Comparative analyses reveal sex-biased gut microbiota in cultured subadult pufferfish Takifugu obscurus

The gut microbiota play an important role in fish nutrition, immunity, development and beyond, and the assemblages and dynamics of the fish gut microbial community can be influenced by a multitude of biotic and/or antibiotic factors. To explore how and to what extent fish sex influenced the structure and function of gut microbiota, we analyzed the sex-biased gut bacterial community via 16S rRNA amplicon sequencing using the sized and sexed subadult obscure pufferfish Takifugu obscurus at different gonadal development stages. Through the microscopic examination of stained gonadal tissue sections, the early initiations of sexual maturation were observed in both sexes. Subsequently, the results of integrative microbiota profiling showed that the sex-biased gut bacterial communities were found to be varied mainly in the relative abundance of certain bacterial taxa rather than the global community-level composition, as was evidenced by no significant differences in either alpha- or beta-diversity metrics, implying a limited degree of influence by early gonadal maturation of the host on gut microbiota. Furthermore, potential sex-specific bacterial taxa, such as the genus Sphingomonas and the order Lactobacillales, were classified by linear discriminant analysis effect size (LEfSe) and random forest classification, respectively. In addition, most of the bacterial phenotypes inferred by the BugBase algorithm were found to be not significantly different between sexes except for aerobes and facultative anaerobes, suggesting that the relative abundance of some sex-specific bacterial taxa influences the limited difference in the bacterial phenotypes. Our results emphasize the importance of host sex in shaping the gut microbiota of subadult obscure pufferfish and provide deeper insight into the aquaculture management of this particular species and other species that are of great economic importance.