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

## TL;DR

Thai mahseer populations show hidden genetic diversity shaped by geography, with four distinct lineages identified across river systems in Thailand.

## Contribution

The study identifies four distinct genetic lineages in Thai mahseer and clarifies taxonomic confusion with specimens from Malaysia and Vietnam.

## Key findings

- Four distinct genetic lineages (clades A-D) were identified in Thai T. tambra populations.
- Southern populations form a separate lineage, influenced by the Isthmus of Kra as a biogeographic barrier.
- Multiple lineages coexist in northern and western river systems, indicating complex evolutionary history.

## Abstract

Thai mahseer (Tor tambra) populations harbor hidden genetic diversity that has important implications for their conservation and management. By analyzing mitochondrial DNA from fish collected across nine Thai river systems, we discovered four distinct genetic lineages with unique geographic distributions. Southern populations formed a separate group from northern populations, reflecting the influence of the Isthmus of Kra as a biogeographic barrier. Northern and western river systems showed complex patterns with multiple lineages coexisting, suggesting a dynamic evolutionary history shaped by ancient river connections during glacial periods. We also resolved a taxonomic puzzle: specimens from Malaysia and Vietnam, previously identified as different species, are actually misidentified T. tambra. These findings highlight the importance of protecting each genetic lineage as a distinct conservation unit and underscore the need for integrated approaches combining molecular and morphological data in mahseer taxonomy.

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.