Estimates of the strain additive, maternal and heterosis genetic effects for harvest body weight of an F2 generation of Oreochromis shiranus
Introduction
Estimates of strain additive and non-additive genetic effects provide information on the choice of a breeding strategy and may also assist in the choice of paternal and/or maternal strain in a crossbreeding program. A review of literature on the estimates of strain additive genetic and strain heterosis effects shows that most of the breeding and genetic selection work in aquaculture has concentrated on estimating these effects from diallel cross experiments involving F1 populations (Gjerde, 1988, Gjerde and Refstie, 1984, Bentsen et al., 1998, Gjerde et al., 2002). Except for some work on common carp, Cyprinus carpio (Wohlfarth, 1983) and Indian major carps (Reddy, 1999) that reported the estimates of heterosis beyond the F1 generation of the species, literature on the estimates of additive and non-additive genetic effects from F2 and further generations of selection for aquaculture species is scarce. Unlike the diallel cross experiments, data from F2 and further generations allow the partitioning of the strain heterosis effects into strain individual, maternal and paternal heterosis. This partitioning of heterosis has not been reported for any fish species, but some estimates are available for other livestock species. In beef cattle, individual heterosis was found to be more important than maternal and paternal heterosis for growth rate, while the maternal heterosis increased calf weaning weight and cow longevity (Long, 1980, Greiner, 2002). In dairy cattle, the average maternal heterosis was important for body weight of heifers (Gregory et al., 1987) while paternal heterosis was important for conception rate, early sexual development and high libido in the male offspring (Dobicki et al., 2002). In swine, paternal heterosis was important for reproductive traits such as testis weight, sperm count and motility and age to first mating in the male offspring (Buchanan, 1987). In sheep, maternal heterosis was important for birth and weaning weights and litter size (Nitter, 1978). The estimation of the different components of heterosis requires properly designed experiments (Fimland, 1983). Accounting for non-additive genetic effects in genetic evaluation models is important in order to obtain unbiased estimates of additive genetic values of strains as well as individuals within strains and thus to maximise response to selection (Falconer and Mackay, 1996).
In Malawi, Oreochromis shiranus is one of the four indigenous tilapias that are being used for small-scale aquaculture. Trewavas (1983) described the species. A tilapia-breeding program for Malawi was initiated in 1996 at the National Aquaculture Centre, Domasi. The base population comprised an F1 generation that was formed from a complete diallel cross experiment with four indigenous wild strains (Maluwa and Gjerde, 2006). The results from the diallel cross showed that for harvest body weight, the strain total heterosis effect was relatively more important than the strain additive genetic effect (accounted for 15.3% and 5.3% of the total harvest body weight variance, respectively). The average heterosis effect was 12.5%. The correlation between the strain additive genetic and total performance (additive genetic plus total heterosis) of all the strain crosses was low (r = 0.25). As a result, the formation of a composite F2 generation comprising the best individuals from the best strain and strain combinations for strain additive genetic effects in the F1 base population was recommended (Maluwa and Gjerde, 2006).
The objective of this study was to estimate the magnitude of the strain additive and maternal genetic and the strain individual, maternal and paternal heterosis effects for harvest body weight of O. shiranus in the composite F2 generation.
Section snippets
Materials and methods
The F2 generation was a composite population that was formed by crossing some selected purebred and crossbred offspring of a complete diallel cross (F1 base population) between four wild strains; i.e. Shire (SR), Nkhotakota (KK), Chilwa (CI) and Chiuta (CU). The F2 generation comprised the crossbreds from the six strain cross combinations SR × KK, SR × CI, SR × CU, KK × CI, KK × CU, CI × CU and their six reciprocal crosses, and the purebreds from the Shire strain. A total of 50 males and 150 females were
Proportion of genes in the F2 generation
The percentages of individuals in the F2 generation possessing genes from the four strains with different gene frequencies are shown in Table 2. A relatively high proportion of the individuals had no genes from the Chiuta (34.6%), Nkhotakota (28.4%) or the Chilwa (27.7%) strains, while a small proportion of the individuals had no genes from the Shire strain (13.1%). A low proportion of the individuals (1.4%) had Shire genes only. Overall, the Shire strain contributed the highest proportion
Growth performance
The mean harvest body weight in the F2 generation was similar to that of the F1 generation (Maluwa and Gjerde, 2006), but substantially higher than that normally obtained (40–50 g) in mixed sex culture by small-scale fish farmers in Malawi under semi-intensive farming system and similar culture period (Maluwa et al., 1995). Males were heavier than females as also reported in the F1 base population (Maluwa and Gjerde, 2006).
Strain additive and maternal genetic effects
For the strain additive genetic effect, the Shire strain ranked highest
Acknowledgements
This study was funded by the UNDP through the World Fish Centre in Penang, Malaysia, under the project entitled; ‘Transfer of Gift Technology from South East Asia to Egypt and Sub-Saharan Africa’. Technical and Managerial assistance during the research period from Dr. Raul Ponzoni, the Project Leader for the Aquaculture and Genetic Improvement Project at the World Fish Centre, in Penang, Malaysia, is acknowledged. The data analysis and write up was carried out at AKVAFORSK, as part of PhD
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