Low energy –
The “powerhouse of the cell” is apparently not necessary for animal life.
That said, there are a small number of single-celled parasites that seem to have lost this energy-producing function over the course of evolution. They typically still have mitochondria-like compartments, but they’ve lost their DNA and role in aerobic metabolism. Instead, these compartments are involved in specialized chemical functions like producing hydrogen. But these were typically parasites that lived in oxygen-free environments and were only distantly related to animal life.
A lot of animal life, including us and most of the animals we spend our time with, are bilaterians — they have a left and a right side. But there are plenty of organisms without this tidy body organization. Jellyfish, sea anemones, and corals belong to a separate branch of the animal kingdom called cnidaria. Cnidarians are typically radially symmetric and don’t have a clear brain to coordinate their activities, but coordinate them they do, as the gentle swimming of jellyfish makes clear.
Henneguya salminicola , which, as its name implies, spends part of its existence preying on salmon. The discovery — or rather lack of discovery — was that it had no mitochondrial genome.
The mitochondria are the remains of what were once free-living bacteria, incorporated inside the cell and adapted for the production of the chemical energy source ATP. While many of the genes needed for mitochondrial function are found in the regular cellular genome located in the nucleus, the mitochondria retain their own genome, which still encodes a variety of proteins that are essential for its role in metabolism. It’s difficult to see how the organism’s mitochondria could function without a genome.
Myxed up myxozoans
The mitochondria are the remains of what were once free-living bacteria, incorporated inside the cell and adapted for the production of the chemical energy source ATP. While many of the genes needed for mitochondrial function are found in the regular cellular genome located in the nucleus, the mitochondria retain their own genome, which still encodes a variety of proteins that are essential for its role in metabolism. It’s difficult to see how the organism’s mitochondria could function without a genome.
So what’s that then?
Yet when the researchers looked using electron microscopy, they were able to see things that looked like mitochondria, having its typical layered membrane structure, including a series of folds in the innermost membrane. This is not completely unexpected; as mentioned above, some single-celled parasites that no longer have mitochondria that perform oxidative metabolism still build similar-looking structures that handle other metabolic functions.
does, however, still contain genes needed for the metabolism that handles the building blocks of DNA, RNA, and proteins, suggesting that there are still some essential functions performed there. So, the function of the mitochondrial remnant remains unclear at this point. But it is possible to speculate how this ended up being the only known animal species without functioning mitochondria. When in fish, the organism takes up residence in the white muscles, which apparently function using anaerobic metabolism. While it’s not clear what the second host is, plenty of worm options also live in anaerobic environments. So, it’s entirely possible that this organism was spending most of its existence without any oxygen to use for metabolism in the first place.
Parasitic species like Henneguya salminicola
often lose features because the species they attack provides so much for them. If this organism rarely sees much oxygen, then losing the genes needed to perform oxygen-dependent reactions would be the expected outcome. It’s also possible that having a smaller genome and less complicated internal structure would be evolutionarily favorable for these organisms.
Parasitic species like Henneguya salminicola
Does this discovery mean that we should rethink the need for oxygen-based metabolisms as a prerequisite for animal life? Not entirely. It’s pretty clear that these organisms would have a hard time surviving without animal hosts to provide many of the things we normally associate with more complicated organisms. So, it’s entirely possible that the oxygen-based metabolism enabled by complicated cells remains essential for the origin of animals. It’s only after those animals exist that it may be dispensable.
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