Through twelve generations of selection, the GIFT breeding program proved to be very successful in increasing the mean level of harvest weight in tilapia. However, it is often desirable not only to improve the mean level of traits, but also to change their variability. In farmed tilapia, individuals display large size differences within a population, but the genetic background of this variability is almost entirely unknown. Selection for more uniform individuals is possible only in the presence of genetic variation in trait variability. This phenomenon is known as genetic heterogeneity of environmental (residual) variance. The aim of this study was to investigate the potential for genetic improvement in variability of harvest weigh in the GIFT strain of Nile tilapia, by applying double hierarchical generalized linear models. This approach offers simultaneous estimation of genetic parameters for the mean and for the residual variance, which is modelled on the exponential scale. Both untransformed and Box-Cox transformed harvest weight were analysed to study the impact of non-normality on estimates of variance components. Analysis involved 6090 fish with records on harvest weight. The estimated heritability of harvest weight had moderate value of 0.44, which increased to 0.46 after the transformation. We found substantial genetic variation in the residual variance of 0.41 which reduced to 0.27 after the Box-Cox transformation. The given estimates are on a log scale. The genetic coefficients of variation for residual variance were 64% and 52% on untransformed and transformed scale respectfully. These values describe the potential decrease in residual variance when achieving one genetic standard deviation by selection. The genetic correlation between mean and variance was 0.67 for untransformed harvest weight, which points to a strong positive relationship between both traits. The genetic correlation for Box-Cox transformed harvest weight was 0.12. The results obtained in this study indicate a good opportunity to improve variability of harvest weight. The high genetic correlation of 0.67 between harvest weight and residual variance suggests necessity for index selection in order to increase harvest weight and reduce its variation at the same time.
|Title of host publication||Abstract Book ISGA XII - The International Symposium on Genetics in AquacultureI-Abstract Book|
|Publication status||Published - 2015|
|Event||ISGA XII, Santiago de Compostella, Spain - |
Duration: 21 Jun 2015 → 27 Jun 2015
|Conference||ISGA XII, Santiago de Compostella, Spain|
|Period||21/06/15 → 27/06/15|
Marjanovic, J., Mulder, H. A., Khaw, H. L., & Bijma, P. (2015). Genetic parameters for uniformity of harvest weight in the gift strain of nile tilapia estimated using double hierarchical generalized linear models. In Abstract Book ISGA XII - The International Symposium on Genetics in AquacultureI-Abstract Book (pp. 71-71)