1. Sars Centre for Marine Molecular Biology, Bergen High Technology Centre, Thormøhlensgaten 55, 5008 Bergen, Norway
2. Max Planck Institute for Molecular Genetics, Ihnestrasse 73, 14195 Berlin, Germany
3. Genoscope-Centre National de Sequencage and CNRS UMR-8030, 91000 Evry, France

Correspondence to: Daniel Chourrout¹ Email: Daniel.Chourrout@sars.uib.no
BAC clone sequences have been deposited in GenBank under accession numbers AY449458–AY449462 and AY613855–AY613856.

Abstract

Tunicate embryos and larvae have small cell numbers and simple anatomical features in comparison with other chordates, including vertebrates. Although they branch near the base of chordate phylogenetic trees¹, their degree of divergence from the common chordate ancestor remains difficult to evaluate. Here we show that the tunicate Oikopleura dioica has a complement of nine Hox genes in which all central genes are lacking but a full vertebrate-like set of posterior genes is present. In contrast to all bilaterians studied so far, Hox genes are not clustered in the Oikopleura genome. Their expression occurs mostly in the tail, with some tissue preference, and a strong partition of expression domains in the nerve cord, in the notochord and in the muscle. In each tissue of the tail, the anteroposterior order of Hox gene expression evokes spatial collinearity, with several alterations. We propose a relationship between the Hox cluster breakdown, the separation of Hox expression domains, and a transition to a determinative mode of development.

1. Sars Centre for Marine Molecular Biology, Bergen High Technology Centre, Thormøhlensgaten 55, 5008 Bergen, Norway
2. Max Planck Institute for Molecular Genetics, Ihnestrasse 73, 14195 Berlin, Germany
3. Genoscope-Centre National de Sequencage and CNRS UMR-8030, 91000 Evry, France

Correspondence to: Daniel Chourrout¹ Email: Daniel.Chourrout@sars.uib.no
BAC clone sequences have been deposited in GenBank under accession numbers AY449458–AY449462 and AY613855–AY613856.

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