Hybridization alters maternal and paternal genome contributions to early plant embryogenesis

After fertilization, zygotic genome activation results in a transcriptionally competent embryo. Hybrid transcriptome experiments in Arabidopsis have concluded that the maternal and paternal genomes make equal contributions to zygotes and embryos, yet embryo defective (emb) mutants in the Columbia (Col) ecotype display early maternal effects. Here, we show that hybridization of Col with Landsberg erecta (Ler) or Cape Verde Islands (Cvi) ecotypes decreases the maternal effects of emb mutants. Reanalysis of Col/Ler and Col/Cvi transcriptomes confirmed equal parental contributions in Col/Cvi early embryos. By contrast, thousands of genes in Col/Ler zygotes and one-cell embryos were biallelic in one cross and monoallelic in the reciprocal cross, with analysis of intron reads pointing to active transcription as responsible for this parent-of-origin bias. Our analysis shows that, contrary to previous conclusions, the maternal and paternal genomes in Col/Ler zygotes are activated in an asymmetric manner. The decrease in maternal effects in hybrid embryos compared with those in isogenic Col along with differences in genome activation between Col/Cvi and Col/Ler suggest that neither of these hybrids accurately reflects the general trends of parent-of-origin regulation in Arabidopsis embryogenesis.