Developmental Biology of Ascidians
Preface: Developmental Biology of Ascidians
Edited by Noriyuki Satoh
Ascidian tunicates are invertebrate chordates that are the closest relatives of vertebrates. Approximately 550 million years ago, their ancestors acquired a set of bacterial genes involved in cellulose biosynthesis. Thus equipped, tunicates became the only animals that can produce cellulose. This material provides the structural basis of the tunic, which protects them from predators and other environmental insults. The tunic enables adult ascidians to survive as sessile filter-feeders, a unique life style among metazoans, but one that is energetically very efficient. Taking advantage of this adult form, ascidians gave rise to nearly 3,000 species sharing a distinctive morphology. In addition, ascidians changed developmental patterns, producing tadpole-like larvae comprising an extraordinarily small number (~2600) of cells. This enabled them to reach adulthood more quickly. However, ascidian larvae possess tissues and organs that represent the basic chordate body plan. Cleavage is bilaterally symmetrical and every blastomere until the gastrula stage has an external surface; thus all cells are distinguishable under a stereo microscope. Cell lineages are well documented and developmental fate is restricted during very early phases of embryogenesis, providing a model to explore molecular mechanisms of cell fate specification.
In 2002, the genome of the ascidian, Ciona intestinalis, was decoded, making it the seventh animal species for which genomic information became available. At the same time, a cDNA resource that covers almost all transcriptomes has been prepared. Molecular techniques have been successfully introduced into the Ciona system so that gene function can be determined at the single-cell level. Together with its simple embryogenesis, Ciona is now a model system to explore genic and genomic mechanisms involved in the formation of the basic chordate body plan.
In addition, ascidians exhibit various phenomena, some unique to this taxon, such as asexual reproduction by budding, and accumulation of vanadium from seawater. These phenomena raise basic questions. What is a stem cell? What is senescence? How do ascidians use compounds from seawater.
Japanese researchers have contributed greatly to the advancement of various fields of ascidian zoology, taking the opportunity to publicize their results in Zoological Science. Descriptions of several new species are included. Zoological Science is a treasure trove of ascidian biology. This special issue provides some history of ascidian research, several papers from which still seem as brilliant as they did at the time of their original publication. Please enjoy the special issue.