# Comparative Transcriptome Analysis Reveals the Seawater Adaptation Mechanism in Pseudaspius hakonensis

## TL;DR

This study compares gene activity in seawater and freshwater fish to understand how Pseudaspius hakonensis adapts to saltwater.

## Contribution

The study provides the first multi-tissue transcriptomic comparison of an anadromous and a freshwater cyprinid, revealing novel molecular mechanisms of seawater adaptation.

## Key findings

- Gill, kidney, and liver tissues showed thousands of differentially expressed genes in seawater-adapted P. hakonensis.
- Key pathways like MAPK signaling, ABC transporters, and glutathione metabolism were consistently activated across tissues.
- Candidate genes such as DUSP10, SLC38A2, ATP8B1, GSTA4, and MGST1 were significantly upregulated in seawater-adapted fish.

## Abstract

Background: The family Cyprinidae is predominantly restricted to freshwater habitats, making the evolution of diadromy and seawater adaptation exceptionally rare within this group. Pseudaspius hakonensis, a rare anadromous cyprinid, and its strictly freshwater congener P. leptocephalus, provide an ideal comparative model to investigate the molecular mechanisms underlying salinity adaptation. This study aimed to elucidate the tissue-specific transcriptional reprogramming, identify candidate genes and key pathways, and explore their association with seawater acclimation in P. hakonensis. Methods: We performed comparative transcriptomic analyses of gill, liver, and kidney tissues from both species using RNA-Seq. Sequencing reads were aligned to a high-quality reference genome of P. hakonensis. Differential expression analysis was conducted using DESeq2, followed by functional enrichment analyses (GO and KEGG) to identify significant biological processes and pathways. Results: A total of 8784, 5965, and 5719 differentially expressed genes (DEGs) were identified in gill, kidney, and liver tissues, respectively, with the gill showing the highest differences. Functional enrichment revealed tissue-specific roles: gill DEGs were associated with protein synthesis and energy metabolism; kidney DEGs with transport and detoxification; and liver DEGs with metabolic regulation and stress signaling. Cross-tissue analysis highlighted three core pathways consistently enriched: MAPK signaling, ABC transporters, and glutathione metabolism. Key candidate genes, including DUSP10, SLC38A2, ATP8B1, GSTA4, and MGST1, were significantly upregulated in P. hakonensis. Conclusions: This first multi-tissue transcriptomic comparison of an anadromous and a freshwater cyprinid reveals pervasive, tissue-specific molecular reprogramming underlying seawater adaptation in P. hakonensis. The coordinated activation of MAPK signaling, glutathione metabolism, and transporter pathways suggests an integrated regulatory network for osmoregulation and stress resistance. These findings provide novel insights into the genetic basis of salinity adaptation in cyprinids and identify candidate genes for future functional validation.