RT Journal
A1 Yokomine, Takaaki
A1 Shirohzu, Hisao
A1 Purbowasito, Wahyu
A1 Toyoda, Atsushi
A1 Iwama, Hisakazu
A1 Ikeo, Kazuho
A1 Hori, Tetsuya
A1 Mizuno, Shigeki
A1 Tsudzuki, Masaoki
A1 Matsuda, Yoh-ichi
A1 Hattori, Masahira
A1 Sakaki, Yoshiyuki
A1 Sasaki, Hiroyuki
T1 Structural and functional analysis of a 0.5-Mb chicken region orthologous to the imprinted mammalian Ascl2/Mash2–Igf2–H19 region
JF Genome Research 
JO Genome Research 
YR 2005 
FD January 01 
VO 15 
IS 1 
SP 154 
OP 165 
DO 10.1101/gr.2609605 
UL http://genome.cshlp.org/content/15/1/154.abstract 
AB 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.