Dissection of developmental processes leading to the chordate body plan – ChordateForm

Embryogenesis in chordates leads to the generation of a tadpole-like organisation, which is characterised by a central notochord and a dorsal hollow neural tube. The tadpole-like body plan is the defining feature of this group of animals which consists of the marine invertebrates, including cephalochordates and tunicates, and vertebrates. Recent molecular analyses have placed cephalochordates as the most basal extant chordates and tunicates as the sister group of vertebrates. Among the invertebrate chordates, a group of tunicates, called ascidians, provide a readily accessible experimental model and our understanding of their developmental mechanisms is increasing rapidly. Comparison of the pre-gastrula fate maps of ascidians and vertebrates reveals a high degree of topological similarity. This similarity is maintained through gastrulation, when embryonic territories are extensively rearranged and the basic tissue organisation of the chordate body plan is established. Remarkably, high conservation at the morphological level does not necessarily reflect an equally strong conservation of the underlying molecular mechanisms. Despite this, the tadpole-like body plan appears to be an immensely robust outcome of chordate embryogenesis. We have been studying ascidian embryogenesis as a model organism, which we believe, due to its overall simplicity, offers an unprecedented opportunity to dissect the entire developmental processes that generate the basic chordate body plan. We focus on two chordate specific structures, notochord and neural tube. In ascidian embryos, the anterior notochord cells and caudal neural plate cells originate from common precursors located in the dorsal marginal zone of the embryo. Our major achievements include a discovery of a novel molecular mechanism that controls an asymmetric cell division generating one notochord and one caudal neural precursors and a detailed description of the signalling events leading to the patterning of the caudal neural plate. In this proposed project, we will develop further these research topics aiming to dissect the entire developmental process leading to notochord and neural tube formation in this simple chordate model. It remains to be addressed how the notochord/neural mother cell is specified via maternal inputs, how the cell division planes and cell rearrangements are so precisely controlled in order to shape embryos, and how the signalling inputs that pattern the ascidian neural plate interact with the genome in order to generate specific cell identities. We are proposing a number of projects that should shed some light on these problems. These projects should collectively enable us to draw a more complete view of how the notochord and neural tube are generated and should provide further insight into how the chordate body plan is patterned and shaped. Methodologies employed in the proposed project include proteomics, functional gene knockdown, promoter analyses and live imaging. In order to tackle each task, we will use two ascidian species, Ciona intestinalis and Phallusia mamillata, each of which provides complementary features as experimental system. The Ciona model is supported with a vast number of molecular, genomics and bioinformatics tools and allows us to conduct functional studies, while the Phallusia model is suitable for live imaging due to the extreme transparency of their eggs and embryos. The complementary use of these two ascidian models places us in a unique advantageous position among research groups in the field.