PhD Candidate University of Georgia Statham, Georgia
Body of Abstract: Plants alternate between multicellular haploid and diploid generations. The sporophyte-to-gametophyte transition (SGT) is a transition from sporophytic to gametophytic expression that occurs during gametophyte development. The SGT is defined by global changes in steady state transcript levels, and can be studied in the context of genome activation. In maize, widespread gametophyte genome activation occurs between late unicellular microspore and bicellular microspore stages, setting the timing of the SGT around pollen mitosis I (Nelms & Walbot, 2022). In male gametophytes, the SGT results in pollen that actively transcribe genes from their haploid genomes, causing them to be exposed to haploid selection and express phenotypes. The timing of the SGT determines the timing and scope of haploid selection, and yet it remains unknown whether the timing is conserved across angiosperms or variable. To address this, we are determining the timing of the SGT in three species spanning the diversity of angiosperm evolution: Arabidopsis, tobacco, and rice. We are using allele-specific RNA-sequencing of single pollen precursors from F1 hybrids, paired with developmental staging via DAPI (4′,6-diamidino-2-phenylindole) staining. Using F1 hybrids allows us to use SNPs in the RNA-seq data to discern whether each pollen precursor had transcripts from one allele or both. Transition from biallelic to monoallelic expression indicates that the haploid gametophyte genome has activated, and the SGT has occurred. Currently, we are preparing RNA-sequencing libraries for Arabidopsis and tobacco and crossing rice inbred lines to produce F1 hybrids. This research will fill fundamental knowledge gaps in the current understanding of the haploid generation of plants, especially regarding the timing and scope of haploid selection, which is relevant to breeding practices and agriculture.