Appendicularia, Thaliaceae and Ascidiacea constitute the three classes of Urochordata. Urochordates (Tunicates), along with cephalochordates and vertebrates, form the phylum Chordata.

EMBRC expertise:

Access

The life cycle can be carried out in the laboratory with production of hundreds of gametes and in vitro fertilisation carried out in beakers.

Available tools

• The life cycle is very short and varies with temperature: 8 days at 15°C and less than 2 days at 29°C. This model can be maintained in the laboratory for several generations in artificial seawater. Pure lines have been produced from repeated crosses between related individuals.
• The embryos are transparent as are the adults and both are thus suitable for cell imaging techniques, in situ in toto hybridisation, and immunohistochemistry.
• Embryos are easy to handle and functional gene analyses are possible; genes can be knocked out by injecting RNAi in oocytes.

Scientific advantages of this model organism

Chordates are characterised by the presence of a dorsal neural tube and a notochord. This body plan is shared by all urochordates but only at the larval stage, with the exception of appendicularians whose body plan is that of a simplified chordate. The adult appendicularian retains its tail, which is used for locomotion. Moreover, appendicularians, like thaliaceans (salps, doliolids, pyrosomes) are pelagic tunicates. These planktonic animals feed by filtering particles of a large size range from the seawater.

• External embryonic development is very rapid: at 22°C the egg divides 15 min after fertilisation and the larva hatches 2.5 h later. Development is synchronous, cleavage is determinate and cell lineages are invariable. The precursors of various tissues have been inventoried and mapped in the embryo. The body is small with a low number of cells: upon hatching, there are 550 cells, of which 20 make up the notochord and 20 are muscle cells in the adult tail. It is an important model for the study of chordate development.
• This organism is a simplified experimental system for the exploration of the mechanisms of cell migration.
• Oikopleura dioica occupies a central position between the two trophic pathways and ensures efficient transfer of matter and energy. It plays a particular role in the functioning of pelagic ecosystems and in the carbon cycle.
• It is useful for modelling the ecophysiology of appendicularians to measure their impact on the ecosystem.
• Oikopleura dioica is the only dioecious appendicularian species. Other species include Oikopleura longicauda, Oikopleura fusiformis and Oikopleura rufescens.

Maintenance of live specimens and culture upon request at the Villefranche-sur-Mer Observatory (cultures are initiated from strains maintained at the SARS Institute, Norway)​

Available genetic resources

EST and BAC libraries

Databases

The genome has been sequenced (Denoeud et al., Science, 2010) and is very small (70 Mb) with a high density of genes (approx. 1 gene every 5 kb), which facilitates molecular and genetic approaches (Nucleic Acids Research, 2013, Vol. 41, Database issue, D845–D853).