Genetic evaluation of four strains of Oreochromis shiranus for harvest body weight in a diallel cross

Estimates of strain additive genetic, total heterosis and general reciprocal effects were obtained from a complete diallel cross with four strains (Shire, Nkhotakota, Chilwa and Chiuta) of Oreochromis shiranus in Malawi. Growth was recorded at harvest on 367 individually tagged progeny from 16 strain combinations after a grow-out period of 180 days across three test environments that represented different agro-climatic zones of the country. Each of the 16 strain combinations was produced from 3 to 5 five single-pair matings; i.e., 6-10 parents per strain combination. In total, the number of contributing sires per strain ranged from 14 to 16 and the number of contributing dams also from 14 to16. Across the test environments, the mean body weight was 75 g and the males had a significantly higher body weight at harvest than the females (23%; P < 0.05). The strain combination by test environment interaction effect was not significantly different from zero (P > 0.05) meaning that with respect to the harvest body weight trait, there is no need to develop specific strains for the different agro-climatic conditions of the country. The strain total heterosis effect accounted for 15.3% (P < 0.001) of the total variance for harvest body weight and was relatively more important than the strain additive genetic (5.3%; P < 0.001) and the strain reciprocal (2.4%; P > 0.05) effects. For the strain additive genetic effect, Shire was the best and Chiuta the poorest strain, performing 16.9% above and 10.3% below the mean of the purebred strains, respectively. Five out of the six strain crosses showed a significant positive total heterosis effect ranging from 12.3% to 17.0% above the mean body weight of the purebred strains. The average heterosis effect was 12.5%. The correlation between the strain additive genetic and the strain additive genetic plus total heterosis performance of the six strain crosses was low (r = 0.25). The results show that for harvest body weight there are substantial additive genetic and total heterosis differences among pure and crossbred strains of O. shiranus. The exploitation of these genetic differences in a breeding program is discussed.