Analysis of a Developmental Gene Regulatory Network in the Sea Urchin Embryo During Normal and Regulative Development

Every zygote follows a characteristic developmental path resulting in a complex multicellular organism, with specialized cells performing specialized functions. This developmental path is faithfully inherited over generations, suggesting that the regulatory code is hardwired in the genome. A unifying theme in developmental biology is to decipher this genomic code. Pioneering work in the sea urchin embryo has lead to the detailed understanding of the gene regulatory network (GRN) that underlies the specification of the skeletogenic cells, cells that secrete the larval skeleton. The sea urchin embryo is also known for its regulative properties and a remarkable feature of the skeletogenic network is that can be ectopically activated in any cell of the embryo during regulative development. This presents a unique opportunity to study the reconfiguration of developmental networks in different cellular contexts.

The work presented in this thesis refines our understanding regarding the initial deployment of this GRN during normal development. The activation of this network is thought to be regulated by a derepression mechanism, which is mediated by the products of the pmar1 and hesC genes. Here, we show that the activation of the skeletogenic network occurs by a mechanism that is distinct from the transcriptional repression of hesC. We provide evidence that unequal cell division in the vegetal blastomeres is tightly linked to the activation of the early regulatory genes. In addition, our analysis of the upstream regulation of the two key transcription factors, alx1 and tbr, reveal that these genes are controlled by a two-phase regulation that can be divided into an activation phase and a maintenance phase.

Furthermore, to dissect the molecular underpinnings of regulative development we have taken advantage of the rich knowledge of the skeletogenic GRN. We have used two experimental paradigms, first, that induces the activation of the skeletogenic GRN in the cells of the non-skeletogenic mesoderm and second, that activates this network in the endodermal cells. Our findings highlight several interesting and significant differences in the initial deployment of this network during regulative development. We provide evidence that, despite these upstream differences the downstream network is faithfully recapitulated. We also show that the NSM subpopulation that activates the skeletogenic GRN is the prospective blastocoelar cells. Finally, we show that mitotic cell division does not play a role in lineage reprogramming (both NSM and endoderm) in the sea urchin embryo. These and other findings described in the following chapters further illuminate our understanding of development GRNs and the evolution of cell lineages.