Chicken Special/Letters

Structural and functional analysis of a 0.5-Mb chicken region orthologous to the imprinted mammalian Ascl2/Mash2–Igf2–H19 region

    • 1 Division of Human Genetics, Department of Integrated Genetics, National Institute of Genetics, Research Organization of Information and Systems
    • 2 Department of Genetics, School of Life Science, Graduate University for Advanced Studies, Mishima 411-8540, Japan
    • 3 Division of Disease Genes, Research Center for Genetic Information, Medical Institute of Bioregulation, Kyushu University, Fukuoka 812-8582, Japan
    • 4 Human Genome Research Group, Genomic Sciences Center, RIKEN Yokohama Institute, Yokohama 230-0045, Japan
    • 5 DNA Data Analysis Laboratory, Center for Information and Biology and DNA Data Bank of Japan, National Institute of Genetics, Research Organization of Information and Systems, Mishima 411-8540, Japan
    • 6 Department of Genetic Engineering, School of Biology-Oriented Science and Technology, Kinki University, Uchita, Wakayama 649-6493, Japan
    • 7 Department of Agricultural and Biological Chemistry, College of Bioresource Sciences, Nihon University, Fujisawa 252-8510, Japan
    • 8 Laboratory of Animal Genetics, Faculty of Applied Biological Science, Hiroshima University, Higashi-Hiroshima 739-8528, Japan
    • 9 Chromosome Research Unit, Faculty of Science, and Laboratory of Cytogenetics, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-081, Japan
    • 10 Kitasato Institute for Life Science, Kitasato University, Sagamihara 228-8555, Japan
    • 11 Human Genome Center, Institute of Medical Science, University of Tokyo, Minato-ku, Tokyo 108-8639, Japan
Published December 8, 2004. Vol 15 Issue 1, pp. 154-165. https://doi.org/10.1101/gr.2609605
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Abstract

Previous studies revealed that Igf2 and Mpr/Igf2r are imprinted in eutherian mammals and marsupials but not in monotremes or birds. Igf2 lies in a large imprinted cluster in eutherians, and its imprinting is regulated by long-range mechanisms. As a step to understand how the imprinted cluster evolved, we have determined a 490-kb chicken sequence containing the orthologs of mammalian Ascl2/Mash2, Ins2 and Igf2. We found that most of the genes in this region are conserved between chickens and mammals, maintaining the same transcriptional polarities and exon–intron structures. However, H19, an imprinted noncoding transcript, was absent from the chicken sequence. Chicken ASCL2/CASH4 and INS, the orthologs of the imprinted mammalian genes, showed biallelic expression, further supporting the notion that imprinting evolved after the divergence of mammals and birds. The H19 imprinting center and many of the local regulatory elements identified in mammals were not found in chickens. Also, a large segment of tandem repeats and retroelements identified between the two imprinted subdomains in mice was not found in chickens. Our findings show that the imprinted genes were clustered before the emergence of imprinting and that the elements associated with imprinting probably evolved after the divergence of mammals and birds.

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