Short-term responses to selection for parameters of the allometric-autoregressive model

Abstract

The allometric-autoregressive model describes growth accurately and is useful in the characterization of growth responses. Hence, the potential of the model for selection purposes was investigated. Rats were used in a selection experiment, where selection was practised for slope (b) and intercept (ln a) of the allometric function: w = ln a + bv [where w = ln (body mass) and v = ln (cumulative feed intake)], and ρ, the autoregression slope of ln (cumulative feed intake). Facilities for five selection groups of 40 rats each were available. Both upward and downward within-family selections were practised for ln a and b, and only downward selection for ρ. In the short term, selection for ln a and b resulted in good direct responses, and it appears that the model can be used to alter the shape of the growth and efficiency curve by selection. The realized heritabilities correspond to the heritabilities of previous studies. Although the heritability estimate of ρ is low, a moderate selection response was realized in the short term. Only the correlated responses to selection for b seem to be of any significance during the four generations of selection. Efficiency during the growth phase in which selection was practised increased markedly (17%), while total efficiency, which includes the estimated efficiency from conception, also increased (13%). Furthermore, selection for b led to an increased growth rate (15%) and a slight increase in body mass (8%), whilst intake tended to decrease (-5%). Normally intake tends to increase as growth rate and mass increase.

Die allometriese-outoregressiemodel beskryf groei akkuraat en is bruikbaar vir die karakterisering van groeiresponsies. Gevolglik is die potensiaal van die model vir seleksiedoeleindes ondersoek. Rotte is gebruik in 'n seleksieeksperiment waarin daar geselekteer is vir helling (b) en afsnit (ln a) van die allometriese funksie: w = ln a + bv [waar w = ln (liggaamsmassa) en v = ln (kumulatiewe voerinname)], sowel as vir ρ, wat die outoregressiehelling van ln (kumulatiewe voerinname) is. Met die beskikbare fasiliteite kon vyf seleksiegroepe van 40 rotte elk geakkommodeer word. Beide opwaartse en afwaartse binne-familieseleksie is vir ln a en b uitgevoer, terwyl daar slegs vir 'n lae ρ geselekteer is. In die korttermyn het seleksie vir ln a en b tot goeie direkte responsies gelei, en dit wil voorkom asof die model gebruik kan word om die vorm van die groei- en doeltreffendheidskurwe deur seleksie te wysig. Die gerealiseerde oorerflikhede stem ooreen met die oorerflikhede van vorige studies. Alhoewel die oorerflikheidsberaming vir ρ laag is, is 'n matige seleksieresponsie oor die korttermyn gerealiseer. Slegs die gekorreleerde responsie van seleksie vir b was van betekenis gedurende seleksie oor vier generasies. Doeltreffendheid in die groeifase waarin geselekteer is het merkbaar verbeter (17%), terwyl totale doeltreffendheid, wat beraamde doeltreffendheid vanaf konsepsie insluit, ook verbeter het (13%). Verder het seleksie vir b gelei tot 'n verhoging in groeitempo (15%) en 'n geringe verhoging in liggaamsmassa (8%), terwyl inname geneig het om af te neem (-5%). Normaalweg neem inname toe met 'n toename in groeitempo en massa.

Keywords: Allometric-autoregressive model, correlated response, efficiency, growth, realized heritability, selection.