Effect of Salinity Stress on Gut–Brain–Gonad Axis in the Red‐Eared Slider (<i>Trachemys scripta elegans</i>)

The red-eared slider (Trachemys scripta elegans) can adapt to brackish water, which can endanger its biodiversity. Spermatogonial stem cells (SSCs) are essential for establishing and maintaining spermatogenesis and are regulated by the gut-brain-gonad axis. However, the effect of salinity on SSCs is unclear. We investigated the influence of salinity stress on the composition of the gut microbiota in T. s. elegans to determine whether it regulates SSC self-renewal and differentiation via the gut-brain-gonad axis. Three salinity groups (0‰, 5‰, and 10‰) were used in this study, and samples were obtained after 6 months of feeding. The mRNA expression of self-renewing genes (GFRα-1, RAS, and ERK) and meiotic initiation genes (RARα, NRG3, and ERBB4) in SSCs decreased with increasing salinity, indicating that salinity affects renewal and differentiation. In addition, harmful bacteria such as Enterococcus and Clostridium were increased in the S10 group, and lower levels of g_norank_f_Eubacteriaceae were negatively associated with γ-aminobutyric acid (GABA), whereas higher Turicibacter levels were positively associated with GABA levels, resulting in increased GABA content in the S5 group. The results show that salinity affects the secretion of neurotransmitters in the brain and negatively regulates the synthesis of reproductive hormones by changing the composition of intestinal microorganisms and metabolites, which affect SSC function. In conclusion, salinity influences the reproductive ability of T. s. elegans through the gut-brain-gonad axis. This study provides a new perspective for understanding the adaptation of T. s. elegans to brackish water.