Temperature-Associated Variation in Evolutionary Rates of FADS1 and FADS2 in Rodents.

Environmental temperature is a major ecological factor shaping physiological and molecular evolution in mammals. Fatty acid desaturase genes (FADS1 and FADS2) play essential roles in the biosynthesis of long-chain polyunsaturated fatty acids (PUFAs), which are critical for membrane structure and metabolic regulation. However, their evolutionary responses to climatic variation in wild mammals remain poorly understood. In this study, we analyzed 27 rodent species distributed across diverse climatic regions to investigate the relationship between environmental temperature and the evolutionary rates of the FADS1 and FADS2 genes. Phylogenetic comparative methods, including phylogenetic ANOVA, phylogenetic generalized least squares (PGLS), and branch model analyses, were applied to estimate ω (dN/dS) ratios and assess selection patterns. Branch-site model analyses were further performed to test for episodic positive selection acting on specific codons in low-temperature-associated lineages. Our results showed that species inhabiting lower-temperature environments exhibited significantly higher ω values for both FADS genes compared with those in warmer environments. PGLS analyses revealed consistent negative associations between temperature variables and evolutionary rates, particularly for BIO5, and branch model analyses further indicated elevated ω values in low-temperature-associated lineages relative to background branches. Branch-site analyses detected significant signatures of episodic positive selection in Spermophilus dauricus and Microtus oregoni, with candidate positively selected codons identified in both lineages. Similar temperature-associated evolutionary rate patterns were also observed in expanded mammalian datasets, suggesting a broader cross-taxa trend. Overall, these findings indicate that environmental temperature is associated with heterogeneous evolutionary rate patterns in FADS genes and suggest that lipid metabolism genes may be recurrently influenced by cold-related selective pressures across mammals.