2023年度第3回セミナークラブ 筑波大・笹倉靖徳

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Ascidians are invertebrate chordates living in the ocean.
They are the closest living relatives of vertebrates, although that
is not easy to deduce from their vase-like shape in the adult stage.
The relationship is more readily understood from the tadpole-like
structure of ascidian larvae. Ascidian larvae share characteristics with
vertebrates, such as the notochord and a dorsally located,
hollow central nervous system. Adult ascidians also possess key
features of chordates, such as a thyroid homologous organ,
gill slits along the throat, and a heart.
A critical goal of the studies using ascidians is the elucidation of
the evolutionary history of vertebrates. Ascidians (strictly speaking,
urochordates including ascidians) form the sister group of
vertebrates. Therefore, by comparing mechanisms between
ascidians and vertebrates, we can deduce the characteristics of
our ancestors. I will show you our recent studies on heart
formation as an example for this topic.
Their simplicity is another advantage of ascidians as
research models,and only a small number of cells form the
tadpole larvae of many ascidians. In the case of the model
ascidian Ciona intestinalis, the total cell number is less than
3,000, including 40 notochord cells and 231 neurons.
Furthermore, the genome of Ciona is short and compact.
The size of its haploid genome is only 160 megabase pairs and
encodes 16,000 genes, meaning that there is one gene in
every 10,000 base pairs. The small cell and gene numbers
enable us to address gene functions with cellular resolution.
Our research addressing the specification mechanisms of
dopaminergic neurons made good use of these advantages of Ciona.
Ascidians also have uniquely acquired characteristics that
distinguish them from vertebrates. One of them is the sessile
lifestyle in the adult stage. The evolution of the sessile lifestyle is
deeply related to the acquisition of the ability to synthesize cellulose,
which was suggested by the genome and genetic studies of Ciona.

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