HAFIZ ISHFAQ AHMAD ^* Xiujuan Zhang Haoting Mai Linmiao Li Haiying Jiang Jinping Chen ^*

• Institute of Zoology, Guangdong Academy of Science (CAS), Guangzhou, Guangdong Province, China

Every mammalian species harbours a gut microbiota, and variation in the gut microbiota within mammalian species can profoundly affect host phenotypes. Understanding the consequences of gut microbiotas in mammalian evolution requires testable hypotheses regarding the specific modes by which they alter the adaptive landscapes experienced by hosts. Mechanisms underlying adaptation to various gut microbiota during the evolutionary process remain poorly understood. This study examines how the host's immune system influences the molecular evolution and adaptation of the gut microbiotagut microbiota adaptation in various mammalian species. We assessed the evidence for the gut microbiota's influence on mammalian evolution and diversification. The maximum likelihood approach was used to identify evidence of positive selection in immune genes. To identify codons that underwent adaptive evolution, we Formatted: Superscript Formatted: Highlight looked for episodic and pervasive positive selection throughout all branches of the mammalian evolutionary tree. Our findings reveal intriguing co-evolutionary processes in which the host's immune system exerts selective pressure on immune genes, resulting in adaptive changes in microbial populations. Our findings suggest that, in most mammalian species, episodic positive selection has played an important essential role in the genetic development of species-specific gene sequences and divergence. Furthermore, we found evidence of broad positive selection during the molecular evolution of immune genes on all branches of the mammalian phylogenetic tree. These results suggest that the gut microbiota plays a crucial role in influencing how mammals adapt to their diet, their ability to change their physical characteristics, the structure of their gastrointestinal system, and their immune response.