Who would get mixed with a hefty chap two meters high? There would be few those who wish. Just imagine that in a moment of danger this “hefty chap” becomes thrice as large... Is it fantasy? It is, if we are talking about people. But if it comes to unicellular algae, it is a typical scenario provided by nature for them. The scenedesmus unicellular alga grows thrice as large in presence of predators - cladoceran and rotifers that eat up unicellular algae. This is a defence mechanism as rotifers are unable to swallow such big cell.
This interesting peculiarity of the scenedesmus unicellular alga was investigated by the researchers of the Zoological Institute (Russian Academy of Sciences) in St. Petersburg. They managed to prove that the cell’s “image change” occurs at the genetic level at the point of transcription - information read-out from DNA.
To ascertain the transformation mechanism the researchers applied the actinomycin D inhibitor, which blocks read-out of hereditary information from DNA and, as a result, protein synthesis. An alga specimen was cultivated in a lean solution of chemical fertilizers. For the first experiment, the alga was transplanted into two glasses with rotifers that started to eat up the algae. The researchers added actinomycin D in one of the vessels, the other remaining intact for comparison.
In the vessel without the stopping agent, the algae began to grow up promptly to avoid being swallowed by the predator. In the vessel with added actinomycin D, big “defensive” forms of scenedesmus almost did not appear, and rotifers could banquet in cold blood.
The next manipulation with algae repeated the first one but vice versa. From the vessel, where the cells had grown up in size for defense purposes, “starving” rotifers were removed. Then the specimen was again divided in two, and actinomycin D was added into one vessel. In inhibitor’s presence, the scenedesmus algae remained big, while in pure water without rotifers the cells grew small again.
The researchers came to the conclusion that actinomycin D blocks the scenedesmus’ genetic programs “switch”, as a result the alga stops reacting to the predator’s appearance or disappearance.
Biologists discovered long ago formation of “defensive forms” with scenedesmus and species close to it in response to appearance of algae-eating cladoceran and rotifers nearby. Besides, for the purpose of defense these low plants often live singly or in small colonies, but as soon as filtrating organisms appear in the water, the cells not only grow up several times larger but they also assemble in more multitudinous colonies. The predator detection signal is the substances excreted by algae into the water. The researchers believe that many millions of years ago such unicellular colonies, that are formed in periods of danger according to genetically coded scheme, could become one of the steps towards multicellular creatures.
Some infusoria have a similar technique for protection from predators. By the way, microbiologists have recently carried out a similar investigation with one of them - Euplotes infusorium. It has turned out that actinomycin D impacts the protozoa’s outward appearance. That means that with infusoria that belong to animal organisms, the choice of strategy in response to predator’s appearance also takes place at the genetic level.
Sergey Komarov | alfa
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