Of all of the strange and wonderful things living with us on this planet Earth, it is the single-celled microscopic organisms known as bacteria which are the strangest and most wondrous. A particularly notable example can be found in bacteria of the genus Epulopiscium.
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| [IMAGE: A brown surgeonfish, also known as a Lavender Tang. Epulopiscium is found only in the guts of surgeonfish.] |
These bacteria are found only the guts of surgeonfish, a group of tropical fish whose members include Dory from Finding Nemo. They make their living as intestinal symbionts, helping their hosts digest food. This kind of mutually-beneficial relationship between bacteria and host is not at all unusual, and surgeonfish aren’t a particularly weird type of fish, but Epulopiscium makes up for all that by being one of the strangest bacteria known to man. They are so bizarre, in fact, that when they were first observed in 1985 by Israeli ichthyologists studying brown surgeonfish in the Red Sea, they were initially identified as eukaryotes.
This is not a minor error: not only are eukaryotes in a completely different evolutionary domain than bacteria, but the two groups look nothing alike. First of all, even single-celled eukaryotes are much, much larger than bacteria, to the point that they require different magnitudes of magnification to be seen clearly. Secondly, bacteria do not possess organelles like eukaryotes do. An organelle is a specialized structure that performs a unique function in the cell, the same way organs do specific jobs in the human body. Most organelles are clearly visible at a glance, so it seems impossible that actual scientists could mix up a small, organelle-less bacteria with a large eukaryote with visible internal structures. However, Epulopiscium’s weirdiosities made such mistake not just possible but probable.
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| [IMAGE: Epulopiscium, alongside a normal sized bacterium (E. coli) and a single-celled eukaryote. Notice that Epulopiscium is so big they couldn’t fit the whole organism in the picture.] |
A single glance at the above picture comparing Epulopiscium with the bacteria E. coli and a single-celled eukaryote reveals the big difference between Epulopiscium and other bacteria: namely, that Epulopiscium is ginormous. It can grow to be as much as 1,000,000 times larger than the largest E. coli. Not does it dwarf other bacteria, but it’s larger than many single-celled eukaryotes as well. In fact, the largest Epulopiscium can be seen with the naked eye. At 0.06 mm or more, they’re approximately half the size of a hyphen.
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| [IMAGE: Epulopiscium’s strange, cigar shaped ‘organelles’. The persistence and clear visibility of these bodies contributed to Epulopiscium’s misclassification as an eukaryote.] |
While bacteria that large are incredibly rare, they aren’t unheard of, and its incredible size alone wasn’t what caused the misidentification. Instead, Epulopiscium was identified as a eukaryote on the basis of of not only its eukaryotic size but upon observation of what appeared to be clearly defined-if unknown-organelles.
It wasn’t until nearly a decade after its discovery that genetic testing revealed Epulopiscium’s miscategorization and true identity as bacteria. This change transformed Epulopiscium from a large, but not abnormally so, single-celled eukaryote into one of the biggest bacteria known to science, and also raised an important question. Bacteria, by definition, don’t have organelles; so what were those rod-shaped intracellular bodies initially identified as organelles? As it turns out, they were baby Epulopiscium.
Typical bacterial reproduction is far removed from anything we see in complex life forms such as ourselves. For one thing, bacteria reproduce asexually, meaning that a single individual copies their genome and passes it to their offspring, whereas in sexual reproduction the DNA of two individuals are combined through sex to produce an offspring that is a genetic mix of both parents. Bacteria also aren’t viviparous. In viviparous organisms, the offspring develops within the body of their parent. With the exception of the platypus and the echidna, all mammals are viviparous, as well as some species of snakes, lizards, fish, and even insects. This strategy, which is restricted to the domain eukarya, is very unlike typical bacterial reproductive strategies.
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| [IMAGE: Binary Fission is the method through with bacteria typically reproduce. Note the green FtsZ ring, which pinches the bacteria into two.] |
Model bacteria like, E. coli and B. subtilis, reproduce through a process called binary fission. In binary fission, the original cell, called the ‘mother cell’, replicates all of its DNA and intracellular contents and moves the old DNA and the new DNA to opposite sides of the cell. Then the mother cell pinches itself in its middle using a ring of protein called FtsZ until it splits in half. The two genetically identical, equally sized cells produced by this process are called ‘daughter cells’.
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| [IMAGE: Epulopiscium’s reproductive strategy, which is so strange that it hasn’t been formally named. Note the tiny FtsZ rings at each end of the cell.] |
Reproduction in Epulopiscium, however, looks a lot like viviparous reproduction at first glance. The daughter cells, which can number from one to twelve depending on the species, grow inside the mother cell like fetuses developing in the womb. However, things soon become nightmarish. Epulopiscium do not give birth; the daughter cells remain inside the mother cell, growing larger and larger until the mother cell can no longer contain them and they burst out into the world. It is a birth that the mother cell does not survive.
This kind of reproductive strategy, in which the mother’s gruesome death at the hands of her own offspring is an inevitability instead of a freak occurrence, isn’t limited to just Epulopiscium and the chestburster monster from the Alien franchise. It has been documented in sea-lice, where the offspring must eat through their still-living mother’s flesh to be born. We even see it in less horrifying but still fatal forms in incredibly intelligent animals such as octopi, who care for their eggs with such intense focus that they invariably starve to death. These strategies, while undeniably awful from a human perspective, make sense from an evolutionary standpoint: as long as the number of offspring that survive thanks to their mother’s death outweigh the loss of the mother and her potential future offspring then it’s a net gain for the species.
The evolutionary processes that transformed viviparous reproduction into cannibalistic matricide in sea-lice, and maternal care into starvation in octopi seem intuitive; they are simply the typical strategies taken to horrifying extremes. However, the same cannot be said for the connection between binary fission in model bacteria like B. subtilis and the reproductive cycle of Epulopiscium. How do you go from cutting yourself in half to reproduce to growing your children inside your own body? What intermediate stages and evolutionary pressures could possibly lead to the novel, pseudo-viviparous bacterial reproduction found in Epulopiscium?
In 2012, a microbiologist named David Miller and his colleagues discovered an important answer to this question. By analyzing the Epulopiscium genome, they were able to trace the origins of Epulopiscium reproduction, not to binary fission which it barely resembles but to a process called ‘sporulation’.
Most of what we know about sporulation we learned from studying the model bacteria and sporulater B. subtilis. Sporulation is a kind of reproductive backup plan for bacteria. It produces not a copy of the mother cell like in binary fission, but a spore, a biologically inert cell that can survive all sorts of environmental stresses that would kill the non-spore form of the bacteria, called a ‘vegetative’ cell. As a spore, the bacteria can survive extreme temperatures, harsh chemicals, drought, and starvation. Then, once the environment has become liveable again, the spore emerges from its hibernation and transforms into a vegetative cell, which carries on reproducing through binary fission until the next crisis.
The spore produced by B. subtilis is an endospore, meaning that it is formed inside the bacterial cell and is only released into the outside world when the vegetative cell wall degrades. In order to accomplish this, the cell follows the first steps of binary fission up until it’s time for the cell to divide. While in binary fission the cell splits in the middle to produce two daughter cells of equal size, in sporulation the cell splits off a tip, creating a much smaller cell which the larger cell than engulfs. This newly engulfed cell develops into a spore.
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| [IMAGE: Engulfment in B. subtilis endosporulation. Note the similarities between this and the top half of the previous image.] |
Sporulation requires many unique proteins to perform all of the tiny intermediate steps necessary for the process, and all of these proteins must be coded for in the bacteria’s DNA. In contrast to organisms such as humans, whose genome is frankly cluttered with unused genes, bacteria maintain a very streamlined genome and only keep genes they need. Considering that Epulopiscium does not create spores, any preserved sporulation genes can be assumed to have been retrofitted for other purposes, such as reproduction. If Epulopiscium pseudo-viviparity is an evolution of endospore formation, than we would expect to see that the bacteria has conserved the genes used to engulf daughter cells, but may have lost the genes responsible for the development of those engulfed cells into spores instead of vegetative cells.
By sequencing the genomes of a species of Epulopiscium and of B. subtilis and comparing the more than 700 genes in B. subtilis known to be involved in the sporulation process with Epulopiscium genes, Miller and colleagues were able to find out how many sporulation genes are present in the bacteria. In agreement with their hypothesis, they found that Epulopiscium had an equivalent for every protein necessary for the engulfment step in spore-formation except two. Interestingly, those two exceptions are also absent in Epulopiscium’s closest endospore forming relative, one Cellulosilyticum lentocellum. This suggests that Epulopiscium and relatives perform those steps of the engulfment process in a different way than B. subtilis does, and so the preservation is even higher than it seems. In contrast, most of the genes used in endospore development in B. subtilis are absent in Epulopiscium. Notably, C. lentocellum has nearly twice as many conserved B. subtilis spore-development genes as its non-endospore forming Epulopiscium relatives.
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| [IMAGE: Comparison of endosporulation and Epulopiscium reproduction based off of Miller et al.’s findings. Notice how Epulopiscium completes every stage involved in engulfment, which it has the genes for, but doesn’t create any spores.] |
The implications of these findings are clear: Epulopiscium’s pseudo-viviparous daughter cells are actually endospores that developed into vegetative cells instead of spores. While the intermediate steps in the evolution of this unusual form of bacterial reproduction in a very unusual bacteria have been illuminated, the selective pressures that produced the change remain unclear. The evolutionary distance between Epulopiscium and B. subtilis should also not be forgotten. The B. subtilis endosporulation model was chosen for this paper mostly because it’s the best understood sporulation model we have. However, learning about sporulation in other bacteria could improve our understanding of how the process might have evolved in Epulopiscium.
Why would an ancestor of Epulopiscium abandon binary fission for sporulation? Sporulation is usually Plan B for bacteria; in what environment would it work better as Plan A? Does this strange reproductive strategy have anything to do with the bacteria’s unusual size or is that just a coincidence? Does it owe anything to the habit of its host species, the surgeonfish? How is this strategy more evolutionarily favorable than binary fission? In the process of answering these and other questions, scientists discover more and more about the wondrous and occasionally nightmarish forms life takes on Earth.
Work Cited:
The genomic basis for the evolution of a novel form of cellular reproduction in the bacterium Epulopiscium. D. A. Miller, G. Suen, K. D. Clements and E. R. Angert. BMC Genomics, 13:265
About the Author:
Clare Collins '19 is a Biology and Spanish double major with a passion for history and medical anthropology. Their special interests include prion diseases, comic books, and the history of medicine in the late 20th century. They plan to further the study of infectious diseases and their treatment in an age of increasing antibiotics resistance.
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