The Evolutionary Foundation Of Genomic Imprinting In Lower Vertebrates

In mammals,genomic imprinting confers developmental asymmetry and complementation on the parental genomes and makes both parental genomes essential for complete development. Genomic imprinting is,therefore,the first regulatory step of genome-wide gene expression of embryogenesis and thought to be the epigenetic foundation of bisexual reproduction.However, how the genomic imprinting is originated,established and maintained during vertebrate evolution remains unknown. Because no endogenous imprinting gene has been identified in non-mammalian vertebrates,genomic imprinting is thought to be a unique evolutionary event of mammals.Here,in order to study the evolutionary origin of genomic imprinting in vertebrates, we examined whether parent-specific methylation occurred in the teleost homologue of mammalian imprinting gene during gametogenesis.Investigating genomic imprinting or the evolutionary foundation of genomic imprinting in the lowest vertebrates would provide insights into how genomeic imprinting was originated,established and maintained during vertebrate evolution. The main contents,results and conclusions of this research are as follows:1.A 1.2 kb sequence of goldfish Igf2 was obtained by 5' genome walking.Sequence analysis showed that the sequence in the open reading frame(ORF) region was identical to the known sequence of goldfish Igf2 and BLAST analysis confirmed that this sequence was the 5' flanking sequence of the goldfish Igf2 gene.This 5' flanking sequence of the goldfish Igf2 was deposited in GenBank(FJ410930).CpG island of the goldfish Igf2 gene was searched and localized in the known goldfish Igf2 sequence,including the known 5' flanking region and the encoding region,with online software Methprimer according to the criteria of a CpG island.CpG-rich region was not identified in the promoter region but a dense cluster of CpG dinucleotides (12 CpG sites) covering the whole exon3 of goldfish Igf2 was found.2.Bisulfate sequencing analysis showed that,as mammalian Igf2 CpG island,goldfish Igf2 CpG island was a parental differentially methylated region(DMR) that was hypermethylated in sperm but unmethylated in eggs.Unlike mammalian imprinting gene DMR,however,the parent-specific methylation pattern of goldfish Igf2 DMR was not maintained during embryogenesis,suggesting that the parent-specific methylation of goldfish Igf2 DMR might be a primitive genomic imprinting in the early period of vertebrate evolution.These results indicate that the evolutionary foundation of genomic imprinting exists in lower vertebrates and genomic imprinting should not be considered as a unique evolutionary event of mammals.