Phylogeographic Structure Reveals Hidden Diversity Patterns in Tor tambra (Cyprinidae) Across Thai River Systems.

Understanding patterns of genetic diversity and population structure in freshwater fishes is crucial for evolutionary studies and conservation planning. Tor tambra is a common freshwater fish species in Thai waters, yet its genetic diversity and population connectivity remain largely unexplored. This study investigated the phylogeographic structure and genetic diversity of Thai T. tambra through phylogenetic analyses and molecular approaches using mitochondrial DNA markers (COI and Cytb). Our analyses revealed four distinct genetic lineages (clades A-D) within Thai T. tambra. Lineage C comprised specimens from southern Thailand, Malaysia, Indonesia, and southern China, while lineages A, B, and D were distributed exclusively within Thailand. These Thai-endemic lineages exhibited pronounced geographic structuring across northern and western regions, with lineage A widespread, lineage B confined to northern areas, and lineage D restricted to western localities. The genetic differentiation and distribution patterns of these lineages suggest influence from the limited north-south river system connections and the biogeographic barrier posed by the Isthmus of Kra. Multiple lineages co-occurred at some locations, indicating a complex evolutionary history. These findings provide important implications for understanding the evolutionary processes and biogeographic factors shaping genetic diversity within T. tambra across the riverine systems of Thailand.