Genetic analysis of specific and non-specific immune response in Oreochomis niloticus L.

Abstract

Gynogenesis or androgenesis were used to produce completely homozygous inbred tish, from which clonal lines were established by subsequent gynogenesis, androgenesis and sib mating. Homozygosity of the inbred lines was verified using multilocus DNA fingerprinting and isozyme locus ADA*, which showed all gynogenetic offspring to have only maternal inheritance. DNA fingerprints of meiotic and mitotic gynogens showed bands consistent with those of their mother but not of their father. Inbred clones also showed identical banding patterns with their mitotic mother and not their father. The outbred clones, in contrast, shared bands with both parents according to Mendelian inheritance. Sex ratios of both meiotic and mitotic gynogenetics were analysed and a significantly higher number of females (P<0.05) were found in both gynogenetic groups. All inbred clones, except one line, were 100% females. MHC class II B genotypes of the clonal lines were determined by PCR. Scale grafting was carried out between clonal lines where two sets of grafts were reciprocally exchanged. Both .sets of grafts were completely rejected by reciprocal recipient fish. The mean survival time of the .second set of grafts was significantly shorter than that of the first set. This suggested that the differences in the MHC haplotypes of different clonal lines exerted strong alloantigenic effects on the foreign grafts which led to their rejection and memory in immune system. Non-specific immune re.spon.se of different inbred and outbred clonal lines was examined by analysing an array of immunological parameters. Significantly different SCI urn lysozyme activity (P<0.05) was found between inbred clonal lines. The outbred clonal lines showed an intermediate level of lysozyme activity to that of their parents. Significant differences were found between the number of macrophages containing 0 and > 10 phagocytosed bacteria between the different groups. In the artificial challenge, significantly different susceptibility to A. hydrophilci infection was found between inbred clonal lines. The outbred clonal lines showed an intermediate infection level between that of their parents, and their disease resistance was higher than that of the lea.st resistant parent. The artificial challenge results revealed that there was a genetic difference in the disease resistance between different clonal groups of fish. It also suggested that disease resistance of an outbred population might be improved by crossbreeding.