Henneguya salminicola: A non-oxygen breathing animal

Diya Adhikary, Amity University Kolkata

Researchers have discovered a “non-oxygen breathing” animal called Henneguya salminicola, which is a cnidarian parasite on some species of salmon fish. This discovery has changed one of science’s assumptions— that all multicellular animals are aerobic (need oxygen to survive) in nature. Other organisms (fungi, amoebas) are found in anaerobic environments as they have lost the ability to breathe over time. According to new studies, something similar might have happened in the case of animals too.

What is Henneguya salminicola?

Henneguya salminicola (earlier named Henneguya zschokkei) is a parasite that lives in salmon tissue and could probably be the last creature to survive on our planet. The taxonomic classification of the animal is as follows: Kingdom Animalia, Phylum Cnidaria, Class Myxosporea, Order Bivalvulida, Family Myxobolideae, Genus Henneguya, and Species salminicola. It is known to be a distant relative of jellyfish, although there are very few functional and anatomical similarities between the two. The parasite can live in various salmon species such as pink salmon, chum salmon, rainbow trout, Atlantic salmon, coho salmon, to name a few. The parasite causes a certain disease called milky flesh or tapioca disease (visible as small white cysts in the flesh of the fish).

Discovery of the non-oxygenic breathing process of the animal

This anaerobic nature of this parasite was accidentally discovered while its genome sequence was being assembled. One of the researchers observed that it lacked a mitochondrial genome. To abate their confusion, the researchers sequenced the DNA of another animal named Myxobolus squamalis (close relatives of Henneguya salminicola) and found evidence of the expected mitochondrial genomes. The parasite did have structures that resembled mitochondria (pseudogenes) but they were not capable of producing the enzymes needed for respiration. This suggests that they have undergone a process of de-evolution. Not only mitochondria but they also have lost nearly all nuclear genes involved in the transcription and replication of the mitochondrial genome. Mitochondria (“powerhouse” of the cell) capture oxygen to produce energy. Its absence in the genome of Henneguya salminicola indicates that it does not breathe oxygen. The researchers did not find any other mechanism for energy production in the cells of Henneguya salminicola but suggested that they might steal energy from their host with the help of some types of proteins.

Life cycle of Henneguya salminicola 

This microscopic parasite infects salmon (fish host), where it undergoes proliferation and sporogenesis in pseudocysts within the white muscle (a tissue known to have anaerobic metabolism). While the obligate invertebrate host of Henneguya salminicola might probably be an annelid (family Naididae), members of the Naididae can grow and reproduce in anoxic environments. When the host dies, the spores get released. These are consumed by worms, which can also serve as hosts for the parasite. When the infected worms are eaten by other salmons, they become infected as the parasite moves into their muscles. Henneguya salminicola infections are commonly referred to as tapioca disease because of the large tapioca-like cysts that the parasite creates in the muscle tissue of fish. They can be seen by fishermen as white, oozing bubbles.

The above image shows the tapioca disease (white cysts) in salmon, caused by the parasite Henneguya salminicola

Presence of MRO without an mt genome but with cristae in this species

A circular mitochondrial (mt) genome composed of a single chromosome but no tRNAs, was recovered from Myxobolus squamalis. In contrast, no such mitochondrial sequence was identified among the contigs of Henneguya salminicola. Living multicellular developing stages of both the parasites were stained with DAPI (4’,6-diamidino-2-phenylindole) to detect the presence of DNA. Staining of both nuclei and mitochondria was observed in the cells of Myxobolus squamalis whereas only nuclear staining was seen in the case of Henneguya salminicola cells. These inferences supported the hypothesis that Henneguya salminicola has lost its mitochondrial genome. Genes involved in cristae organization (DNAJC11 and MTX1) were also detected in the genome of both species. Genes (51 and 57) involved in mitochondrial metabolic pathways were found in both the parasites, which implies that the MROs (mitochondria-related organelles) of Henneguya salminicola still perform diverse metabolic pathways. Yet Henneguya salminicola genome is devoid of almost all nuclear-encoded proteins responsible for mitochondrial genome replication and translation.

This discovery validates that adaptation to an anaerobic environment is no more unique to single-celled eukaryotes, but has also evolved in a multicellular, parasitic animal. Henneguya salminicola provides an opportunity for understanding the evolutionary transition from aerobic to exclusively anaerobic metabolism.

Also read: Drugs like ivermectin, doxycycline, Hydroxychloroquine removed in revised Covid-19 guidelines

Reference:

1. Yahalomi, D., Atkinson, S. D., Neuhof, M., Chang, E. S., Philippe, H., Cartwright, P., Bartholomew, J. L., & Huchon, D. (2020). A cnidarian parasite of salmon (Myxozoa: Henneguya) lacks a mitochondrial genome. Proceedings of the National Academy of Sciences of the United States of America, 117(10), 5358–5363. https://doi.org/10.1073/pnas.1909907117

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