ABSTRACT: In animals, maternal gene products deposited into eggs regulate embryonic development before activation of the zygotic genome. In plants, an analogous period of prolonged maternal control over embryogenesis is thought to occur based on gene-expression studies. However, other gene-expression studies and genetic analyses show that some transcripts must derive from the early zygotic genome, implying that the prevailing model does not fully explain the nature of zygotic genome activation in plants. To determine the maternal, paternal and zygotic contributions to the early embryonic transcriptome, we sequenced the transcripts of hybrid embryos from crosses between two polymorphic inbred lines of Arabidopsis thaliana and used single-nucleotide polymorphisms (SNPs) diagnostic of each parental line to quantify parental contributions. Although some transcripts appeared to be either inherited from primarily one parent or transcribed from imprinted embryonic loci, the vast majority of transcripts were produced in near-equal amounts from both maternal and paternal alleles, even during the initial stages of embryogenesis. Results of reporter experiments and analyses of transcripts from genes that are not expressed in sperm and egg indicate early and widespread zygotic transcription. Thus, in contrast to early animal embryogenesis, early plant embryogenesis is mostly under zygotic control. Col-0 and Cvi-0 seed stocks were obtained from the Arabidopsis Biological Resource Center. Plants were grown at 22M-BM-0 C in a Conviron growth chamber with a 16-h light/8-h dark cycle. Flowers were emasculated one day before crossing, and pools of twenty 1-cell/2-cell, 8-cell and ~32-cell embryos were hand-dissected approximately 40, 64 and 78 hours after pollination, respectively. Embryo dissections, RNA isolation, linear amplification of poly(A) RNA and strand-specific RNA-Seq was as described (Nodine and Bartel, 2010, Genes & Development), except that embryos were extensively washed prior to RNA isolation. After removing adaptor sequences, reads were mapped to both the A. thaliana genome and transcript models (Col-0 genome reference, TAIR10) with the Bowtie short-read aligner. Reads were also mapped to a M-bM-^@M-^XpseudoM-bM-^@M-^Y Cvi-0 genome and Cvi-0 transcript models, in which SNPs in the Col-0 genome and transcript models were replaced with Cvi-0 variants (ftp://ftp.arabidopsis.org/home/tair/Sequences/Ecker_Cvi_snps.txt) generated by the 1,001 Genomes Project. Reads that both perfectly and uniquely matched either the genomes or transcript models were retained, and those overlapping transcribed regions of annotated genes were evaluated for overlap with SNPs. To normalize for differences in library sizes, the numbers of reads representing each transcript were divided by the total number of reads matching the genome and transcript models. SNP-overlapping reads were assigned to one of the parental genomes and tallied for each transcript, combining tallies for multiple SNPs within the same transcript.
