Mixed model analysis of a selection experiment for food intake in mice

Abstract

Data from 23 generations of mice selected for increased and reduced appetite were analysed by Restricted Maximum Likelihood fitting an animal model with litters as additional random effects. Traits considered were food intake between 4 and 6 weeks of age adjusted for 4-week body weight (AFI), the selection criterion, and body weight at 6 weeks (6WW). Selection was carried out within families. A high and a low selection line and a control were maintained in each of three replicates. Analyses were performed for each replicate separately taking subsets of the data spanning different numbers of generations. Overall estimates of heritabilities were 0-15 for AFI, which agreed well with realized heritability estimates, and 0-42 for 6WW. The litter variance, expressed as a proportion of the phenotypic variance, was 0-21 for both traits, yielding intraclass correlations of full-sibs of 0-29 and 0-42, respectively. Similar results were obtained for variances of each trait using univariate and multivariate analyses. From the latter, estimates of correlations between the two traits were 0-46 for additive genetic, -019 for litter and 0-31 for residual effects, resulting in a phenotypic correlation of 0-23. Analyses of data from generations 2-7, 8-13 and 14-23 separately showed a marked decrease in genetic variance and heritability in later generations for both traits. Heritabilities of AFI, for instance, were 0-24, 010 and 007, respectively. These changes could not be attributed to the effects of inbreeding or of selection in an infinitesimal model and suggested that some change in variance due to change in gene frequency had occurred during the course of the experiment.

Long-term selection experiment with Afrikaner cattle 3. Selection applied and response in calf growth traits

“…In theory, REML techniques using an animal model and complete relationship matrix yield unbiased estimated of base population parameters (Johnson et al, 1995). However, Meyer & Hill (1991) found that REML analyses did not account for a decrease in genetic variance in the analyses of 23 generations of selection in mice. …”

Persistence of Changes in the Genetic Covariance Matrix After a Bottleneck

“…Bryant and Meffert (1988) found that G changed in shape following population bottlenecks in the housefly Musca domestica, and we have shown that G can change drastically as a result of population bottlenecks in D. melanogaster (Phillips et al 2001). Many other studies have shown changes in the additive genetic variance during selection (Wilkinson et al 1990;Meyer and Hill 1991; Beniwal et al 1992a,b;Shaw et al 1995) or after inbreeding or population bottlenecks (Frankham 1980;Bryant et al 1986;Briscoe et al 1992; Whitlock and Fowler 1999 and references therein). These changes in G after inbreeding may be transient, though, because the evolutionary forces such as selection and mutation that act to determine G may return it to its previous shape (although see Camara et al 2000).…”

Evolution of the additive genetic variance–covariance matrix under continuous directional selection on a complex behavioural phenotype

“…In this case, the animal model may not yield a strict estimate of G in the base parental population, but one that is 'biased' towards subsequent generations for which data are included. Further research is required to better understand exactly how animal models perform in such situations [25]. To ensure the differences found were attributable to selection per se and not genetic drift, we also ran MCMC models on a line-by-line basis, which yielded a total of 24 G matrices for comparison (i.e.…”