Crossing of African catfish, Clarias gariepinus (Burchell, 1822), strains based on strain selection using genetic diversity data

African catfish, Clarias gariepinus (Burchell, 1822), was introduced to Thailand for aquaculture in 1987. The stocks have been maintained without genetic management, which may be responsible for the deterioration of economically important traits observed by farmers. One possible solution is crossbreeding between genetically distinct strains. The present study employed six microsatellite loci to elucidate genetic diversity of four African catfish strains in Thailand (Suphanburi, Angthong, Nakornpathom and Nakornsawan). The results revealed two distinct groups of strains (Nakornpathom-Nakornsawan and Suphanburi-Angthong) as shown by FST of 0.1444 (P < 0.01), RST of 0.2195 (P < 0.01) and a phylogenic tree. Genetic variation within populations was moderate (A = 4.67–12.17; Ae = 3.15–5.80; Ar = 4.67–9.65; Ho = 0.50–0.69; He = 0.67–0.80). Crossbreeding between a representative population of the two genetically distinct groups (Angthong and Nakornpathom, nine males and nine females from each strain) showed improved mean phagocytosis activity of the crossbreds over the mean of the parents (heterosis = 70.97%; P < 0.01). Nevertheless, no differences between the crossbreds and the parents were observed in other traits, including body weight and length, the specific immune response to Aeromonas hydrophila (antibody titre) at the peak of response, phagocytic index and male reproductive performance. It was surmised that the immune genetic variation within the crossbreds was increased and may be responsible for the slight enhancement of the immune response. The variation of the MHC class I gene of the crossbred was higher than one parental line, but was not different from the other parental line.