Abstract

Animal development from an egg involves a fast increase in cell number, the designation of a specific spatial location of the cells and the specification of their cell fate. This is achieved following different developmental strategies, for example the spiralian development. Characteristic of the spiralian development is the spiral cleavage with its peculiar twist during cell division and the early cell fate determination as a consequence of its fixed development and cell lineages (as in the nematode C. elegans). The spiralians are therefore interesting models for understanding cell division mechanisms and for understanding the cellular and molecular mechanisms underlying the differentiation of cells and tissues. However, although close to half of the animal phyla follow a spiralian development, little is known about the cellular mechanisms and behavior underlying this developmental strategy. To approach these questions, we use the spiralian annelid Platynereis dumerilii. This marine worm has a transparent embryo, a swimming larva with relatively few cells (~700 cells) and is amenable for laboratory and molecular biology techniques.

In the talk, I will present our research on two aspects of the spiralian development. First, which are the cellular mechanisms relevant during the early cleavages (the spiral cleavage) and thus important for correct cell positioning? Second, which are the cellular and molecular trajectories in the differentiation of cell types and tissues? To study the first topic, we fluorescently label cells and cellular compartments during the early development of P. dumerilii, we monitor the cellular dynamics by live-imaging using spinning disk confocal microscopy and perform quantitative measurements of the cortical flows occurring during cell division. We show that the cortical actomyosin is a main driver of the spiral cleavage. For our second research topic, we first perform whole-embryo live-imaging of the developingP. dumerilii by light-sheet microscopy. With these recordings, we generate the complete developmental cell lineage of the swimming larva. This information has led to multiple insights, for example, the detection of left-right body asymmetries at the cellular level during the formation of the trunk neuroectoderm. We are currently aligning gene expression patterns from fixed embryos onto the cell lineages to describe the molecular trajectories underlying the specification and differentiation of cell types and tissues during development.

To sum up, our research describes the first whole-embryo analysis at cellular resolution of the development of a spiralian larva. With our approaches, we gain insights into the molecular and cellular trajectories and the underlying cellular and biophysical mechanisms involved in the formation of a spiralian larva.