Origin and evolution of metamerism in animals: posterior growth growth, stem cells and segmental polarity in the annelid Platynereis – METAMERE

A number of striking similarities have been found in genetic networks that regulate key processes of early embryogenesis as well as organogenesis between insects and vertebrates. To understand these similarities, it has been proposed that broad aspects of body plan organisation were already present in Urbilateria, the last common ancestor of bilaterians, a segmented coelomate animal, looking like an annelid. We are working on this still controversial hypothesis by studying the development of the marine annelid Platynereis dumerilii. We are particularly interested in the genetic mechanisms of metameric segmentation of annelids in order to compare them with analogous networks in insects and vertebrates. We believe this project may provide crucial evidence to understand the early evolution of vertebrates as the « complex Urbilateria » theory stipulates that vertebrates evolved from active annelid-like ancestors.
We propose to analyse two fundamental aspects of segment ontogenesis:
1°) How is segment polarity regulated in the annelid ? Expression data suggest the involvement of wnt signalling. We are going to put this idea to the test by interfering with all potential signalling pathways regulated by the Wnt secreted proteins. With the help of small interfering molecules, we are going to test the potential involvement of the Notch pathway in the mesodermal segment polarity to compare with analogous vertebrate processes.
In parallel with this candidate gene approach, we are going to develop an unbiased transcriptomic screen to discover the genes regulated by the Wnt/beta-catenin pathway during the establishment of segmental polarity. This approach will be based on quantitative sequencing of multiple cDNA samples of treated larvae as well as posterior segment addition zones of juvenile worms with pro- and anti-beta-catenin drugs. Combined with time series, we hope we will identify the direct transcriptional targets of the beta-catenin pathway.
2°) How does posterior addition work in the annelid ? We are going to characterize the stem cells that are responsible for the addition of new segments during the annelid life. We have shown previously that the ectoderm of new segments at least is produced by the synchronized mitotic activity of a posterior ectodermal ring of stem cells. Using clonal tracing (diI), we are going to determine whether another niche of stem cells is responsible for mesodermal growth as suggested by a set of markers. We are going to characterize systematically the action of hedgehog, Wnt, Notch and FGF pathways on the maintenance and mitotic activity of these stem cell pools.
Platynereis, like other annelids, is capable of caudal regeneration. Posterior stem cells therefore can be regenerated from existing segments. We are going to explore with mitotic activity labelling, specific gene markers of stemness and clonal analysis, the origin of the cells that are making the regeneration blastema, We are going in particular to establish whether the blastem is formed by cell dedifferentiation or by pre-existing stocks of segmentel stem cells. We are going to analyse the action of signalling pathways on the formation of the blastema, notably to find out whether they can influence the migratory behaviour of proliferating cells. We hope that we can discover similarities with human adult stem cells and thus put to the test the idea that these human cells are descended from ancestral pluripotent metazoan stem cells.