Some truths about the universe and our experience in it seems to be unchanging over time. These facts about the universe have been prevailing since years and no existing life on earth could ever question them so easily. There are also many traits that unite life on this planet. Every life is made up of cells. Multicellular lives need oxygen to live. Nature has built the perfect system for turning oxygen into energy. Breathing is so natural that most of the time we are not even aware that we are doing it. There is, however, one known exception. Scientists have just discovered a jellyfish like parasite that does not have a mitochondrial genome. It is the first known multicellular organism without it. It does not breathe at all, in fact, it lives its life completely free of oxygen. Mithochondrial respiration is one of the most common characteristics of multicellur organisms on earth. It is the process by which oxygen is used to generate adenosine triphosphate (ATP), which is the fuel used to power cellular processes. Generation of ATP takes place in mitochondria which has its own genome that is separate from the main genome in the cell nucleus.
But now, there is an exception – Henneguya Salminicola. Over many years, they have basically developed from a free living jellyfish ancestor into much more simpler parasite we see today. Like the single celled organisms, it had evolved mitochondria-related organelles, but these are unusual too. They have folds in the inner membrane which are not usually seen. When Professor Dorothee Huchon and her colleagues sequenced the DNA of Henneguya Salminicola, which is related to jellyfish, they thought that they had made a mistake because they found no mitochondrial DNA at all.
Further studies confirmed the finding. When the team stained H. Salminicola with blue fluorescent dye that binds to DNA, no DNA was visible in cells outside the nucleus. In contrast, when they stained a closely related parasite, blue dots corresponding to mitochondrial genomes were visible outside the nucleus. The white cells of H. Salminicola have structures that look like mitochondria, but they cannot make enzymes needed to use oxygen to produce ATP. Thus Huchon and her team concluded that these are not true mitochondria.
Exactly how it survives is still something of a mystery. It could probably be by leeching adenosine triphosphate from its host, but much more is yet to be determined. According to an international team of researchers, H. Salminicola is a microscopic parasite that infects salmon. When the host dies, spores are released that are consumed by worms, which can also serve as hosts for the parasite. When salmon eat the worms, they become infected as the parasite moves into their muscles. These can be seen by fishermen as white, oozing bubbles.
These are salmons with H. Salminicola infections and are sometimes said to have tapioca disease. Although the parasite is harmless to humans, it is a major problem for fish farmers because it creates unsightly white spots in the flesh of infected fish. We don’t know why H. Salmincola has lost this ability while all its immediate relatives use oxygen. As these parasites move through their life cycle, they may also live inside a worm host where they may virtually get no oxygen as well. However the worm host of H. Salminicola has never been identified. According to Huchon, the host may live in sediments with very low oxygen levels.
They have lost most of the original jellyfish genome, but retaining a complex structure resembling jellyfish stinging cells. They don’t use these to sting but to cling to their hosts. It is an evolutionary adaptation from the free living jellyfish’s needs to the parasites. Although it is harmless to humans, no one wants to buy salmon riddled with tiny weird jellyfish. Thus these discoveries could help fisheries adapt their strategies for dealing with the parasite. But it is also a heck of discovery for helping us to understand how life works as it confirms that adaptation to an anaerobic environment is not only unique to single celled eukaryotes but has also evolved in a multicellular parasitic animal. Hence H. Salminicola provides us opportunities for understanding the evolutionary transition from an aerobic to an exclusive anaerobic metabolism.