Lotic specialization in modern Asian newts (Caudata: Salamandridae): Phylogeny, historical biogeography, and ancestral traits based on combined DNA data

Modern Asian newts, including Cynops, Laotriton, Pachytriton, and Paramesotriton, are primarily distributed across East Asia, displaying a broad range of environmental adaptations, including generalists inhabiting both lotic and lentic habitats and lotic specializations. Despite recent studies on their biogeographic history, how these species adapted to geographical changes remains unclear. In this study, divergence times and reconstructed ancestral traits were estimated based on phylogenetic analysis of mitochondrial and nuclear DNA sequences, totaling 4048 bp. Results indicated that the divergence between lotic-specialized and generalist species occurred during the middle to late Miocene (95% confidence interval (CI): 16–13 million years ago (Ma)), correlating with the long-lasting arid period (14–12 Ma) and uplift of the Wuyi-Nanling mountains (15–11 Ma). Ancestral trait reconstruction revealed notable morphological changes associated with the emergence of lotic-specialized lineages. Specifically, male body size increased from 130 mm to 140 mm, and epibranchial traits shifted from thin to thick structures. Additionally, variation in tongue movability and skin texture were observed among lotic species. Each morphological shift is hypothesized to have an ecological basis, facilitating adaptation to specific environmental conditions. Overall, this study represents the first comprehensive attempt to elucidate the evolution of ancestral traits among divergent species of modern Asian newts, providing a deeper understanding of their adaptive responses to historical geographical and climatic changes.