Structural and functional analysis of a 0.5-Mb chicken region orthologous to the imprinted mammalian Ascl2/Mash2-Igf2-H19 region.
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.
Yokomine
Takaaki
T
Division of Human Genetics, Department of Integrated Genetics, National Institute of Genetics, Research Organization of Information and Systems, Mishima 411-8540, Japan.
Shirohzu
Hisao
H
Purbowasito
Wahyu
W
Toyoda
Atsushi
A
Iwama
Hisakazu
H
Ikeo
Kazuho
K
Hori
Tetsuya
T
Mizuno
Shigeki
S
Tsudzuki
Masaoki
M
Matsuda
Yoh-ichi
Y
Hattori
Masahira
M
Sakaki
Yoshiyuki
Y
Sasaki
Hiroyuki
H
D000595:Amino Acid Sequence / Q000235:genetics; D000818:Animals; D015703:Antigens, CD / Q000235:genetics; D001483:Base Sequence / Q000235:genetics; D002642:Chick Embryo; D002874:Chromosome Mapping / Q000379:methods*; D022202:Chromosomes, Artificial, Bacterial / Q000235:genetics; D018899:CpG Islands / Q000235:genetics; D019175:DNA Methylation; D005819:Genetic Markers / Q000235:genetics; D018392:Genomic Imprinting / Q000235:genetics*; D006801:Humans; D007335:Insulin-Like Growth Factor II / Q000235:genetics*; D008565:Membrane Proteins / Q000235:genetics; D051379:Mice; D024101:Mitochondrial Proteins / Q000235:genetics; D008969:Molecular Sequence Data; D022661:RNA, Untranslated / Q000235:genetics*; D012045:Regulatory Sequences, Nucleic Acid / Q000235:genetics; D012269:Ribosomal Proteins / Q000235:genetics; D017422:Sequence Analysis, DNA / Q000379:methods; D012689:Sequence Homology, Nucleic Acid; D020080:Tandem Repeat Sequences / Q000235:genetics; D060225:Tetraspanin 28
D015703:Antigens, CD; C554798:CD81 protein, human; C554799:Cd81 protein, mouse; D005819:Genetic Markers; D008565:Membrane Proteins; D024101:Mitochondrial Proteins; D022661:RNA, Untranslated; D012269:Ribosomal Proteins; D060225:Tetraspanin 28; D007335:Insulin-Like Growth Factor II