Haplotype-resolved and near-T2T assembly of the African catfish (Clarias gariepinus)

Airbreathing catfishes are a group of stenohaline freshwater fish that can withstand various environmental conditions and farming practices, including the ability to breathe atmospheric oxygen. This unique ability has allowed them to thrive in semi-terrestrial habitats. However, the genomic mechanisms underlying their adaptation to adverse ecological conditions remain to fully investigate, due to the absence of gold standard reference genomes. The present study aimed to sequence and characterize the genome of the African catfish (Clarias gariepinus), a representative air-breathing catfish, to elucidate the genomic underpinnings of its remarkable adaptability. By generating a near telomere-to-telomere (T2T) assembly with high-resolution haplotypes, we sought to identify genomic and evolutionary features that may have contributed to its ability to withstand adverse conditions and transition to semi-terrestrial life. \textbf{Methods:} We conducted a comprehensive genomic analysis of the African catfish using a multi-platform sequencing approach, integrating Oxford Nanopore, PacBio HiFi, Illumina, and Hi-C technologies to achieve a haplotype-resolved chromosome-scale genome assembly. Functional annotations and comparative genomic analyses, including gene family evolution and positive selection studies, were performed to identify the genomic mechanisms underlying the species' resilience and adaptation to diverse environments. Results: This multifaceted approach has provided novel insights into the African catfish's complex genomic architecture and adaptive strategies. The near-T2T diploid assembly yielded 48 contigs spanning 969.62 Mb with a contig N50 of 33.71 Mb. We report 25,655 predicted protein-coding genes and 43.94\% repetitive elements in the African catfish genome. Several gene families involved in ion transport, osmoregulation, oxidative stress response, and muscle metabolism were expanded and positively selected in clariids, suggesting a potential role in their transition and adaptation to semi-terrestrial habitats. Conclusion: Our study provides a comprehensive genomic resource for \textit{Clarias gariepinus}, shedding light on the genetic and genomic mechanisms of clariids' adaptation to adverse ecological environments. The findings enhance our understanding of resilience in C. gariepinus and offer valuable insights for improving aquaculture and studying related teleosts.