Sometimes evolution leads us in crooked ways, and indicates a cellular structure equivalent to Russian dolls (babushka) - a cell within a cell within a cell
The nucleus of a single-celled organism called chlorarachniophyte (a species of green algae), surrounded by a chloroplast painted here in red.Credit: Patrick Keeling, University of British Columbia
The chloroplast, a cell organelle with the ability to carry out photosynthesis, was ever in the past an independent bacterium, which was swallowed and enslaved by another, larger microbe. This event paved the way for the development of plants and algae. The engulfed cell and the engulfing cell developed a relationship of endosymbiosis between them (endo = internal). The mutual relations probably benefited each of the parties - one benefited from the products of photosynthesis and the other benefited from a protected environment. However, unlike other species that maintain mutual relations between them, and are separate from each other, in this case one of the species is inside the host species - endosymbiosis.
Ages later, in a process called secondary endosymbiosis, carnivorous protozoa swallowed the green algae (which, as you recall, developed from a cell that itself swallowed the photosynthetic bacteria).
Now, after determining the sequence of bases in the DNA of the chloroplast, it is possible to show that this second event happened not once but at least twice.
The study focused on the group of protozoa known as chlorarachniophytes. Creatures belonging to Protozoa, constitute a subkingdom of tiny, single-celled, animal-like eukaryotic organisms in the kingdom Protista. (*)
Chlorarachniophytes have flagellated, amoeba-like cells that can connect to each other by fusing their minute protrusions to form a network of cells, which together trap prey, which is usually other protozoa. The name chlorarachniophytes means "green spider" in Greek. Chlorarachniophytes, despite being protozoa, can also perform photosynthesis (a characteristic of a plant cell, not an animal-like cell). They belong to one of the two groups of protozoa, which at any stage in their history, acquired chloroplasts by ingestion and internal symbiosis with green algae. The second group that acquired this ability is called euglenids.
There is disagreement among different researchers about how the two groups acquired their green algae. Some argue that this happened to a common ancestor, from which these two groups of protozoa later diverged, while others believe that these "acquisitions" were independent and independent of each other. It is especially interesting to study the Chlorarachniophytes since these microbes still retain part of the nucleus of the green algae, a fact that indicates that the process of secondary endosymbiosis still occurs in our time.
Patrick Keeling, an evolutionary biologist at the University of British Columbia in Vancouver, Canada, and his colleagues sequenced the chloroplast genome in one of the Chlorarachniophytes species. The genome is only 692,000 bases long, making it the shortest chloroplast genome currently known. It was found that the genome contains 57 protein-coding genes. A comparison between this sequence and sequences of chloroplasts from various green algae and the euglenids family shows that the chlorarachniophytes group acquired their chloroplast independently, probably from the bottom of any lake. Thomas Cavalier-Smith of the University of Oxford estimates that the event occurred 135-380 million years ago. On the other hand, if this endosymbiosis occurred only once (a common ancestor), then it is possible that these microbes harnessed the sun's energy for their needs as early as 540 million years ago.
"These are indeed very ancient events and it is challenging to exhaust the evolutionary history of endosymbiosis in this way," says John Achibald, a comparative geneticist at Dalhousie University in Halifax, Canada. The new analysis seems to indicate that green algae were assimilated into the cells of carnivorous protozoa at least twice, independently, he says.
* Creatures belonging to Protozoa, a subkingdom of tiny, single-celled, animal-like eukaryotic organisms in the kingdom Protista. The vast majority of protists are single-celled, and their size varies between 0.01 and 0.5 mm (ie, much larger than bacteria). Among the multicellular protists we can mention the brown and red algae, for example. The protists are very common in aquatic and soil environments. Most of them are heterotrophs and some live as parasites, for example inside the human body. Among the better known protists, besides the algae mentioned, many types of mold and amoeba can be named. The number of protist species is estimated at about 200,000. Modern taxonomic classifications again tend to eliminate the protist kingdom and divide it between the animals (amoeba) and the plants (algae). The animal-like protists (amoebas, mainly) are called protozoa.
