The big-oui hybrid catfish (female C. macrocephalus x male C. gariepinus) was successfully achieved using artificial hybridization. This hybrid combines the superior taste of the C. macrocephalus with the faster growth rate and higher resistance to environmental conditions of the C. gariepinus and is now the biggest fresh-water aquaculture product in Thailand. In this thesis the results of experiments involving hybridization and genetic manipulation were used to investigate the nature of the hybridization event. This information was used to develop broodstock for Clarias catfish breeding programme. Allozyme studies resolved 18 protein loci encoding different enzyme systems in C. batrachus, C. macrocephalus, C. gariepinus and the big-oui hybrid. GPI-2*, MDH-2* and LDH-l* show clearly variation between the hybrid and the parental species and were used for species diagnostic loci. Comparisons of fertilisation and survival of the big-oui hybrid, reciprocal cross hybrid, F2 hybrid, back cross hybrid and their parental species were carried out. The only cross involving the F 1 big-oui hybrid still gave viable embryos and fry was between female F 1 hybrid and male C. gariepinus. The F2 hybrid never developed to hatch. Karyotyping studies showed a modal chromosome number of 2n=54 in C. macrocephalus; 2n=56 in C. gariepinus; 2n=55 in the big-oui hybrid, the reciprocal hybrid and the back cross hybrid. Using male C. gariepinus from two other stocks ('Malawee' and 'Wageningen') were carried out to produce the big-oui hybrid. Cold shock at 2°C administered for 15 mins duration and 4 mins after fertilisation was the most effective in inducing 100% triploidy in big-oui hybrid while heat and pressure shock were less effective. Growth performance of diploid and triploid hybrids was not significantly different. The triploid hybrid were shown to be functionally and endocrinologically sterile. Gynogenetic diploids were produced by fertilizing C. macrocephalus eggs with C. gariepinus sperm that had been genetically inactivated with ultraviolet (UV) light, and then cold shocking the eggs after fertilisation. The UV dose of 200 Il W cm -2 for 2 mins using a sperm concentration of 2.5 x 108 mrl was optimal in genetically inactivating sperm without seriously compromising motility. Cold shock at 2°C, started at 4 mins after fertilisation and 15 mins duration gave the maximum number of meiotic gynogenetic offspring. The parental contribution in the gynogenetic offspring was check by using the species diagnostic allozyme loci and showed no evidence of male contribution. Gynogenetic offspring were grown on to investigate the sex ratio. All gynogenetic offspring were female suggesting female homogamety in this species. The possible implications of the above results of hybridization and genetic manipulation studies in Clarias catfish culture are discussed.

D.Sc. / Please refer to full text to view abstract