Structural and functional analysis of a 0.5-Mb chicken region orthologous to the imprinted mammalianAscl2/Mash2–Igf2–H19region

Previous studies revealed thatIgf2andMpr/Igf2rare imprinted in eutherian mammals and marsupials but not in monotremes or birds.Igf2lies 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 mammalianAscl2/Mash2, Ins2andIgf2. 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. ChickenASCL2/CASH4andINS, the orthologs of the imprinted mammalian genes, showed biallelic expression, further supporting the notion that imprinting evolved after the divergence of mammals and birds. TheH19imprinting 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.