A multiplexed transcriptomic analysis of a plant embryonic hourglass

Zoologists have adduced morphological convergence among embryonic stages of closely related taxa, which has been called the phylotypic stage of embryogenesis. Transcriptomic analyses reveal a phylotypic hourglass pattern of gene expression during plant as well as animal embryogenesis, characterized by the accumulation of evolutionarily older and conserved transcripts during mid-embryogenesis, whereas younger less-conserved transcripts predominate at earlier and later embryonic stages. However, widespread comparisons of embryonic gene expression across animal phyla describe an inverse hourglass pattern, where gene expression is correlated during early and late stages but not during mid-development. Here, multiplexed spatial-transcriptomic analyses is used to investigate embryo ontogeny and homology in maize, which has novel, grass-specific morphology. An embryonic-organ genetic network is identified, replete for ancient/conserved genes manifesting a phylotypic hourglass during mid-embryogenesis. Transcriptomic comparisons of grass embryo ontogeny with that of a moss Physcomitrium patens identify a conserved, inverse hourglass pattern across plant phyla, as in animals. The data indicate that the plant phylotypic stage and that of animals is characterized by ancient gene network expression during early histo- and morphogenesis and polarized embryonic development. The data reveal an ancient, convergent mechanism for the evolution of morphological novelty.