Research
Larvacean as “the simplified chordate”.
Our work is based on the chordate, Oikopleura dioica, for understanding development and morphogenesis of animals.
The figure above shows its life history. O. dioica belongs to the phylum Chordata, which includes vertebrates. It has a simplified body with approximately 4,500 cells, and rapid development to form tadpole shape with three hours after fertilization and completed morphogenesis and becomes a juvenile with 10 hours. It has a smallest genome (56-70Mb) ever found in a non-parasitic multicellular organism, and life cycle is extremely short as ~ 5 days (@ 19-20°).
For these features, O. dioica can be regarded as “the simplified chordates” with a swimming tadpole body plan.
Based on its potential as a novel experimental animal, we have developed experimental platforms, including follows:
(1) Functional screening of genes (genetic approaches);
(2) Live imaging of development (visualizing how individual cells behave);
(3) Genome and transcriptomes databases;
(4) histological methods and 3D imagings.
Hereafter, we introduce two main research areas.
(1) House formation
Larvaceans are living in the “House”, a three-dimensional and food-concentration apparatus. This house contains cellulose fibers.
The above figure represents movements of seawater and food particles taken up into the house. When “black ink” were fed instead of foods, you can see that ink particles are concentrated into the food-trapping filter and transported to the stomach.
The house is not just a water bag, but a well-organized 3D structure for concentrating foods by circulating natural seawater.
This figure shows a high magnification views of a house. Each part of the greenhouse has a differently shaped mesh made of fibres.
As shown in the diagram, the house is secreted from the trunk epidermis as a folded form. This folded house is called as “house rudiment”. The trunk epidermis is covered with a few house rudiments. A new house is formed by inflating the outer house rudiments. A new house is constructed within a few minutes by inflating the outermost house rudiment. These means that houses are disposable, and re-built about 5-8 times a day.
To secrete the house, the trunk epidermis has a unique and stereotyped cell arrangement, which is invariant in cell numbers and shapes (Kishi et al., 2017).
The left image shows arrangement of trunk epidermal cells, of which nuclei were stained with DAPI. The right image shows opening of the epidermis by cutting ventral side. It is hypothesized that fibres are secreted from this oikoplastic epidermis to design the three-dimensional shape of the house.
To understand processes of house formation, we are focusing on follows:
- Synthesis of cellulose fibres (nm scale)
- Mesh formation with fibres (µm scale)
- House morphology (mm scale)
House construction is a new area in biology. Given that “knitting cellulose fibres dictates their three-dimensional shape”, it also has potential as an engineering technique. We are trying to understand this morphogenesis not only through biological experiments, but also by learning from mathematical models in collaboration with experts in mathematics, physics and engineering.
(2) Left-right patterning
Animals have three orthogonal axes (anterior-posterior, dorsal-ventral, and left-right). How the left-right (LR) axis is formed is one of the fundamental questions in developmental biology. More than a century ago, it has been described that O. dioica embryo has an exceptional LR-asymmetry.
Upper images are 2- and 4-cell embryos (vegetative view), and lower diagrams are tadpole larva (dorsal view) of O. dioica (left) and the zebrafish Danio rerio (right), respectively. At the 4-cell stages, blastomeres are shifted invariantly to same orientation (red arrow). In larva, the nerve cord is placed on the left side, not dorsal side, of the tail. We obtained evidence supporting that this LR-patterning process may be mediated through approximately 90° rotation of the dorso-ventral pathway (Onuma et al., 2020).
This example represents that this ”simplest chordate” possess a new principle of LR patterning. In future, we are planning to investigate molecular and cellular events to understand how left and right sides arise. These studies will gain insights into how the developmental process can change while maintaining formation of the tadpole-like body in chordates.
We value many other research projects by focusing on unique biological features of larvaceans. Also, we are promoting collaborations by providing samples, resources, and techniques. Larvaceans are also useful material for scientific education and outreach activities. If you have any questions, please do not hesitate to contact us.
Policy
All students are responsible for at least one research project.
In consultation with the PI and laboratory members, experiments and analyses are expected to be carried out on his/her own hands. We consider that “Honesty” and “Consultation” are necessary to conduct research based on your intellectual curiosity. Through this process, I hope members to gain skills in planning research, preparing experiments, interpreting results, presenting at conferences, and also writing original papers. Graduate students are also encouraged to write a paper as first author.
