The Thai walking catfish, Clarias macrocephalus Gunther, 1864, is economically important to Thailand. It occupies marshes and swamps that are severely endangered due to population expansion and natural populations are thought to be suffering from massive back-crossing with the C macrocephalus x C gariepinus hybrids. Therefore, a study on genetic diversity of this species is required to enable efficient conservation and management plans. In this study, 25 natural populations were collected throughout the country, 12 populations from provinces locate in the Chaophraya river basin in the center of the country, 5 from the Mekong river basin, 1 from the east and 7 from the south. One population of hatchery origin was obtained from the Department of Aquaculture, Kasetsart University in Bangkok. Twelve isozymes and one protein system were analyzed. Among 18 loci resolved, 8 were polymorphic. The number of alleles per locus, average polymorphism and individual polymorphic were significantly higher in collections from the Chaophraya river basin than from the Mekong, east and south. The hatchery population also had relative high genetic variation. Six out of twenty-six populations differed significantly from Hardy-Weinberg equilibrium after Bonferroni correction. None of loci pairs showed significant linkage disequilibrium after Bonferroni correction. The F-st value across loci was highly significant from zero. A neighbor-joining tree reveals that populations from the south were genetically distinct from the remaining populations. Alleles peculiar to the African catfish [C gariepinus (Burchell, 1822)] genome were observed in 12 of the natural populations and the hatchery, population. This is evidence of genetic introgression which has probably persisted for several generations, since there was no significant genotype disequilibrium between the macrocephalus and gariepinus alleles at three diagnostic loci. (C) 2004 Elsevier B.V All rights reserved.
