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Consequences of selection for improving production traits on the frequency of deleterious alleles for fitness

J. F. Kearney^*, P. Navarro, C. S. Haley and B. Villanueva^*

^* Scottish Agricultural College, West Mains Road, Edinburgh, EH9 3JG, UK , Institute of Evolutionary Biology, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JT, UK Roslin Institute (Edinburgh), Roslin, Midlothian, EH25 9PS, UK

beatriz.villanueva{at}sac.ac.uk

Abstract

In this study the effect of artificial selection on BLUP estimated breeding values for production traits on the allele frequencies of a pleiotropic QTL (quantitative trait locus) affecting both production and disease susceptibility was investigated. Stochastic simulations were used to model artificial selection on a production trait that is controlled, in part, by a biallelic QTL that also controls susceptibility to disease. The QTL allele increasing production also increased susceptibility to disease. Different modes of action and proportions of variation accounted for by the QTL were assessed for the production trait. The main results indicated that alleles that confer susceptibility to the disease can be maintained in the population over a long period of time, depending on the mode of action of the QTL. In addition, the results of the study indicate that, under various conditions, it is possible to find pleiotropic QTL that control 2 traits despite these traits appearing to be uncorrelated. Therefore, in practice, an estimate of the genetic correlation between 2 traits may be misleading when the presence of such a QTL exists. The results of this study have implications for breeding programs. For example, if a pleiotropic QTL exists that favors heterozygotes for a production trait, it would be very difficult to remove disease susceptibility alleles via traditional selection methods.

Key Words: fitness • health related traits • pleiotropy • QTL • selection