It is crucial for a tissue to stop growing, once there is enough of it. The proportions of every living entity are predefined for a reason – and not simply for the sake of aesthetics. If our brains didn’t stop growing, we would become top heavy and our heads would tilt and droop helplessly on one side, making life somewhat inconvenient. The same goes for the slime mould Dictyostelium discoideum.
D. discoideum is unicellular and lives off soil bacteria. When there is not enough food, the mould shifts into starvation mode and aggregates with other cells to form a ‘fruiting body’ which looks like a balloon on the end of a string, i.e. a slim stalk on the tip of which is balanced a mass of spore cells. The length of the stalk is important; the longer it is, the further the spore cells are from the ground and the more chance they will have of being distributed far. The size of the balloon-like mass of spores at its end is also significant – if it is too big, the stalk will bend under the weight and spore dissemination will be poor.
What decides on the proportions of D. discoideum’s fruiting body? Who’s counting? When the mould encounters dire conditions, single cells aggregate into a stream which breaks up into smaller entities. Each entity then becomes a fruiting body. If the stream is untouched, a giant fruiting body will develop. Proteins known as countins are involved in dividing the stream into smaller parts. Typically, a fruiting body under countin influence will be an aggregate of about 10 000 cells which then differentiate into the stalk and the spore cells.
Countins are secreted proteins which have the ability to regulate cell-cell adhesion – and thus define growth limits. Their activity is based on a fib; floating in the intercellular matrix, countins make cells believe that they are in contact with a large number of other cells. Consequently, cell aggregation is checked. Countins seem to function as a multimeric protein complex which binds one way or another to a cell receptor. Not much is known yet and only time will tell. In the meantime, we can ponder on the thought that our proportions may be the fruit of a molecular fib.