Genetic homogeneity of migratory catfish Pangasius conchophilus (Siluriformes: Pangasiidae) across the Lower Mekong River

Pangasius conchophilus is a migratory catfish species of high ecological and economic importance in the Lower Mekong basin. However, its population structure and levels of genetic diversity remain poorly understood. To address these questions, we sequenced the mtDNA cytochrome b gene (Cytb) from 83 wild fish samples collected at five sites along the Mekong River, including two upstream locations in Laos (Paksan and Pakse), one in Cambodia, and two downstream sites in the Mekong Delta, Viet Nam (An Giang and Can Tho). Analysis of 657-bp Cytb fragments revealed a high level of genetic diversity, with haplotype diversity and nucleotide diversity values ranging from 0.700 to 0.958 and 0.0037 to 0.0073, respectively. This high genetic diversity was consistent with signatures of population expansion, as indicated by mismatch distribution tests. Analysis of molecular variance showed that most of the genetic variation occurred within populations, which was supported by overlapping within- and between-population genetic distances and by the absence of genetic structuring in the phylogenetic tree and haplotype network. These results indicate extensive gene flow among geographically distant populations, leading to genetic homogeneity, and suggest long-distance migratory behaviour of this species along the Mekong mainstream. Overall, these findings have critical implications for the regional management and conservation of P. conchophilus.