Real plants, even without DNA

Whether it is human beings or animals, plants or algae: the cells of most life forms contain special structures responsible for the production of # 39; energy. Called mitochondria, they normally have their own genetic material, in addition to that found in the nucleus. Uwe John and his colleagues at the Alfred Wegener Institute have now identified the very first exception to this rule for a single cell parasite. The mitochondria of dinoflagellate Amoebophrya ceratii seem to produce energy just like our own mitochondria, but without any genetic material, as reported by the team in the newspaper Progress of science.

They are successful, varied and practically ubiquitous: dinoflagellates make up the bulk of the plankton present in our oceans and have a wide range of lifestyles. About half of the two thousand known species use photosynthesis as plants, while others are predators or alternate between different forms of food, depending on availability. Finally, the group also includes parasites. An international team from France, Canada, Egypt, the United Kingdom, Czech Republic and Germany, led by Uwe John, biologist at the Alfred Wegener Institute of the Helmholtz Center for Research Polar and Marine (AWI), recently took a closer look at one of these pests. "Genetic material – and were surprised by what they found.

The researchers originally discovered their object of study in the cells of other dinoflagellates of the genus Alexandrium, which includes many species that are infamous sea alchemists and have a tendency to form proliferations. Toxic algae. In some cases, whole carpets of these unicellular organisms cover the surface of the water and produce saxitoxin, a neurotoxin that can be dangerous to humans. But there are also parasites that can control these flowers, for example. the species Amoebophrya ceratii, which is at the heart of this study.

"These unicellular organisms float in water under the name of dinospores until they find a suitable host," says John Uwe. They then catch their victim, enter it and eat it from the inside. During the process, they become larger and eventually reach a stage with multiple nuclei. Like a worm, they come out of the dead host and dissolve to form hundreds of new dinospores. This cycle of infection takes only three to four days and can decimate Alexandrium populations. "In fact, it is perhaps through these parasites that toxic algal blooms are now less common off the coast of Brittany than a few years ago," says John, which makes the investigation of the destructive toxic algae even more exciting.

In this context, the team sequenced the genome (all the genetic material) of Amoebophryawhich consists of about 100 million base pairs. This is extremely low by dinoflagellate standards: the genome of other species is a thousand times larger, which is much larger than even the human genome. That being said, a small genome is not really unusual for a parasite. Many who practice this lifestyle do not produce all the metabolic products they need to survive. they just steal from their host. While this makes them dependent on the host, it also means that there are many genes that they can simply do without. But it's not the road that Amoebophrya ceratii chose to go down. "In this species, almost all metabolic processes are still functioning, so it should also be able to cope very well by itself," said Uwe John. And it can do it with a lot less genetic material than any other dinoflagellate.

This reduction is particularly pronounced in the part of the genome located outside the nucleus. In plants and algae, their own DNA is not only found in the nucleus and mitochondria, but also in plastids, which they need for photosynthesis. For dinoflagellates in general, this plastid DNA is quite basic and contains only 14 genes. Again Amoebophrya ceratii would seem to have completely eliminated plastids and all their genes except one.

However, the "budget cuts" that the parasite has continued with regard to its mitochondria are even more impressive. In related species, the DNA of these tiny power plants still contains three genes, which experts generally considered indispensable. But Amoebophrya ceratii has apparently completely cut the genome of the mitochondria; Despite painstaking efforts, the team could not find any trace. Two of the three genes were not found, and the third, cytochrome c oxidase 1 (COX1 or COI), migrated to the nucleus. "I'm absolutely amazed," says Uwe John, "because no other form of oxygen-breathing life has ever been recorded that does not have clean genetic material in its mitochondria."

This reduction may be useful when parasites need to rapidly form a large number of new dinospores. According to John, "it might then be more efficient to regulate all processes using the kernel.This is probably also the most efficient way to use the resources of the host." But none of this would do any good if the energy supply collapsed. Still, this seems unlikely: the mitochondria work pretty well at all stages, allowing even the dinospores to swim quickly when looking for a host. "Apparently, these parasites have found their own strategy for energy production," says Uwe John, a researcher at AWI. "To produce energy, they only need one of the five known protein complexes present in the mitochondria of humans and all animals."

Experts hope their findings will help us understand the evolution of dinoflagellates and their relatives. This would also be interesting because these include other parasites and pathogens that cause diseases such as malaria. In addition, the findings could offer new insights into the evolutionary history of mitochondria and plastids: they were at the origin of independent life forms, which were absorbed by other monocellular organisms in the body. Primal past and lived there under the name of endosymbiontes. Over time, their genetic material was progressively reduced and they became essentially "service providers" who were no longer able to survive on their own. Again Amoebophrya ceratii seems to have pushed this development a step further and removed the last vestiges of the genetic autonomy of its endosymbiots.