A Chinese scientist group working in College of Life Science, Zhejiang University, has shown that, as mammalian Igf2 CpG island, goldfish Igf2 CpG island has a parental differentially methylated region (DMR). 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. The study is reported in volume 54 (Issue 8, April, 2009) of Chinese Science Bulletin.
Genomic imprinting has been identified in eutherians, marsupials and flowering plants, but no endogenous imprinted gene has been reported in other vertebrates and invertebrates. Therefore, genomic imprinting is thought a unique phenomenon of placental mammals and a regulatory mechanism to keep a balance of supply and demand for maternal nutrients between mother and fetus in mammalian species. However, how genomeic imprinting was originated, established and maintained during vertebrate evolution remains unclear.
In this study, the authors examined the methylation status of the teleost homologue of mammalian imprinting gene Igf2 during gametogenesis. Their results showed that, as mammalian Igf2 CpG island, goldfish Igf2 CpG island was a parental differentially methylated region that was hypermethylated in sperm but unmethylated in eggs. The authors also indicated that, unlike mammalian imprinting gene DMR, the unmethylated DMR of maternal allele could not resist methylation reprogramming and started de novo methylation at the cleavage stage, while the paternal Igf2 allele appeared to maintain its paternal methylation imprints during embryogenesis. These results suggested that the parent-specific methylation of goldfish Igf2 DMR might be a primitive genomic imprinting in the early period of vertebrate evolution.
The investigation "provided the first molecular evidence that the evolutionary foundation of genomic imprinting exists in lower vertebrates", said one of the journal reviewer.
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