Vertebrates possess several distinguishing features that are not present in non-vertebrate chordates. These features include a head with paired sensory organs, a three-part brain with paired cranial nerves, a skull of cartilage or bone; and specialized organs, such as pancreas, liver, kidneys with nephrons, and a closed circulatory system. Many of these features have been considered vertebrate novelties, whose evolution was maybe favored by new genes provided by the genome amplification that occured during early vertebrate evolution. The origins of these features, however, remain unclear, and whether some of these features were present in the ancestral eochordate is still controversial.
Investigation of two non-vertebrate chordate systems, amphioxus (Cephalochordata) and ascidians (Urochordata), has contributed enormously to understanding the origins of vertebrate innovations (Cañestro et al., 2003).
There is a substantial need, however, to broaden the phylogenetic distribution of models to lever the problem of vertebrate origins.
Ascidian life cycle makes developmental shortcuts that may obscure some developmental mechanisms present in stem chordates. Ascidian larvae cannot feed; they quickly settle from the plankton and undergo an elaborate metamorphosis in which they reabsorb their notochord and tail, and dramatically restructure their CNS. When the animal has a notochord and dorsal CNS, the digestive system is undifferentiated, and it has no pharyngeal slits. When the digestive system and pharyngeal slits become functional, the CNS is dramatically restructured and simplified, and the notochord is dismantled. The distinct possibility exists that some ancestral developmental genetic mechanisms may have become compromised in ascidians to accommodate these important and interesting features of their life cycles. Study of a urochordate that retains chordate features as an adult, however, would complement data from ascidians.
Larvaceans have been shaped by different evolutionary forces than their ascidian relatives and must be considered in any generalization about urochordate evolution. Larvaceans are motile as adults; they display a relatively large number of behaviors; and they have a catalogue of sensory cell types. It is vital to investigate whether larvaceans retain evidence of vertebrate characters, but that might now be obscured by developmental shortcuts in urochordates with extensive metamorphosis.
We propose an evolutionary approach based in two steps: first we compare between larvacean and ascidian features to infer the ancestral urochordate status, and second, features held in common would be candidates for those in stem chordates, while lineage-specific features would force further thinking about ancestral features.
Our research focus on the developmental roles of genes in Oikopleura and their duplicated orthologues in vertebrates, especially those genes necessary for the development of the notochord (Bassham and Postlethwait, 2000), placodes (Bassham and Postlethwait, 2005), tripartite brains (Cañestro et al., 2005) and neural crest cells, to understand the relationships of gene duplication and the evolution of novel morphologies.
Oikopleura Research at the University of Oregon
Under Construction
Last updated on March 24, 2006.
If you have questions about this page contact the webmaster.