Effects of parental strains and heterosis of the hybrid between Clarias macrocephalus and Clarias gariepinus

The hybrid between Clarias macrocephalus female×C. gariepinus male has been cultured in Thailand for more than 20years, but the effects of parental strains have not been studied. In the present study, differences among three strains each of C. macrocephalus and C. gariepinus were shown by the analyses of five microsatellite loci (FST=0.174±0.037). Pairwise FST within species was significantly higher than zero (P=0.0000) for every population pair, and ranged from 0.027 to 0.041 for C. macrocephalus and from 0.087 to 0.254 for C. gariepinus. Interspecific hybridization was performed using females from C. macrocephalus and male C. gariepinus, resulting in nine hybrid crosses. Performance traits of the hybrids were compared with the offspring of three crosses each within C. macrocephalus and C. gariepinus. The experiments were done in tanks from 28 to 84days old, and in cages fixed in an earthen pond from 85 to 175days old. The results showed that the effects of both parental strains and their interaction on the absolute growth rate and specific growth rate (SGR) from 28 to 84days old were significant. Absolute growth rates during 85 to 175days were significantly affected only by strains of C. gariepinus, while neither parental strains affected SGR from 85 to 175days old of the hybrids. The effects of the parental strains and their interaction were not significant for other traits, including the survival rate from 28 to 84days old and 85 to 175days old, dress-out percentage, fillet yield, phagocytic activity and phagocytic index. It was noteworthy that the difference in traits among crosses within each genetic group (C. macrocephalus, C. gariepinus, and hybrids) was significant only for growth traits (absolute growth rates from 28 to 84days and from 84 to 175days among strains of C. gariepinus, and among hybrids; the SGR from 28 to 84days old and 85 to 175days old within C. macrocephalus, and the SGR from 28 to 84days old within the hybrids). Mean heterosis was low and not notable (<1%) in most cases, except for the survival rate from 28 to 84days old and 85 to 175days old (+2.43±2.14 and +10.71±4.71%, respectively), absolute growth rate from 85 to 175days (+10.85±11.35%), and percent fillet yield (+1.34±1.73%). Variation of heterosis among the hybrid crosses was observed. Among the nine hybrid crosses, the cross between female C. macrocephalus from Udorn Thani province (Cma-UD)×C. gariepinus from the Department of Aquaculture (Cga-AQ) showed the highest proportion of positive heterosis across ten traits (90%).