Enlarge /. Mitochondria that were previously found in all animals are now in all but one animal.
Eukaryotes are the branch of the tree of life with complex cells that contain a separate DNA compartment, many internal compartments, and mitochondria that use oxygen to provide a lot of energy. These functions were so successful that we cannot find any trace of a eukaryotic ancestor who lacks one of them. The ability of the mitochondria to mobilize energy has been suggested to be an integral part of animal life.
However, there are a small number of unicellular parasites that appear to have lost this energy-producing function in the course of evolution. They usually still have mitochondrial-like compartments, but they have lost their DNA and role in aerobic metabolism. Instead, these compartments are involved in special chemical functions such as the generation of hydrogen. However, these were typically parasites that lived in oxygen-free environments and were only distantly related to animal life.
Now researchers have identified an animal that is also a parasite that lives in low-oxygen environments. And it too got rid of its mitochondria and is the first known example of an animal that it lacks.
The strange cousins we never talk about
Many animals, including us and most of the animals we spend our time with, are Bilaterians – they have left and right sides. But there are many organisms without this proper body organization. Jellyfish, sea anemones and corals belong to a separate branch of the animal kingdom called Cnidaria. Cnidarians are usually radially symmetrical and do not have a clear brain to coordinate their activities, but they coordinate them, as the gentle swimming of jellyfish shows.
If you find these typical cnidarians a bit bizarre, the myxozoa are even beyond the confines of the familiar. They are obligate parasites and their life cycle requires at least two hosts, typically a fish and a worm. Perhaps there are actually many of them due to this particular life cycle; So far, scientists have described over 1,300 species. Although they do not infect people (at least we know about them), they cause us problems because their penchant for fish makes them a problem for commercial fish farms.
The only reason we realized that they are cnidarians is because we got some DNA sequences from them. It is therefore not surprising that researchers have started to sequence the entire genome of these organisms. The real surprise came when the researchers came to the genome of the Henneguya salminicola species, which, as the name suggests, spends part of its existence chasing salmon. The discovery – or rather the lack of discovery – was that it didn't have a mitochondrial genome.
The mitochondria are the remains of formerly free-living bacteria that were built into the cell and adapted for the production of the chemical energy source ATP. While many of the genes required for mitochondrial function are found in the regular cell genome in the nucleus, the mitochondria keep their own genome, which still encodes a variety of proteins that are essential for their role in metabolism. It is difficult to see how the organism's mitochondria could function without a genome.
To ensure that not all myxozoa or their own laboratory procedures have anything strange, the researchers sequenced the DNA of a related organism. The researchers found that the DNA had a perfectly reasonable mitochondrial sequence. They then examined the nuclear genome sequence they received and looked for genes that would encode proteins needed to copy the mitochondrial DNA. While a related species had about 50 of these genes, Henneguya salminicola only had six. The gene that encodes the actual enzyme that does the copying was there, but it had three mutations that would make it inoperable.
The search for DNA in the cell using a fluorescent molecule attached to it showed that it was only in the nucleus of Henneguya salminicola. The related species had glowing mitochondria after exposure to the dye. As the researchers were able to best judge, there were no mitochondria in this species.
Then what is that?
However, when the researchers looked through electron microscopy, they could see what looked like mitochondria and had a typical layered membrane structure, including a series of folds in the innermost membrane. This is not entirely unexpected; As mentioned above, some unicellular parasites that no longer have mitochondria that perform oxidative metabolism still build similar looking structures that perform other metabolic functions.
To get a feel for what is possible in Henneguya salminicola, the researchers searched for genes that encode components of the electron transport chain that contribute to the production of ATP. Most of them do not appear to be present in this organism, which indicates that no ATP is formed regardless of this structure. In many unicellular parasites, the mitochondrial residue releases hydrogen; The genes required for this also do not appear to be present. However, H. salminicola still contains genes that are required for the metabolism that handles the building blocks of DNA, RNA and proteins, which suggests that some essential functions are still performed there.
Therefore, the function of the mitochondrial residue remains unclear at this point. However, it is possible to speculate that this was the only known species with no functioning mitochondria. In fish, the organism sits in the white muscles, which apparently function via the anaerobic metabolism. Although it is not clear what the second host is, many worm options also live in anaerobic environments. So it is quite possible that this organism spent most of its existence without oxygen in order to use it for metabolism at all.
Parasitic species like Henneguya salminicola often lose traits because the species they attack offer so much. If this organism rarely sees a lot of oxygen, the loss of the genes required for oxygen-dependent reactions would be the expected result. It is also possible that a smaller genome and a less complicated internal structure would be evolutionarily favorable for these organisms.
Does this discovery mean that we should rethink the need for an oxygen-based metabolism as a prerequisite for animal life? Incomplete. It's pretty clear that these organisms would have a hard time surviving without animal hosts to provide many of the things that we normally associate with more complicated organisms. It is therefore quite possible that the oxygen-based metabolism made possible by complicated cells remains essential for the origin of the animals. Only when these animals exist can it be unnecessary.
PNAS, 2019. DOI: 10.1073 / pnas.1909907117 (About DOIs).