Molecular embryology of a larvacean Urochordate, Oikopleura dioica, and the origin of chordate innovations.

摘要

Phyla are by recognized by unifying characters, some of which are unique evolutionary innovations, but how do new structures and body plans evolve? Defining characters of the phylum Chordata include the notochord, dorsal hollow nerve cord, and post-anal tail. Despite these commonalities, the chordate subphyla Vertebrata, Cephalochordata, and Urochordata span a broad range of morphological diversity. Vertebrates are distinguished from the other chordates by an elaborated brain, paired sense organs, and a skeleton. Two embryonic tissues, neural crest and placodes, are essential for these features. It became widely accepted that crest and placodes are vertebrate innovations because no clear homologues of these tissues or their derivatives had been observed in other chordates. Further speculation suggested that acquisition of neural crest and placodes permitted or propelled the evolution of raptorial vertebrates from a sessile, filter-feeding ancestor. By comparing ontogenies of extant chordates and their outgroups, we hope to reconstruct the timing and mechanism of the origin of chordate novelties.;Within the most basal chordate lineage, Urochordata, larvaceans uniquely allow us to analyze development of adult characters in a chordate body plan that does not become reconfigured by sweeping metamorphic events. This dissertation examines the evolutionary history of two chordate innovations, notochord and placodes. I present DNA sequences and expression patterns for larvacean homologues of several vertebrate genes important in notochord and placode development: brachyury, pitx, eya, and members of the pax and six gene families. Larvacean brachyury is expressed in notochord, but also in posterior endoderm. Because deuterostome and protostome outgroups display a similar pattern, chordate brachyury may have a conserved role in endoderm patterning more ancient than its role in the notochord. I also show that several genes important for the development of a subset of vertebrate placodes are expressed in the developing ciliary funnel and ventral organ in larvaceans. Ultrastructure and topography had previously suggested that these organs are candidate homologues of pituitary and olfactory placodes. My results are consistent with an origin of these placodes in the common ancestor of modern chordates, predating the evolution of vertebrates.;This dissertation includes both my previously published and my co-authored materials.