Abstract
The double fertilization of the female gametophyte initiates embryogenesis and endosperm development in seeds via the activation of genes involved in cell differentiation, organ patterning, and growth. A subset of genes expressed in endosperm exhibit imprinted expression, and the correct balance of gene expression between parental alleles is critical for proper endosperm and seed development. We use a transcriptional time series analysis to identify genes that are associated with key shifts in seed development, including genes associated with secondary cell wall synthesis, mitotic cell cycle, chromatin organization, auxin synthesis, fatty acid metabolism, and seed maturation. We relate these genes to morphological changes in Mimulus seeds. We also identify four endosperm-expressed transcripts that display imprinted (paternal) expression bias. The imprinted status of these four genes is conserved in other flowering plants, suggesting that they are functionally important in endosperm development. Our study explores gene regulatory dynamics in a species with ab initio cellular endosperm development, broadening the taxonomic focus of the literature on gene expression in seeds. Moreover, it is the first to validate genes with imprinted endosperm expression in Mimulus guttatus, and will inform future studies on the genetic causes of seed failure in this model system.
Highlights
Upon their emergence in the Early Cretaceous, seed-bearing plants diversified rapidly, displacing older plant lineages and colonizing nearly every terrestrial habitat (Lidgard and Crane, 1988; Crane and Lidgard, 1989; Magallón and Castillo, 2009)
We examined hybrid seed from a compatible cross between two members of the M. guttatus complex: a serpentine-adapted annual M. guttatus and Mimulus pardalis, a facultatively selfing annual that is fully interfertile with the outcrossing M. guttatus
The number of genes expressed in any stage was highest in ovules (16,681) and lowest in heartstage seeds (8 days after pollination (DAP): 14,784), but we found no relationship between gene expression diversity and collection time point (ANOVA F4,15 = 2.522, p = 0.085) (Figure S3)
Summary
Upon their emergence in the Early Cretaceous, seed-bearing plants diversified rapidly, displacing older plant lineages and colonizing nearly every terrestrial habitat (Lidgard and Crane, 1988; Crane and Lidgard, 1989; Magallón and Castillo, 2009). Among the many factors enabling their swift rise to dominance was the emergence of seeds, a major evolutionary and reproductive innovation that frees vascular plants from a dependence on water for gametophytic dispersal and enables the sporophytic generation to delay germination until conditions are favorable for growth and reproduction. Angiosperm seeds are formed by a unique process called double fertilization, wherein two haploid sperm nuclei contained within a pollen grain act separately to fuse with the haploid egg cell and the homo-diploid central cell of the female megagametophyte to form a diploid embryo and triploid endosperm. Angiosperm seeds undergo processes of cell differentiation, patterning, and growth. Once fully formed, the mature embryo will enter a period of developmental arrest in preparation for dormancy
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