Ascidians, or sea squirts, are invertebrate chordates, which diverged from the vertebrate lineage near the root of the chordate phylogenetic tree. Their larvae have a tadpole structure that closely resembles lower vertebrate larvae: they have a dorsal hollow nerve cord and a central notochord flanked by skeletal muscle cells. Comparison of present day ascidians with the lower Cambrian fossil record indicates that the adult morphology of these animals has not significantly changed since these remote times. Their study is likely to give us key information about the emergence of the chordate lineage.
In addition to their evolutionary interest, ascidian larvae are composed of a very small number of cells (2600 for the tadpole of Ciona intestinalis) and are therefore simpler to study than vertebrate embryos. They have an invariant cleavage pattern, a rapid development, and their lineage has been determined up to the gastrula stage. These characteristics suggest that it is possible to describe ascidian development at the single-cell level of resolution, a level of precision that has so far only been reached in nematodes.
This morphological simplicity is paralleled by the simplicity of the Ciona intestinalis genome, which has been sequenced and assembled (Joint Genome Institute, USA and Kyoto university, Japan, http://genome.jgi-psf.org/Cioin2/Cioin2.home.html). This genome is similar in size and gene density to Drosophila and C. elegans and most genes present in 2-4 copies in the vertebrate genomes have a single Ciona orthologue (Dehal et al., 2002). The genome of a second species, Ciona savignyi, has recently been released by the MIT (http://www.broadinstitute.org/annotation/ciona/), opening the way to cross species comparisons. This genomic effort has been complemented by several large-scale in situ hybridisation (ISH) screens. These screens have provided a wealth of molecular markers or effectors for the different embryonic tissues.
Recent years have seen the development of powerful molecular techniques such as mRNA injections, gene loss-of-function by injection of antisense morpholino oligonucleotides and transient transgenesis by egg electroporation. In particular, transgenesis by electroporation allows a rapid characterisation of cis-regulatory sequences. In most cases so far studied, the key regulatory sequences that drive the expression of tissue specific genes are found within 2kb of the transcription start site. As a result, precise direct regulatory networks are emerging for the notochord, endoderm and muscle lineages and it is expected that ascidians will make a major contribution to the deciphering of chordate gene regulatory networks. Finally, Ciona is hermaphroditic with a life cycle of about 2 months, two features that encourage the use of genetic approaches to study ascidian embryonic patterning. The combination of these favourable experimental characteristics and of a key phylogenetic position justifies the renewed interest for this model system.
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