Elucidating the Evolutionary Origin of the Neural Crest

Abstract

The evolutionary origin of the neural crest, an embryonic stem cell population unique to vertebrates, has eluded biologists since its discovery. The neural crest is characterized by its epithelial to mesenchymal transition (EMT), migration, and differentiation into stereotyped tissues of the embryo. These processes require an intricate gene regulatory network (GRN) that controls the signaling required for successful neural crest formation and differentiation into target tissue types. It is hypothesized that the neural crest, like other complex tissues, arose from co-option of existing developmental GRNs, but this has not been tested. Here, I will use an invertebrate deuterostome, the sea urchin L. variegatus, to look for ancestrally conserved circuits of the neural crest GRN. I hypothesize that genes operating in the neural crest GRN will be found in cells of the L. variegatus embryo that undergo similar processes to vertebrate neural crest cells (EMT, migration, etc.), namely primary mesenchyme cells (PMCs), secondary mesenchyme cells (SMCs), pigment cells, and neurons. I have cloned orthologs of vertebrate neural crest genes in the developing embryo of L. variegatus including foxd, phb1, musk, elk3, egr/krox20, and csnrp. Using RNA in situ hybridization, I have found that these genes are expressed in the predicted cell types in sea urchin embryos. Double in situs were then performed for musk / pks and foxd / phb1 to demonstrate co-expression of the gene pairs. Both pairs of genes were co-expressed, indicating that they may be part of the same GRNs. If these connections are shared with the neural crest GRN, it will provide evidence that these small GRNs are ancestral to deuterostomes and were co-opted into a single tissue in the vertebrate lineage, which gave rise to the neural crest.