Genetic impacts of hybrid catfish farming (Clarias macrocephalus x C. gariepinus) on native catfish populations in central Thailand

Escaped hybrid catfish (female Thai walking catfish, Clarias macrocephalus x male African catfish, C. gariepinus) from farms in central Thailand may interbreed with C. macrocephalus individuals in the wild. We assessed genetic introgression of C. gariepinus genes into four wild and two broodstock populations of C. macrocephalus based on diagnostic alleles at six allozyme loci and one microsatellite locus. A total of 22 out of 515 C. macrocephalus individuals examined had C. gariepinus alleles. One individual had C. gariepinus alleles at all six loci while the 21 remaining individuals had C. gariepinus alleles at one to two loci. In each population, the diagnostic markers detected between one and five individuals (1% to 11% of the sample) bearing hybrid genotypes. Farmers' inadvertent use of introgressed C. macrocephalus individuals as broodstock in producing hybrid catfish could result in loss of hybrid vigor for growth or disease resistance. We also evaluated existing genetic diversity within and among wild and broodstock C. macrocephalus populations using three microsatellite DNA markers. Compared to other Clarias species, C. macrocephalus had moderately high genetic variation within and among populations. Within each population, the number of alleles per locus ranged from 4 to 18 and heterozygosities ranged from 0.34 to 0.89. Narathivas, a southern population had the lowest microsatellite variation, allelic diversity (three alleles per locus) and heterozygosity (HO=0.34). Within the central Thailand populations, the Ang Thong broodstock population expressed the lowest genetic diversity. Pair-wise exact tests suggested genetic divergence among most studied populations (31 of 36 tests, p<0.01). Cavalli-Sforza's chord genetic distances indicated strong genetic divergence between the Narathivas population (i.e., southern Thailand) and the rest of the sampled populations. Within central and northern Thailand, the Ang Thong Broodstock (central), Ayutthaya (central), Phitsanulok 1(northern) and Phitsanulok 2 (northern) populations were genetically diverged from the rest of the central populations. Our results suggested low levels of genetic introgression within examined wild and broodstock populations of C. macrocephalus and moderately high genetic diversity within and among these populations. Therefore, these populations could still be properly managed to prevent further genetic introgression and to maintain existing genetic diversity.