Ascidians are sessile marine invertebrates commonly called sea squirts or tunicates because they filter and squirt water through siphons in their tunics. Some 2000 species populate the oceans by travelling on boat hulls. Ascidians are hermaphrodites and comprise most of the urochordate phylum, which includes two other classes: larvaceans (appendicularians) and thaliaceans (salps). Several species of ascidian are used for research: Ciona, Phallusia, Halocynthia, Ascidiella, Styela. Ascidian embyros develop like vertebrate embryos into simple tadpoles made of only 3000 cells and 6 tissues. After 2 or 3 days they attach to a surface and metamorphose into a juvenile that becomes a sessile adult in 2-3 months.They have been a favorite of embryologists for over a century, ever since Conklin described the lineage and discovered myoplasm, a cortical region of the egg that becomes the tail muscles of the swimming tadpole.
Phallusia mammillata is our favorite species for studying fertilization and development. We collect these ascidians in great numbers in Sete on the Mediterranean and find them in smaller numbers in the bay of Villefranche.
They also grow as big as 10 cm along the Atlantic coasts of Europe.Phallusias are hermaphrodites and make gametes in abundance throughout the year (best months are may through november). We can control fertilization and culture large numbers of synchronous embryos (see our methods). Phallusia eggs develop into tadpoles in 15 hours and into juveniles in 3 days. The transparent eggs and embryos are ideal for imaging.
The eggs and embryos of ascidians (seasquirts) are well suited for studying how cell cycle control mechanisms have been integrated into the processes of meiosis, fertilization, and embryonic development. Ascidians are simple marine invertebrates that develop rapidly from an egg to a swimming tadpole but unlike other invertebrate model organisms ascidians are in fact chordates and the closest extant relative of the vertebrates. We work primarily with the species Phallusia mammillata which is favorable for live imaging because their cells are completely transparent and their eggs efficiently translate exogenous mRNA encoding fluorescent fusions. Moreover this species produces copious amounts of gametes year round, facilitating biochemical and proteomic approaches. We also use the ascidian Ciona intestinalis and complementary models including mouse eggs, sea urchin embryos, and tissue culture cells.