GENOME-WIDE MAPPING OF UNSELECTED TRANSCRIPTS FROM EXTRAEMBRYONIC TISSUE OF 7.5-DAY MOUSE EMBRYOS REVEALS ENRICHMENT IN THE T-COMPLEX AND UNDER-REPRESENTATION ON THE X-CHROMOSOME

Mammalian embryos can only survive if they attach to the uterus (impla ntation) and establish proper maternal-fetal interactions. To understa nd this complex implantation pathway, we have initiated genomic analys is with a systematic study of the cohort of genes expressed in extraem bryonic cells that are derived from the conceptus and play a major rol e in this process. A total of 2103 cDNAs from the extraembryonic porti on of 7.5-day post-conception mouse embryos yielded 3186 expressed seq uence tags, similar to 40% of which were novel to the sequence databas es. Furthermore, when 155 of the cDNA clones with no homology to previ ously detected genes were genetically mapped, apparent clustering of t hese expressed genes was detected in subregions of chromosomes 2, 7, 9 and 17, with 6.5% of the observed genes localized in the t-complex re gion of chromosome 17, which represents only -1.5% of the mouse genome . In contrast, X-linked genes were under-represented. Semi-quantitativ e RT-PCR analyses of the mapped genes demonstrated that one third of t he genes were expressed solely in extraembryonic tissue and an additio nal one third of the genes were expressed predominantly in the extraem bryonic tissues, The over-representation of extraembryonic-expressed g enes in dosage-sensitive autosomal imprinted regions and under-represe ntation on the dosage-compensated X chromosome may reflect a need for tight quantitative control of expression during development.