Viviparity is sufficient for the evolution of genomic imprinting.
DESCRIPTION
Genomic imprinting is an epigenetic phenomenon whereby one parental allele is preferentially expressed. Despite the assumption that extraembryonic annexes are necessary for the innovation of genomic imprinting, the evolutionary conditions under which embryonic genomic imprinting can evolve are unclear.
We use the viviparous model Marchantia polymorpha to assess whether imprinting can occur in embryos that develop inside the mother but lack complex extraembryonic tissues.
Contrasting with the limited number of imprinted loci in flowering plants and mammals, the paternal genome is globally repressed in Marchantia embryos. Silencing is mediated by Polycomb-deposited methylation of histone H3 at lysine 27 after gametogenesis but before karyogamy.
Regulation of this new form of imprinting is essential for embryonic development. This report provides the proof of principle that imprinting can regulate embryogenesis in a larger range of viviparous organisms and extends our view on the diverse forms which imprinting can take.
