Animal breeding programs have been very successful in improving the mean levels of traits through selection. However, in recent decades, reducing the variability of trait levels between individuals has become a highly desirable objective. Reaching this objective through genetic selection requires that there is genetic variation in the variability of trait levels, a phenomenon known as genetic heterogeneity of environmental (residual) variance. The aim of our study was to investigate the potential for genetic improvement of uniformity of harvest weight and body size traits (length, depth, and width) in the genetically improved farmed tilapia (GIFT) strain. In order to quantify the genetic variation in uniformity of traits and estimate the genetic correlations between level and variance of the traits, double hierarchical generalized linear models were applied to individual trait values.
Catla catla (Hamilton) fertilised spawn was collected from the Halda, Jamuna and Padma rivers in Bangladesh from which approximately 900 individuals were retained as 'candidate founders' of a breeding population. These fish were fin-clipped and genotyped using the DArTseq platform to obtain, 3048 single nucleotide polymorphisms (SNPs) and 4726 silicoDArT markers. Using SNP data, individuals that shared no putative parents were identified using the program COLONY, i.e. 140, 47 and 23 from the Halda, Jamuna and Padma rivers, respectively. Allele frequencies from these individuals were considered as representative of those of the river populations, and genomic relationship matrices were generated. Then, half-sibling and full-sibling relationships between individuals were assigned manually based on the genomic relationship matrices. Many putative half-sibling and full-sibling relationships were found between individuals from the Halda and Jamuna rivers, which suggests that catla sampled from rivers as spawn are not necessarily representative of river populations. This has implications for the interpretation of past population genetics studies, the sampling strategies to be adopted in future studies and the management of broodstock sourced as river spawn in commercial hatcheries. Using data from individuals that shared no putative parents, overall multi-locus pairwise estimates of Wright's fixation index (FST) were low (≤ 0.013) and the optimum number of clusters using unsupervised K-means clustering was equal to 1, which indicates little genetic divergence among the SNPs included in our study within and among river populations.