# Ammonia Stress Induces Transcriptional Expression Changes in the Mature Eggs of the Acipenser baerii

## TL;DR

Ammonia stress harms sturgeon egg development by causing structural damage and disrupting hormone and antioxidant gene expression.

## Contribution

This study identifies specific gene expression changes in sturgeon eggs under ammonia stress, linking them to oxidative stress and hormone disruption.

## Key findings

- High ammonia concentrations cause follicle membrane rupture and yolk granule adhesion in sturgeon eggs.
- Ammonia stress suppresses antioxidant and steroidogenesis-related genes, impairing oocyte function.
- Transcriptome analysis reveals significant changes in extracellular matrix and amino acid metabolism pathways.

## Abstract

Ammonia stress has become a significant stress factor in sturgeon aquaculture. In this study, we found that exposure to different concentrations of ammonia stress affected the fourth stage of female sturgeon egg development, primarily causing rupture of the follicle membrane, adhesion of yolk granules, and damage to the surface structure. Transcriptome analysis revealed that ammonia exposure may enrich cellular components in the extracellular space, further disrupting the function of the extracellular matrix. Antioxidant gene expression revealed that antioxidant genes were significantly downregulated under ammonia stress, and gene expression of enzymes involved in steroidogenesis was suppressed. This suggests that high ammonia concentrations impair oocyte function by inducing oxidative stress and interfering with hormone synthesis. This result enables us to understand the effect of ammonia stress on sturgeon egg development from a molecular biological perspective, providing valuable insights for future sturgeon breeding and hatching efforts.

Ammonia is a key factor in the water, impacting the physiological functions of aquatic organisms. To explore the effect of ammonia stress on mature eggs, female A. baerii at the end of the fourth stage of ovarian development were subjected to varying ammonia concentrations (0 mg/L (control, C), 10 mg/L (low concentration, T1), and 50 mg/L (high concentration, T2)) for 96 h. After 96 h of stress, histological analysis revealed that the follicular membranes of group T1 remained intact and clear compared to group C, although the vacuole fusion had begun. In contrast, the T2 group exhibited ruptured follicular membranes and adhered yolk granules compared to the C group, indicating structural damage. Transcriptome analysis generated 97.89 Gb of clean data, with each sample yielding over 6.09 Gb. A total of 5576, 3719, and 9446 differentially expressed genes (DEGS) were screened from T1 vs. C, T2 vs. C, and T2 vs. T1 comparisons, respectively. Gene Ontology analysis (GO) functional enrichment analysis showed that DEGS were significantly enriched in multicellular organism processes (T1 vs. C), cell surface receptor signaling pathways (T2 vs. C), and immune system processes (T2 vs. T1) during biological processes. It indicates that ammonia exposure may enrich cellular components in the extracellular space, potentially disrupting the function of the extracellular matrix. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment indicated significant impacts on amino acid metabolism, particularly glutamate and arginine pathways, as well as key pathways involved in steroid biosynthesis and antioxidation. Weighted gene co-expression network analysis (WGCNA) revealed that a total of 26,369 DEGs were divided into 29 distinct modules, displaying obvious associations with their traits. In the T2 vs. C group, antioxidation-related genes such as GST and GCLM were significantly downregulated, and the expressions of key enzymes for steroid synthesis, such as CYP11A1, CYP17, and CYP19A1 were suppressed, indicating that high ammonia nitrogen concentrations impair oocyte function by inducing oxidative stress and disrupting hormone synthesis. This study provides a comprehensive repertoire of candidate genes associated with ammonia stress in the mature egg of A. baerii, which will be useful for development of sturgeon breeding and reproduction.