The Non-vertebrate Chordate Oikopleura and Evolution of Vertebrate Developmental Innovations

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NSF0345203 The Non-vertebrate Chordate Oikopleura and Evolution of Vertebrate Developmental InnovationsDevelopmental innovations transformed ancient filter-feeding chordates into voracious predators, the ancestors of the vertebrates. These innovations are proposed to include a three-part brain, neural crest, and epidermal placodes, and to be associated with extensive genome amplification. A central problem is an understanding of the molecular genetic changes that permitted the evolution of new developmental mechanisms responsible for the vertebrate bauplan. In addition to the Vertebrata (or Craniata), the phylum Chordata contains two other sub-phyla, Cephalochordata (the sister group to the Vertebrata), and the basally diverging Urochordata. Urochordate larvae have a typical chordate morphology including notochord, dorsal nerve cord, pharyngeal slit primordia, and a muscular post-anal tail. Urochordates of the class Ascidiacea are well studied classically, molecularly, and genomically, but their larvae usually lack a complete one-way digestive system, and metamorphosis destroys much of the central nervous system as the body plan reorganizes into a sessile, filter-feeding adult. In contrast, urochordates of the class Larvacea (or Appendicularia), including Oikopleura dioica, maintain a complete chordate body plan as free-living adults, which may be an advantage for the investigation of the chordate bauplan. Furthermore, in contrast to cephalochordates, Oikopleura embryos are available year round and can be maintained many generations in the lab. The overall objective of this proposal is to help understand the evolutionary origin of vertebrate developmental innovations utilizing Oikopleura dioica to complement investigations on ascidians and cephalochordates. Intellectual Merit Objectives: Aim 1. To determine the fate map of Oikopleura embryos. Understanding the fate map is essential for investigations of cell fate determination. Methods include injection of fluorescent dyes into blastomeres at various ages, and four-dimensional reconstructions in multi-photon confocal microscopy. Aim 2. To test in Oikopleura the developmental role of orthologs of genes shown to be required for the development of vertebrate neural crest and placodes. Methods for Aim 2 include down-regulating gene function with morpholino antisense oligonucleotides, and testing gene function by transient transgenesis. Broader Impact Objectives: Aim 3. To train two minority undergraduates studying Oikopleura molecular developmental genetics in a ten-week summer research program, and one minority undergraduate for academic year research. Aim 4. To increase knowledge of the basic biology of a major player in global carbon sequestration. Larvaceans bypass most of the microbial food web by converting picophytoplankton into biomass directly available to predatory fish and into fast-sinking carbon trapped in their discarded houses that are transported to ocean depths. In the face of global warming, likely due at least in part to rising atmospheric carbon dioxide, it is crucial to understand better the biology of this key global link in the carbon cycle.