The Pacific oyster (Crassostrea gigas) is a bivalve mollusc with vital roles in coastal ecosystems and aquaculture globally. While extensive genomic tools are available for C. gigas, highly contiguous reference genomes are required to support both fundamental and applied research. Herein we report the creation and annotation of a chromosome-level assembly for C. gigas. High-coverage long- and short-read sequence data generated on Pacific Biosciences and Illumina platforms were used to generate an initial assembly, which was then scaffolded into 10 pseudo-chromosomes using both Hi-C sequencing and a high-density linkage map. The assembly has a scaffold N50 of 58.4 Mb and a contig N50 of 1.8 Mb, representing a step advance on the previously published C. gigas assembly. Annotation based on Pacific Biosciences Iso-Seq and Illumina RNA-Seq resulted in identification of ∼30,000 putative protein-coding genes. Annotation of putative repeat elements highlighted an enrichment of Helitron rolling-circle transposable elements, suggesting their potential role in shaping the evolution of the C. gigas genome. This new chromosome-level assembly will be an enabling resource for genetics and genomics studies to support fundamental insight into bivalve biology, as well as for selective breeding of C. gigas in aquaculture.

Diploid and triploid Atlantic salmon show distinct physiological differences including heart, brain, and digestive system morphology, propensity for certain deformities, temperature tolerance as eggs and once hatched, and different nutritional requirements. Whilst several studies have looked in detail at the rate of embryogenesis in diploid salmon, no study has compared the rate of embryogenesis between ploidies from fertilisation to hatch. This study based its assessment on a seminal paper by Gorodilov (1996) and used the same techniques to compare the rate at which triploid and diploid embryos developed morphological characteristics. Whilst no significant difference was found, this study provides well-needed justification for the assumption that both ploidies develop at the same rate and gives scientific weight to studies which involve manipulation at these stages of development. Two factors that did differ, however, were the timing of hatch, and mortality. Triploids hatched more quickly than diploids and reached 50% hatch at a significantly earlier point. Triploids also suffered from a significantly higher rate of mortality.

The production of lumpfish (Cyclopterus lumpus) has become crucial in controlling sea lice levels in salmonid aquaculture. To improve their breeding, there is a need for early sex identification. The genomic region containing the anti-Müllerian hormone (amh) gene was suggested as the candidate master sex-determining gene in lumpfish. However, the genome of lumpfish contains three copies of amh with ambiguous sex specificity, designated amh1, amh2, and amh3. The study aims to analyse the male-specific region between these amh paralogues for its application as a sex marker. In this study, we utilised polymerase chain reaction (PCR)-based assays to identify the male-specific amh markers in lumpfish and estimate the length of the male-specific region in the lumpfish genome. Our results indicate that a specific genomic region of approximately 27 kilobases (kb), encompassing amh1 and amh2 genes, exhibits male specificity, whereas amh3 is present in both sexes. The developed PCR-based genetic sex identification assays targeting amh1 and amh2 exhibited over 97% concordance with phenotypic records. Further experiments in other members of the Cyclopteridae: Aptocyclus ventricosus, Eumicrotremus taranetzi, and E. asperrimus revealed male-specific amh genome region only in A. ventricosus. Phylogenetic analyses using the available Cyclopteridae amh sequences suggest that male-specific amh arose early in the Cyclopteridae lineage. Our findings, along with the development of the PCR test, hold great promise for the field of lumpfish aquaculture and will also contribute significantly to future investigations aiming to enhance our understanding of the sex-determination system and the evolution of sex chromosomes in teleostean fish.

Production of lumpfish (Cyclopterus lumpus) has become crucial in controlling sea lice levels in salmonid aquaculture. To improve their breeding, there is a need for early sex identification. The genomic region containing anti-Müllerian hormone (amh) gene was suggested as the candidate sex-determining gene in lumpfish. However, the genome of lumpfish contains three copies of amh with ambiguous sex specificity, designated amh1, amh2, and amh3. The study aims to analyse the male-specific region between these amh paralogues, for its application as a sex marker. In this study, we utilised polymerase chain reaction (PCR)-based assays to identify the male-specific amh markers in lumpfish and estimate the length of the male-specific region in the lumpfish genome. Our results suggest that a region of approximately 26 kilobase (kb) region containing amh1 and amh2 is male-specific, while both sexes share amh3. The developed PCR-based genetic sex identification assays targeting amh1 and amh2 exhibited more than 97% accuracy. Further experiments in other members of the Cyclopteridae: Aptocyclus ventricosus, Eumicrotremus taranetzi, and E. asperrimus revealed male-specific amh fragments only in A. ventricosus. Phylogenetic analyses using the available Cyclopteridae amh sequences suggest that male-specific amh arose early in the Cyclopteridae lineage. These findings and development of the PCR test will be of service to lumpfish aquaculture as well as to future studies attempting to further elucidate the sex-determination system and sex chromosome evolution in lumpfish.