polydactylus macrochir (Günther; 1867) is a member of the family Polynemidae. The placement of Polynemidae among teleosts has varied over the years. Methods: Therefore, in this study, we sequenced the complete mitochondrial genome of P. macrochir, analyzed the characterization of the mitochondrial genome, and investigated the phylogenetic relationships of Polynemidae. Results: The length of the P. macrochir mitogenome was 16,738 bp, with a typical order. Nucleotide composition analysis showed that the P. macrochir mitogenome was AT-biased (54.15%), and the PCGs tended to use A and C rather than T and G at the third codon. All the PCGs started with the regular codon ATG, except for cox1, which started with GTG. The termination codon varied across the PCGs. It was shown that the ka/ks ratios of all the PCGs were less than one. Phylogenetic analysis, based on the maximum likelihood (ML) and Bayesian inference (BI) methods, indicated that eight threadfins formed a well-supported monophyletic cluster. Polynemidae and Sphyraenidae clustered together as a monophyletic group. According to TimeTree analyses, the most recent common ancestor (MRCA) of Polynemidae was traced back to about 52.81 million years ago (MYA), while six species within Polynemidae diverged from 11.70 MYA to 20.05 MYA. Conclusions: The present study provides valuable mitochondrial information for the classification of P. macrochir and new insights into the phylogenetic relationships of Polynemidae.

Polydactylus macrochir (Günther, 1867) is a number of family Polynemidae. The placement of the Polynemidae among teleosts has varied over the years. Therefore, in this study we sequenced the complete mitochondrial genome of P. macrochir, analyzed the characterization of the mito-chondrial genome, and investigated the phylogenetic relationships of Polynemidae. The lengths of the P. macrochir mitogenomes was 16,738 bp with the typical order. Nucleotide composition analysis showed that P. macrochir mitogenome was AT biased (54.15%), and the PCGs tended to use A and C than T and G at the third codon. All the PCGs started with the regular codon ATG, except for the cox1 which started with GTG. The termination codon varied across PCGs. It was shown that the ka/ks ratios of all PCGs were less than 1. The phylogenetic analysis, based on the maximum likelihood (ML) and Bayesian inference (BI) methods, indicated that 8 Threadfins spe-cies formed a well-supported monophyletic cluster. Polynemidae and Sphyraenidae clustered together as a monophyletic group. According to TimeTree analyses, the most recent common ancestor (MRCA) of Polynemidae was traced back to about 52.81 million years ago (MYA), while 6 species within Polynemidae diverged from 11.70 MYA to 20.05 MYA. The present study provides valuable mitochondrial information for the classiffcation of P. macrochir and new insights in phylogenetic relationships of Polynemidae.