YU, J.-K.*; HOLLAND, N.D.; HOLLAND, L.Z.; University of California, San Diego; University of California, San Diego; University of California, San Diego: FoxD protein evolution and the origin of vertebrate neural crest

Neural crest is unique to vertebrates. Embryologically, neural crest derives from the edges of the neural plate and adjacent non-neural ectoderm, and undergoes an epithelial/mesenchymal transition to become a migratory population of cells that gives rise to numerous differentiated cell types. The evolution of neural crest is thought to be responsible for many of the complex structures of the vertebrate head which are lacking in invertebrate chordates. Amphioxus is the closest living invertebrate relative of the vertebrates, and is key to understanding the evolution of vertebrates from an invertebrate ancestor. Our previous comparative study on amphioxus and vertebrate FoxD genes has suggested that multiplication of FoxD genes in the vertebrate lineage and subsequent changes in their regulation might have been important for the evolution of neural crest. However, whether the FoxD protein function is also changed during this process is not known. To address this question, we ectopically expressed the amphioxus FoxD protein in Xenopus embryos and compared its function to Xenopus FoxD proteins. Overexpression of endogenous FoxD3 in frog embryo induces expression of neural crest markers. In striking contrast, ectopic expression of amphioxus FoxD resulted in a great reduction of neural crest markers. Our result suggests that in addition to the co-option of duplicated FoxD genes into new expression domains, functional changes in this regulatory protein are also likely to play an important role in the evolution of neural crest.