Ascidie Phallusia mammillata (sea squirt)
Phallusia mammillata is a solitary ascidian. This marine invertebrate belongs to the subphylum Urochordata (Tunicates). Appendicularia, Thaliaceae and Ascidiacea constitute the three Urochordata classes.
Phallusia mammillata is found on European coasts, along the Atlantic and Mediterranean seaboards.
- Found in abundance during most of the year in marinas.
- The life cycle can be carried out in the laboratory with the production of hundreds of gametes and in vitro fertilisation can be carried out in beakers. Requires concomitant production of microalgae to feed the juveniles.
- So-called wild lines of Phallusia can be maintained in the laboratory and cultivation of 6th generation endogamous lines.
- This organism is hermaphroditic but external fertilisation is allogamous. The gametes are easy to collect and in vitro fertilisation is easy to perform in the laboratory. Phallusia produces gametes in abundance all year round, but embryos develop better in the spring and in the autumn.
- Animals can be maintained in aquariums ready to produce gametes for several months if they are held in constant light and fed with Artemia and microplankton.
- Embyro development is external, making them accessible for experiments and rapidly available (less than 12 h).
- Eggs and embryos are remarkably transparent and extremely practical for studies of developmental processes and experiments on cell lines in 3D confocal microscopy, epifluorescence microscopy or other techniques (SPIM), in toto hybridisation, immunohistochemistry and for the use of fluorescent markers.
- They are easy to handle and functional gene analysis is straightforward. The genome has been sequenced and is available for users (see http://octopus.obs-vlfr.fr/ et http://www.aniseed.cnrs.fr/). The EST or RNSseq libraries are also available for several embryo stages (see http://octopus.obs-vlfr.fr/). Many molecular tools have been developed: micro-injections of morpholinos or mRNA at the one- or two-cell stage, electroporation of plasmids to obtain hundreds or even thousands of transgenic embryos using simple electric pulses, and double hybrids.
Scientific advantages of this model organism
Urochordates (Tunicates), along with cephalochordates and vertebrates, form the phylum Chordata. 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 adult appendicularians which retain their tail. Upon metamorphosis, the larval notochord, the tail and the neural tube disappear. Adults (sessile; solitary or colonial) have two siphons, an incurrent buccal siphon and an excurrent atrial siphon, to filter seawater. They are sheathed in a tunic secreted by the epidermis. This tunic contains cellulose, an exceptional occurrence in the animal kingdom.
- Tunicates hold a key evolutionary position because they are considered to be the invertebrates the closely related to vertebrates. Their genome has not undergone total duplication, facilitating functional gene analyses due to the near absence of redundancy. Phallusia mammillata has proved to be very practical for characterising gene regulatory networks (GRN) to determine structure and function. The existence of other ascidian species as model organisms offers the opportunity to carry out evo-devo studies on cellular, molecular and genetic mechanisms.
- The cell lineages are invariant; although Ciona and Phallusia diverged around 300 million years ago, they share this characteristic.
- Phallusia mammillata, due to meiotic maturation and very rapid early development, is an excellent model organism for the study of early embryogenesis mechanisms and regulation of chromosome segregation during meiosis (metaphase I is complete in 10 min whereas it lasts several hours in the mouse).
Available genetic resources
The mitochondrial genome of Phallusia mammillata has been sequenced. The genome is currently being sequenced.
Much information is available on the following site: http://www.biodev.obs-vlfr.fr/en/research_groups/fertilization_meiotic_cell_cycle_control_and_development.html