The first report on mapping the polled locus to bovine chromosome 1 (BTA1) was given by GEORGES et al. (1993). The polled locus was shown to be linked with the markers GMPOLL-1 (TGLA49) and GMPOLL-2 (AGLA17). With a somatic cell hybrid panel TGLA49 and AGLA17 were arranged near to the superoxid dismutase 1 gene (SOD1), that was mapped on BTA1 (O’BRIEN et al., 1991). The two-point linkage analysis between the two markers mentioned above and the polled phenotype revealed that the position of the polled locus was not between these two markers, but the relative position of the polled locus proximal or distal of the two microsatellite markers could not be determined. The precise position of the polled locus could not be resolved on the bovine genetic map of BTA1. A linkage analysis in five Charolais families including the markers TGLA49 and BM6438 confirmed the localization postulated by GEORGES et al. (1993) as no recombination was found between the two markers and the polled phenotype (SCHMUTZ et al., 1995). Due to the low density of
markers in the proximal region of BTA1, the relative position of the polled locus to the markers used could not be determined. BRENNEMAN et al. (1996) performed a linkage analysis with all available microsatellite markers from the whole BTA1. Only the marker TGLA49 exhibited a significant linkage with the polled phenotype. This was the first time a genetic map of the proximal region of BTA1 for the polled locus was released (Fig. 1). But the handicap of the previously described study was that there were no flanking markers proximal the polled locus. A further linkage study including the markers BM6438, TGLA49, IFNAR, KAP8, INRA212 and INRA117 from cattle chromosome 1 (HARLIZIUS et al., 1997) showed close linkage of the polled phenotype to all markers used, but as in previous studies the order of the polled locus relative to the markers used remained unclear. The localization of the polled locus was improved by using mapping results of 28 informative meiosis from the USDA reference families (U.S. Department of Agriculture). The polled locus was then mapped between the marker BM6438 and TGLA49 (Fig. 1; KAPPES et al., 1998) and released as horned/polled syndrome (HPS) in the bovine database BovMap (URL:http://locus.jouy.inra.fr/; EGGEN, 1998). EICHLER et al. (1999) analyzed 12 markers in 366 offspring of heterozygous males from German Fleckvieh, Holstein Friesian, Pinzgauer, Welsh Black and hybrids of these breeds. The polled locus could be mapped relative to four polymorphic markers covering a 4.3 cM interval. The polled locus was placed outside of this interval 2.0 cM proximal of BM6438 (Fig. 1). In a further work, BADER et al. (2001) genotyped 10 microsatellite markers in 7 half-sib families of heterozygous bulls German Fleckvieh. In agreement with KAPPES et al. (1998) the polled locus mapped between the markers BM6438 and TGLA49, whereby the order of markers was determined by radiation hybrid mapping and fixed in the linkage analysis (Fig. 1).
2.7 BRENNEMAN et al. 1996
TEXAN14 BM1312
56.2 59.7
KAPPES et al. 1998
TGLA49
Figure 1 Supposed localization of the polled locus on the proximal part of bovine chromosome 1.
In conclusion the most likely position of the polled locus on BTA1 is between the markers BM6438 and TGLA49. Although the map published by KAPPES et al. (1997) and the regularly updated MARC/USDA in the internet map (http://www.marc.usda.gov/) comprised 1250 markers with an average distance of 2,5 centiMorgan (cM) between the markers, and incorporating 2990 cM, especially the density of markers at the proximal region of BTA1 was very low. For the last ten years no new genetic markers have been published for this genomic region. So a further refinement of the position of the polled locus was not possible before the development of new markers for the polled region. The sequence data of the long arm of human chromosome 21 (HATTORI et al., 2000) opened a new perspective to increase the marker density for BTA1 through comparative gene mapping in cattle. The first two comparative maps between the proximal part of BTA1 and HSA21q22 were published by REXROAD and WOMACK (1999) and BAND et al. (2000) (Fig. 2), and a third comparative map between BTA1 and HSA21q22 followed by DRÖGEMÜLLER et al. (2002). The human bovine comparative maps of BTA1 were not consistent with regard to the existence of breakpoints and the arrangement of the markers. The reason for this may be due to the low resolution of the maps constructed. Although, construction of a genome-wide BAC contig for cattle is in progress (LARKIN et al., 2003; SCHIBLER et al., 2004), especially for the proximal part of BTA1 no new genes were identified. In a second generation comparative map constructed by EVERTS-VAN DER WIND et al. (2004) five new positions of genes were mapped in the proximal region of BTA1. The most important step towards a very high
resolution map for the proximal part of BTA1 was the construction of a bovine bacterial artificial chromosome (BAC) contig for this genomic region (Chapter 3). In total, 31 genes were assigned in this bovine BAC contig, of which 16 genes had been newly mapped. This contig provides a valuable tool towards the determination of the position of the polled locus ing and analysing the genes mapped in the contig. This bovine BAC contig built was facilitated by the fact that in the last years more and ore expressed sequence tags (EST) were released in the bovine databases (TAKASUGA et
e mentioned markers. As all ulls and their progeny exhibited the same haplotype for the markers BM6438 and SOD1MICRO2 segregating with the polled alleles, the conclusion was drawn that only the better two markers would be necessary for an i direct gene test for polledness. The reliability of this indirect gene test was not tested in larger samples or breeds other than German on cattle chromosome 1 and the basis for sequenc
m
al., 2001; STONE et al., 2002) and the sequence data of the syntenic region of HSA21q22.