Analysis of response to 20 generations of selection for body composition in mice: fit to infinitesimal model assumptions

Abstract

-Data were analysed from a divergent selection experiment for an indicator of body composition in the mouse, the ratio of gonadal fat pad to body weight (GFPR). Lines were selected for 20 generations for fat (F), lean (L) or were unselected (C), with three replicates of each. Selection was within full-sib families, 16 families per replicate for the first seven generations, eight subsequently. At generation 20, GFPR in the F lines was twice and in the L lines half that of C. A log transformation removed both asymmetry of response and heterogeneity of variance among lines, and so was used throughout. Estimates of genetic variance and heritability (approximately 50%) obtained using REML with an animal model were very similar, whether estimated from the first few generations of selection, or from all 20 generations, or from late generations having fitted pedigree. The estimates were also similar when estimated from selected or control lines. Estimates from REML also agreed with estimates of realised heritability. The results all accord with expectations under the infinitesimal model, despite the four-fold changes in mean. Relaxed selection lines, derived from generation 20, showed little regression in fatness after 40 generations without selection. Les données provenant d'un programme de sélection divergente ontété analysées pour un indicateur de la composition corporelle des souris : la proportion de tissus adipeux gonadal par rapport au poids corporel (GFPR). Trois répliques de chacune des lignées ontété sélectionnées pendant 20 générations pour l'engraissement (F), la minceur (L), ou non sélectionnées. La sélection fut réalisée dans des familles de pleinfrères, 16 familles par réplique durant les sept premières générations et huit pour les suivantes. A la vingtième génération, le GFPR des lignées (F) et (L)étaient respectivement le double et la moitié de celui de (C). Une transformation logarithmique * Correspondence and reprints E-mail: Victor.Martinez@ed.ac.uk 4 V. Martinez et al.permet de supprimer l'asymétrie de la réponse et l'hétérogénéité des variances entre ces deux lignées. Les estimateurs de la variance génétique et de l'héritabilité (approximativement de 50 %) obtenus par le REML avec un modèle animal sont semblablesà ceux obtenus en utilisant les premières générations de sélection, les 20 générations de sélection ou les dernières en employant l'information sur le pedigree jusqu'à la population de base. De plus, en utilisant les lignées sélectionnées et les lignées de contrôle, les estimateurs sont similaires. Les estimations REML sont conformesà celles de l'héritabilité. Tous les résultats sont conformesà ceux attendus sous un modèle infinitésimal malgré une variation de quatre fois la moyenne. Les lignées soumisesà une pression de sélection plus faibleà la vingtième génération, montrent peu de diminution en engraissement après 40 générations sans sélection.sélection / modèle infinitésimal / variance génétique / composition corporelle / souris

The effect of non-additive genetic interactions on selection in multi-locus genetic models

“…But, if smaller family-based pedigrees are used, differences in genetic variance components could more easily be detected (e.g. Martinez et al, 2000). The change in genetic variances in this study would probably have been smaller if we had based our calculations on the whole pedigree, but to what degree is difficult to say.…”

“…However, some theoretical studies of selection suggest that the level of additive variance can be sustained or even increased when non-additive variance is present (Gimelfarb 1989;Fuerst et al, 1997;Jannink, 2003;Carter et al, 2005), in a manner similar to the action of genetic drift. Experimental evidence for this phenomenon was found by Martinez et al (2000) when they selected for body fat in mice, and by Sorensen and Hill (1982) who performed a short-term selection experiment for abdominal bristle number in D. melanogaster. Furthermore, in a recent study, Carlborg et al (2006) showed that epistatic interactions between four loci mediated a considerably higher response to selection of growth in chicken than predicted by a single-locus model.…”

Applications of Population Genetics to Animal Breeding, from Wright, Fisher and Lush to Genomic Prediction

“…Results of our analyses differed somewhat according to the trait selected: we obtained a near-perfect fit (for log-transformed data) to infinitesimal model assumptions for selected lines of mice differing fourfold in a fat measure between high and low after 20 generations (Martinez et al 2000), but selection for body weight from the same founder stock revealed a QTL with large effect on the X chromosome (Rance et al 1997) and poorer fit.…”

On flexible finite polygenic models for multiple-trait evaluation

“…TIM has been successfully applied in animal breeding and proved to be robust when analysing long-term selection responses (e.g. Martinez et al, 2000). Thompson & Skolnick (1977) proposed a finite polygenic model (FPM) to estimate genetic variance components by fitting a finite number of unlinked polygenic loci that describe the genetic covariance among pedigree members.…”