Tunicate tails, stolons, and the origin of the vertebrate trunk

Authors
Citation
Tc. Lacalli, Tunicate tails, stolons, and the origin of the vertebrate trunk, BIOL REV, 74(2), 1999, pp. 177-198
Citations number
142
Language
INGLESE
art.tipo
Review
Categorie Soggetti
Biology,"Experimental Biology
Journal title
BIOLOGICAL REVIEWS OF THE CAMBRIDGE PHILOSOPHICAL SOCIETY
ISSN journal
1464-7931 → ACNP
Volume
74
Issue
2
Year of publication
1999
Pages
177 - 198
Database
ISI
SICI code
1464-7931(199905)74:2<177:TTSATO>2.0.ZU;2-N
Abstract
Tunicates are primitive chordates that develop a transient 'tail' in the la rval stage that is generally interpreted as a rudimentary version of the ve rtebrate trunk. Not all tunicates have tails, however. The groups that lack them, salps and pyrosomes, instead have a trunk-like reproductive stolen l ocated approximately where the tail would otherwise be. In salps, files of blastozooids are formed along the sides of the stolen. The tail and caudal trunk in more advanced chordates could have evolved from a stolen of this t ype, an idea referred to here as the 'stolon hypothesis'. This means the ve rtebrate body could be a composite structure, since there is the potential for each somite to incorporate elements originally derived from a complete functional zooid. If indeed this has,occurred, it should be reflected in so me fashion in gene expression patterns in the vertebrate trunk. Selected mo rphological and molecular data are reviewed to show that they provide some circumstantial support for the stolen hypothesis. The case would be stronge r if it could be demonstrated that salps and/or pyrosomes are ancestral to other tunicates. The molecular phylogenies so far available generally suppo rt the idea of a pelagic ancestor, but offer only limited guidance as to wh ich of the surviving pelagic groups most closely resembles it. The principa l testable prediction of the stolen hypothesis is that head structures (or their homologues) should be duplicated in series in the trunk in advanced c hordates, and vice versa, i.e. trunk structures should occur in the head. T he distribution of both rhabdomeric photoreceptors and nephridia in amphiox us conform with this prediction. Equally striking is the involvement of the Pax2 gene in the development of both the inner ear and nephric ducts in ve rtebrates. The stolen hypothesis would explain this as a consequence of the common origin of otic capsules and excretory ducts from atrial rudiments: from the paired rudiments of the parent oozooid in the case of the otic cap sule (these express Pax2 according to recent ascidian data), and from tubul ar rudiments in the stolen in the case of the excretory ducts.