The first integrated physical and genetic linkage map encompassing the entire swine chromosome 7 (SSC7) reveals that the porcine MHC (SLA) spans the centromere. A SLA class !I antigen gene lies on the q arm, whereas class I and !!1 genes lie on the p arm, suggesting that the presence of a centromere within the SLA does not preclude a functional complex. The SLA appears smaller than other mammalian MHC, as the genetic distance across two class !, three class !1, and three class !!i SLA gene markers is only I.I cM. There are significant variations in recombination rates as a function of position along the chromosome, and the SLA lies in the region with the lowest rate. Furthermore, the directed integration approach used in this study was more efficient than previous efforts that emphasized the screening of large insert libraries for random microsatellites.Genetic maps of livestock species are currently being developed to provide a worldwide resource for mapping quantitative trait loci, comparative mapping in evolutionary studies, and mapping of pertinent loci in animal models of human disorders. These objectives require a framework of informative markers that can be genotyped economically. Integration of this framework with a cytogenetic map allows evaluation of coverage, genome size, and recombination rates along the chromosome, as well as establishing syntenic relationships among species. Two linkage maps of swine chromosome 7 (SSC7) have been reported, one that includes 11 Archibald et al. 1995) and one with 17 ) microsatellite markers. The two maps have only one marker in common, and both contain intervals >30 cM. The cytogenetic map includes only four relatively low resolution assignments of linked markers with -30% coverage (Ed-

We examined the chromosomal basis for the synthesis of tissue (ovary, endometrium/placenta, and peri-implantation blastocyst) isoforms of cytochrome P450 aromatase in the pig. DNA fragments derived from three distinct porcine aromatase chromosomal genes were cloned and characterized. The porcine type III aromatase gene encoding the blastocyst aromatase isoform was found to consist of nine coding exons and two mutually exclusive, 5' untranslated exons (designated E1A and E1B), collectively spanning 30 kb or more. The porcine type II aromatase gene, encoding the endometrial/placental aromatase isoform, was identified by cloning of a genomic DNA fragment spanning the corresponding exons 7, 8, and 9. The DNA inserts of two other phage clones encompassed exons 2, 3, and 4 of a third chromosomal gene (type I) encoding the ovarian aromatase isoform. All intron-exon junctions in these genomic fragments were found to be identical in relative positions to those of the single-copy human aromatase gene. Comparisons of cDNA and genomic sequences indicated that nucleotide sequence variation was not uniform across the corresponding exons of these genes and that the corresponding intronic sequences were conserved. The type II and type III aromatase genes were localized to the same regional location (q16-17) on swine chromosome 1, which is homologous to the human chromosome 15 region (q21.1) in which the human aromatase gene resides. Results demonstrate that the three aromatase genes characterized in the present study appear to be similar in their overall structural organization and most likely are clustered, which could have resulted from at least two independent gene duplication events. The presence of multiple aromatase genes constitutes a newly described mechanism by which aromatase enzyme biosynthesis and functional activity can be regulated in a tissue and temporal fashion and serves to highlight further the complexity of aromatase gene expression in mammals. Moreover, the presence of a unique aromatase gene that is highly expressed in pig blastocysts may constitute a paradigm for other mammals (e.g., equids, rabbit, hamster) whose peri-implantation blastocysts are estrogenic.

Superoxide dismutase 1 (SOD1) was mapped to cattle chromosome 1q12→q14 by in situ methods. Both traditional in situ hybridization using tritium and a new technique, direct in-situ single copy PCR (DISC-PCR), were used in two separate laboratories. Both human and bovine SOD1 clones were tritium labeled for radioactive in situ hybridization. A primer pair based on the bovine SOD1 gene (Barendse et al., 1994b) was used for the DISC-PCR procedure. The map location of SOD1 is close to collagen 6A1. SOD1 is a potentially important type 1 anchor locus in the region where the gene for horns in cattle was recently mapped (Georges et al., 1993; Schmutz et al., 1995).