|M.Sc Student||Anavy Leon|
|Subject||A Phylotypic Stage for all Animals - a Cross Phyla|
|Department||Department of Biology||Supervisor||Professor Yael Mandel-Gutfreun|
|Full Thesis text|
The evolution of embryogenesis is biased since many variations affect vital processes and lead to nonviable organisms. Identifying such processes from species comparisons has been a goal of evolutionary developmental biology, as it promises to reveal the mechanism of evolutionary inventions. For the animal embryo, it has been proposed that a stage in mid-development is conserved throughout the kingdom. Here we test this hypothesis by identifying the developmental transcriptomes of ten species, each of a different animal phylum, including Ctenophores, Annelids, Chordates, Platyhelminthes, Tardigrades, Nematodes, Arthropods, Echinoderms, Cnidarians, and Poriferas. Comparing the expression profiles of orthologous genes between distant species reveals interesting patterns of conservation. We find that animal embryonic development comprises the coupling of two conserved gene expression modules. Genes expressed in the early module are enriched for proliferation related functions, while cell type specific function are characterize the genes expressed in the late module. A sharp transition between the two modules comprises of signaling pathways and transcription regulators that control the shift in developmental phase. A phylostratigraphic analysis revealed that the early module shows the usage of older genes than those expressed in the later module. This pattern of conservation led us to suggest an inverse hourglass model for the evolution of animal development. According to this model, development is constrained in consisting of two phases, an early phase of cell proliferation, followed by a later stage of cell differentiation. The transition between this two phases needs to be tightly control although the specific mechanism in which each lineage controls the transition can vary. Our results provide a map of embryology upon which the ancestral constraints are mapped and distinguished from the stages in which variations are available to the agency of natural selection.