is a member of the tunicates, a group of sessile marine invertebrates. Despite appearances () the tunicates are members of our own phylum, the Chordata
. Indeed recent analysis indicate that the tunicates are the sister group to the vertebrates ()[1
]. Their close evolutionary relationship to the vertebrates is most evident during embryonic stages. Three chordate characters develop within the tunicate tadpole larvae, a dorsal hollow nerve cord, notochord and post-anal tail (). Upon attachment to a substrate, the tail and associated chordate structures are resorbed while rudiments within the larval head differentiate into the adult organs, including the heart, body wall musculature, and gut.
Fig. 1 (A) Adult Ciona intestinalis. B. Current chordate phylogeny. C. Ciona intestinalis larvae with cross-sectional diagram of tail tissues. (A and C modified from )
The cellular simplicity of tunicate embryos has long made them an attractive model system for studying fundamental properties of animal development. Ciona
embryos are a typical example, developing with extraordinarily low cell numbers. Gastrulation is initiated at the 110-cell stage, and the fully formed tadpole larva contains less then 3000 cells. This is in marked contrast to even the simplest vertebrate embryos, in which gastrulation takes place in the context of tens to hundreds of thousands of cells and larval/fetal stages consist of millions of cells. Additionally, Ciona
cell lineages are invariant and an exhaustive cell fate map has been compiled [2
]. Despite this extreme simplicity, embryonic development in Ciona
is strikingly similar to that of vertebrate embryos. This similarity is particularly evident in comparing gastrulation and neurulation [3
] and includes early steps of heart development (see below).
is also simple on a genetic level. Two Ciona
genomes (Ciona intestinalis
) have recently been sequenced and assembled. The tunicates (Ciona
included) diverged prior to vertebrate-specific gene duplications [4
]. The resultant lack of genetic redundancy simplifies testing of gene function. Additionally, Ciona
regulatory DNA is highly compact [5
]. Availability of the fully sequenced Ciona savignyi
genome allows for phylogenetic comparisons that can greatly assist in the identification of conserved, potential regulatory non-coding DNA [6
]. Furthermore, putative regulatory regions can be quickly characterized due to the ease of generating transgenic embryos through electroporation. Despite this highly compact genome, genetic pathways underlying fundamental aspects of Ciona
/vertebrate development, including cardiogenesis, are surprisingly well-conserved [7
Recent efforts have leveraged Ciona’s
simplicity and tractability to initiate construction of comprehensive gene regulatory networks. Almost all of the transcription and signaling factors identified in the genome have been systematically assayed for their developmental expression patterns and many of them have also been functionally tested by morpholino knock down assays leading to the generation of detailed gene networks for early embryos [9
]. Techniques have been developed to permit generation and rearing of stable transgenic lines [11
]. Ascidian stock centers for the creation and maintenance of stable transgenic lines are being developed in California (Smith Lab), Japan (Sasakura Lab) and France (Joly Lab).