Positive Selection of <scp>TLR2</scp> and <scp>MyD88</scp> Genes Provides Insights Into the Molecular Basis of Immunological Adaptation in Amphibians

The transition from water to land of amphibians is evolutionarily significant in the history of vertebrates, and immunological adaptation is an important challenge for amphibians to respond to the dramatic changes of the environmental pathogens during their origin and diversification. Toll-like receptors (TLRs) are important pattern recognition receptors for the innate immune response and TLRs signaling pathway play essential roles in the immune responses to pathogens and inflammatory reaction. However, the evolutionary patterns and molecular mechanisms underlying their adaptation in amphibians are poorly documented to date. Here, we determined the coding regions, expression patterns of TLR2 and Myeloid differentiation factor 88 (MyD88) in the large treefrog (<i>Zhangixalus dennysi</i>), and explored the evolutionary patterns of these two genes in amphibians. Quantitative Real-time PCR analyses showed that the TLR2 and MyD88 mRNA were expressed in all the organs/tissues examined, both with the highest levels in the heart and the lowest levels in the body fat for TLR2 and lung for MyD88. The highly conservation and functional significance of these two genes in amphibians were supported based on the sequence characteristics and evolutionary analyses. Significantly positive selection was found to be acting on TLR2 and MyD88 in amphibians based on different site models. Strong signal of positive selection among different amphibian lineages for these two genes was also detected and a series of positively selected sites were identified based on the branch-site analysis. Our results suggest that amphibians have adapted to different pathogenic microorganisms during their transition from the aquatic to terrestrial environment and diversification into various habitats. The present study will provide new insights into the evolutionary process and molecular basis underlying the immunological adaptation in vertebrates.