# A Hypothesis of Gut–Liver Mediated Heterosis: Multi-Omics Insights into Hybrid Taimen Immunometabolism (Hucho taimen ♀ × Brachymystax lenok ♂)

## TL;DR

Hybrid taimen fish survive gut infections better than their parents, possibly due to beneficial gut microbes and active immune and energy genes in their intestines and livers.

## Contribution

This study proposes a gut–liver mediated heterosis hypothesis by linking hybrid taimen's gut microbiota and host gene expression to improved disease resistance.

## Key findings

- Hybrid taimen showed higher survival rates after gut infections compared to purebred parents.
- Hybrids had higher levels of bacteria linked to antimicrobial compounds and short-chain fatty acid metabolism.
- Hybrid intestines and livers exhibited increased immune and energy-related gene activity.

## Abstract

Hybrid taimen (Hucho taimen ♀ × Brachymystax lenok ♂) survive gut infections better than their purebred parents, but the reason was unclear. We compared the gut bacteria and host gene expression of hybrid and pure fish reared under identical conditions. Hybrids carried more bacteria predicted to make antimicrobial and short-chain fatty acid metabolites, and their intestines and livers showed more vigorous immune and energy-supply gene activity. While these correlations suggest a beneficial gut–liver axis, future controlled experiments are needed to prove that specific microbes or genes actually cause the improved survival.

This study investigated the molecular and microbial factors behind the higher disease resistance of hybrid taimen by combining gut microbiome profiling with host transcriptomic analysis of intestinal and liver tissues. Both hybrid taimen and H. taimen were raised under the same recirculating aquaculture system (RAS) conditions. After recording survival rates following three enteritis outbreaks, samples of intestinal contents and tissues were collected from both groups. The gut microbiota was analyzed using full-length 16S rRNA sequencing in PacBio, and host gene expression was assessed with Illumina RNA-seq. Functional predictions were made using PICRUSt2 and Gene Set Enrichment Analysis (GSEA). Results showed that hybrids had significantly higher survival rates after enteritis (p < 0.05). Although microbial alpha diversity was similar, beta diversity revealed slight compositional differences. Hybrids showed higher levels of Hapalosiphon and Tepidimicrobium, microbes associated with antimicrobial compounds and the metabolism of short-chain fatty acids (SCFAs). Functional predictions indicated enrichment in selenocompound metabolism and ansamycin biosynthesis in hybrids. Transcriptomic analysis identified 4233 differentially expressed genes (DEGs) in the intestine and 3980 in the liver. In hybrids, intestinal tissues exhibited increased expression of immune pathways, including complement activation, lysosomal activity, and the transforming growth factor-beta (TGF-β) signaling pathway. Liver tissues demonstrated higher expression of genes related to cholesterol synthesis, fatty acid degradation, and the peroxisome proliferator-activated receptor (PPAR) signaling pathway. qRT-PCR validated the expression patterns of 20 selected DEGs. These findings tentatively suggest that the elevated disease resistance of hybrid taimen may be linked, at least in part, to a combination of microbial taxa inferred to produce antimicrobial metabolites and short-chain fatty acids, as well as an apparent intensification of intestinal immune and barrier-related gene expression, and hepatic pathways that possibly support energy supply and steroid-based immunity. However, this multi-omics data set is only correlational. We still do not know whether a single strain or a few host genes are enough to produce the resistant phenotype. Gnotobiotic trials, microbiota transplants, and targeted metabolomics will be necessary to turn these interesting associations into solid evidence.