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Alfred Ochanza Harold Maluwa - abstract fra doktorgradsavhandling
Ane Gro Siri SkjelfjordAbstract fra avhandlingen
Genetic selection for growth of a Malawian indigenous tilapia, Oreochromis shiranus / Seleksjon for tilvekst hos den malawiske tilapia-arten Oreochromis shiranus
English language abstract
The aim of this study was to generate knowledge that supports the design, implementation and monitoring of Oreochromis shiranus selective breeding program whose goal is to increase the growth of the species. Genetic evaluation of four strains of the species for growth in a diallele cross was carried out. The F1 growth performance tests were conducted at three test farm environments that represent the agro-climatic conditions of Malawi. For harvest body weight, the total heterosis accounted for more variation (15.3%) than the additive (5.3%) and general reciprocal effects (2.4%) . In addition, the correlation between strain total performance and strain additive genetic effect was low (r=0.25). High to moderate rank correlations of strain combinations between the high and medium (r=0.89), high and low (r=0.87) and medium and low (r=0.61) altitude test farms showed non-significant strain by environment interaction. A composite or synthetic F2 population was formed comprising the best individuals within strain combinations for the additive genetic effect.
In the F2, the strain additive and maternal genetic and heterosis effects for harvest body weight were estimated. The additive genetic effects accounted for more variation (8.5%) than the maternal genetic (1.2%), individual heterosis (3.4%), maternal heterosis (2.9%) and paternal heterosis (2.2%). The magnitude of the additive and non-additive effects was similar. The correlation between the additive genetic performance and total performance of the strain combinations was high (r=0.75). Another synthetic F3 population comprising the best individuals on the additive genetic effects was formed.
The magnitude of the individual genotype by environment interaction for harvest body weight in the three different test farms and genetic parameter estimates within and across the farms were estimated for the F1, F2 and F3 generations. For the harvest body weight trait, the genetic correlations between the high and medium (r=0.95±0.15), high and low (r=0.74±0.17) and medium and low (r=0.63±0.20) altitude test farms, showed a low genotype by environment interaction among the progeny groups. The magnitude of heritability estimates within farms was moderate (h2=0.25; 0.24 and 0.21) for the high, medium and low altitude farms, respectively. The age effect on harvest body weight was not significantly different from zero (P>0.05). There were consistent sex effects in each generation with males being 12-43% heavier than the females at harvest.
Genetic gain from F1 to F2 and F2 to F3 was estimated as the difference in the least squares means of the selected and control populations (realised genetic gain) and also predicted using the difference in the mean breeding values of adjacent generations (predicted genetic gain). The cumulative realised (13.2%) and predicted (14.4%) genetic gains over the two generations were not significantly different (P>0.05). The intensity of selection (0.53-0.74) was low. The results show substantial genetic gain for the two generations of selection.
Pr. desember 2005:
Alfred O. Maluwa, National Aquaculture Centre, P.O.Box 44, Domasi, Malawi
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Publisert: 01.12.08
Oppdatert: 07.01.09
Utskriftsvennlig versjon
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