A simple life

Since the summer, a strange creature is growing in our laboratory. Trichoplax adhaerens is the simplest known multi-cellular animal (metazoan) on the planet. At most 1-2 millimetres in size, it consists of two layers of cells and not much else. It leaves on rocks in coastal waters, and is quite abundant in the Mediterranean sea.

The pictures above are 20x magnifications of the same animal taken at 1 minute interval. When searching food, the animals change shape continuously. These pictures were taken immediately on arrival of the animals from the laboratory of Bernd Schierwater in Hamburg. Since then, Trichoplax grows happily in our laboratory.

Most relevant for us, Trichoplax adhaerens has few cell types (4-5), no muscle, no neuron, and a small genome with few genes (in the range of 11,000). Trichoplax adhaerens is the simplest system capable of maintaining a cell identity. Many unicellular organisms can differentiate (like the Plasmodium malaria parasites for instance), but maintaining a cell identity is a different challenge. In unicellular organisms, differentiation is invariably triggered by the environment. This makes differentiation a sensing problem. The more accurate the sensors the better.

The purpose of maintaining a cell identity is quite the opposite. It is to not differentiate, no matter what the conditions. In this regard, we could see it as anti-sensing, or cutting part of the cell network from its environment. How is this achieved? Does this require advanced epigenetic silencing mechanisms? This is what we would like to answer.

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