Toward deciphering the phylotranscriptomic hourglass in plant embryogenesis

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Embryogenesis is an essential developmental processes that enables complex multicellular life. A fundamental pattern of embryogenesis, the developmental hourglass, dates back to pioneering work by Karl Ernst von Baer on vertebrates in the early 19th century. The developmental hourglass describes a seemingly universal pattern that animal embryos from various taxa appear different in early stages, converge to a similar form during mid-embryogenesis, and again diverge in later stages. This morphological pattern has recently received molecular support by demonstrating that mid-embryogenesis is also maximally conserved on the transcriptomic level.Interestingly, the classic morphological hourglass pattern had never been reported for the plant lineage, the second kingdom that evolved embryogenesis. However, by combining phylogenetic and transcriptional information we were recently able to demonstrate the existence of a phylotranscriptomic hourglass during embryogenesis of the model plant Arabidopsis thaliana. As the last common ancestor of plants and animals was unicellular, this strongly suggested convergent evolution of the transcriptomic hourglass as a fundamental developmental profile controlling the expression of evolutionarily young or rapidly evolving genes across kingdoms. While we speculate that such a mechanism may be required for enabling spatio-temporal organization and differentiation of complex multicellular life in general, we do not yet understand the molecular origin responsible for the hourglass pattern. In this proposal, we aim to investigate whether the phylotranscriptomic hourglass pattern also exists in other plant species or whether it is specific for the model plant A. thaliana. We will address this question by generating RNA-seq data sets that cover embryogenesis in A. thaliana and two additional species, the closely related Brassicaceae Capsella rubella and the distantly related moss Physcomitrella patens. We will combine these transcriptional data with various evolutionary, phylogenomic, and sequence information to unravel factors that shape phylotranscriptomic patterns in these three species. By applying information theoretic approaches, we expect to begin to understand the molecular origin and evolution of the hourglass or possible other patterns in plant embryogenesis.