Estimates of additive genetic effects, maternal effects, specific combining ability, maternal heterosis, and egg cytoplasm effects for cold tolerance in Oreochromis niloticus (L.)

Abstract

. Cold tolerances of Auburn University-Egypt and Auburn University-Ivory Coast strains of Oreochromis niloticus (L.) and their F1. F2, and backcross hybrids were analysed by multiple regression to estimate strain additive genetic effects, strain maternal effects, specific combining ability (individual heterosis), maternal heterosis, and strain egg cytoplasm effects for cold tolerance. The Egypt strain was more cold tolerant than the Ivory Coast strain. Cold tolerance of F1 hybrids was intermediate between those of the parental strains. Heterosis for cold tolerances of F1, F2, and backcross hybrids was -1.39%, -0.83%, and -2.08% respectively. Additive genetic effects had the greatest effect on cold tolerance; Egypt genes for cold tolerance were superior to Ivory Coast genes for cold tolerance. Regressions for specific combining ability and maternal heterosis were small and were not different from zero, which explains why F1, F2, and backcross hybrids exhibited no hybrid vigour for cold tolerance. Cold tolerance in O. niloticus is controlled by additive rather than by dominance genetic variance; consequently, selection is the breeding technique that should be used to maximize cold tolerance. Regressions were used to predict cold tolerances of all possible Egypt-Ivory Coast hybrids. Predicted cold tolerances suggested that no possible Egypt-Ivory Coast hybrid would be more cold tolerant than the Egypt strain.