Dairy Cattle by Exploiting Progeny Testing

We have exploited “progeny testing” to map quantitative trait loci (QTL) underlying the genetic variation of milk production in a selected dairy cattle population. A total of 1,518 sires, with progeny tests based on the milking performances of > 150,000 daughtersjointly, was genotyped for 159 autosomal microsatellites bracketing 1645 centimorgan or approximately two thirds of the bovine genome. Using a maximum likelihood multilocus linkage analysis accounting for variance heterogeneity of the phenotypes, we identified five chromosomes giving very strong evidence (LOD score 2 3) for the presence of a QTL controlling milk production: chromosomes 1 , 6, 9, 10 and 20. These findings demonstrate that loci with considerable effects on milk production are still segregating in highly selected populations and pave the way toward marker-assisted selection in dairy cattle breeding. I N dairy cattle, milk yield and composition are typical polygenic traits. Phenotypes are continuously distributed and reflect the joint action of large numbers of polygenes or quantitative trait loci (QTL) confounded with environmental effects. In the populations of interest, milk production traits have narrow sense heritabilities in the 25-50% range ( PEARSON et al. 1990). Despite early efforts to map QTL for milk production using small numbers of genetic markers ( e.g., GELDERMANN et al. 1985; COWAN et al. 1990; HOESCHELE and MEINERT 1990; BOVENHUIS 1992; ANDERSON-EKLUND and RENDEL 1993; SCHUTZ et al. 1993), the nature of the genes underlying the genetic variance of milk production remains essentially unknown. Since the discovery of microsatellite markers that can be typed using the polymerase chain reaction (WEBER and MAY 1989) , a systematic genetic dissection of milk production and other production traits in livestock has become feasible. Characterization of these QTL may lead to more efficient breeding programs using marker-assisted selection ( SOLLER and BECKMAN 1982) and may contribute to a better understanding of lactational physiology. Most successful QTL mapping efforts described to date have exploited F2 or backcrosses obtained from parental populations divergent for the traits of interest (e .g . , PATEWON et al . 1989; HILBERT et al. 1991 ) . Although a similar approach might help to understand Caespondingauthw: Michel Georges, Department of Genetics, Faculty of Veterinaly Medicine, B43, University of Litge, 20, Boulevard de Colonster, 4000 Litge (Sart Tilman) , Belgium. E-mail: [email protected] Genetics 139 907-920 (Februq , 1995) the genetic differences between high and low producing breeds, our objective was to map QTL segregating within elite dairy cattle populations, as these are the molecular substrate of ongoing selection programs. However, because these populations have been intensely selected for milk production, it is generally assumed that polygenes with large effects are near or at fixation, whereas those still segregating are believed to have minor effects. As the individual contribution of such QTL to the overall phenotypic variance would be modest, their mapping is considerably complicated. Recently, however, a number of strategies have been proposed to increase the power of QTL mapping. These strategies include selective genotyping (LANDER and BOTSTEIN 1989), progeny testing (LANDER and BOTSTEIN 1989), interval mapping (LANDER and BOTSTEIN 1989), the simultaneous search for multiple QTL (LANDER and BOTSTEIN 1989) , the use of DNA pools ( ARNHEIM et al. 1985) and the study of diseasetagged QTL ( GEORGES et al. 1993). In this work, we illustrate the use of “progeny testing” in combination with interval mapping to map QTL controlling milk production in an elite Holstein dairy cattle population selected intensely for increased milk production for several generations. MATERIALS AND METHODS Exploiting progeny testing: the “granddaughter design”: During the last 20 years, annual milk production per cow in the United States has increased from -4,500 to 6,800 kg. This remarkable progress, which in recent years is mainly genetic in nature (PEARSON et al. 1990), is due to the extensive use M. Georges et al. 908