Why Ascidian?
If one were to imagine an ideal experimental model organism it would contain many of the features found in ascidians. Ascidians are invertebrate members of the chordate phylum. The close evolutionary relationship of the ascidians to the vertebrates is evident in their morphology and physiology. Furthermore, the developmental biology of ascidians has many parallels at the cellular and molecular level with vertebrates.
Despite their kinship with the vertebrates, the ascidians have many features that are more like those found in invertebrate model organisms such as C. elegans or Drosophila. For example, the free-swimming ascidian larva of the species Ciona intestinalis contains only about 2,500 cells. In the development of the larva, cells making up tissues such as the notochord, muscle, mesenchyme and endoderm follow an invariant lineage. Overall, the ascidian larva has much fewer cell types than vertebrates, and larval tissues and organs are composed of very few cells. For example, C. intestinalis larvae contain only 40 notochord cells, 36 muscle cells and approximately 330 cells in the central nervous system. In my lab we have been conducting forward genetic screens in two species of ascidians, Ciona savignyi and C. intestinalis, taking advantage of the ability of the hermaphroditic adults to self-fertilize to identify recessive mutations in a single generation.