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I'm reading this paper about transmissible cancer cells in clams (Metzger et al. 2015) and I was wondering if there are any single-cellular organisms that are around today that are suspected as having descended from a multicellular organism
My question is different from this question as I'm not asking about cancer specifically but rather single-celled organisms evolving from multicellular organisms in general.
I would consider HeLa cells to be an example of a unicellular eukaryotic organism that evolved from humans. It can survive independently and replicate within cell culture plates, but cannot survive in the wild, however.
HeLa cells are, like in your example, cancer cells, in this specific case human cervical cancer cells. They were propagated as an immortal cell line, and completely match the replication characteristics of eukaryotic cell lines when propagated in the laboratory.
It can be even considered the case that HeLa cells are actually a kind of unicellular eukaryotic model organism not unlike S. cerevisiae, since they are widely used in experiments involving human-like cells.
As WYSIWYG has said in the comments, some biologists have even assigned the binomial name Helacyton gartleri to the cells (albeit with some controversy).
Inspired by the answer by @MarchHo, I came to think of the contagious cancer that attacks Tasmanian Devils - Devil facial tumour disease - which should provide a very similar example to the clams in your question. I don't think it has been given a species name though, but for most purposes it functions as an independent species. This "organism" lives as a parasite on Tasmanian devils, but has originated from a cancer tumour in an individual Tasmanian devil. You could maybe argue that it is not single-celled, since it develops into tumours, but it most likely spreads as single cells and I suspect that the cells functions as independent entities. However, I don't know enought about the disease to really say if e.g. cells in the tumours differentiate into different types. The cancer is spread by bites, and only affects Tasmanian devils (see http://www.tassiedevil.com: The Disease). This disease is apparently one of only three known cases of contagious cancers (also in dogs and hamsters), but this is maybe 4 now if the clams are a similar case).
(picture from Wikipedia)
The Evolution of different Cell Types in Multicellular Organisms
Evolution favors efficiency. Since living things compete for living space, food, and mates, efficient organisms that are better competitors are more likely to leave successful offspring. The division of labor leads to efficiency in living things as it does in society, and therefore, different cell types in multicellular organisms evolved.
Since multicellularity is so adaptive, plants, animals and fungi each developed it independently. Multicellular members of each kingdom competed more successfully by breeding offspring with special protective, digestive, or neural cells.
Evolution of multicellular life
There are different theories about how multicellular life evolved. Symbiosis, sharing of resources as in lichens, may have led to fusion, according to one theory. The syncytal theory suggests that cells developed multiple nuclei, and then partitioned off each nucleus with membranes into separate cells
The colonial theory, though, is the one most biologists favor. They point to actual colonies of single celled organisms as proof of the concept. Eudoria live in colonies of up to 128 cells. They, and many other colonial species, are proof that unicellular organisms prospered by grouping together. These are still not multicellular organisms, because there is no differentiation of function among the colonists, no division of labor. Each cell behaves exactly as the one next to it does.
Volvox is an alga that lives in a hollow spherical colony in water. It has cells that are asexual, and also cells that divide and reproduce, just as animals have. Once a daughter colony has been created, it matures inside the hollow Volvox sphere, which then breaks open to (in effect) give birth to it.
To a biologist, a multicellular organism has cells with different functions, that all share the same genetic material. Therefore, a mere group of cells that are all the same is not multicellular.
Choanoflagellates may have been the first step in the evolution of many-celled animals with different types of cells. Unicellular, they live in underwater colonies. They cling to rock and use their whip-like flagellum to draw in prey in a water vortex. Their colonies are not a single organism though, because there is no specialization, it is every cell for itself.
Sponges are metazoans, multicellular animals. They reproduce sexually. If certain pieces of sponge are broken off though, each can recombine and live on. This is because a sponge has no organs, no liver or heart to be destroyed.
In addition, many of the cells of the sponge, while differentiated, are flexible. They can change roles and positions. A sponge does not have tissues as other animals do. One kind of sponge cell is essentially a choanoflagellate.
Ediacarans were possibly the first animals. Not much is known about them. Scientists believe some ediacarans formed a sort of loose bag around water and algae, and lived off the food that they absorbed from the sea, with the help of the symbiotic alga. They came in other forms as well.
They mostly disappeared at the beginning of the Cambrian era. Perhaps predators got these sluggish creatures. Perhaps they were out-competed. Perhaps there was a universal catastrophe.
About 530 million years ago, the diversity of animal life seems to have exploded. Like a child who slowly learns to read proficiently, and then suddenly acquires access to all knowledge, life began to take numberless forms, or so it seems. Charles Darwin himself worried about it, thinking the sudden explosion could be seen as an argument against the subtle shaping of life through generations that his theory described.
Of course, the Cambrian explosion took many lifetimes, but it was short in geologic time. There are many explanations offered for the Cambrian explosion, including, for one, that it is an illusion. There were more species before the explosion than we are aware of, some scientists say, and so the increase is not as great as it seems.
Another possible explanation, though, is that the spread of multicellular organisms had enabled each new species to compete more fiercely, and thus frantically increased the arms race among competing organisms, requiring each to compete, to specialize, or to die away.
Multicellular organisms are able to protect themselves better against predators. They compete more successfully for living space. They are more efficient, and in general better adapted. Therefore, evolution produced more, and more varied multicellular organisms with more specialized cells.
Evolution of different cell types allowed kelp to anchor itself to the ocean floor, land animals to grow protective shells, plants to grow roots, and primates to grow brains.
Scientists Have Witnessed a Single-Celled Algae Evolve Into a Multicellular Organism
Most of us know that at some point in our evolutionary history around 600 million years ago, single-celled organisms evolved into more complex multicellular life.
But knowing that happened and actually seeing it happen in real-time in front of you is an entirely different matter altogether.
And that's exactly what researchers from the Georgia Institute of Technology and University of Montana have witnessed - and captured in the breathtaking, time-lapse footage below.
The evolution took just 50 weeks, and was triggered by the introduction of a simple predator.
The team captured nine, 14-second time-lapse videos of the transition, which you can view in this playlist:
In this incredible experiment, the team was trying to figure out exactly what drove single-celled organisms to become multicellular all those years ago.
One hypothesis is that it was predation that put selective pressure on single-celled organisms, causing them to become more complex.
So to test the validity of this in the lab, the team led by evolutionary biologist William Ratcliff, took populations of single-celled green alga Chlamydomonas reinhardtii.
They then put a single-celled filter-feeding predator in the mix, Paramecium tetraurelia and watched what happened.
Incredibly, the researchers watched as in just 50 weeks - less than the span of a year - two out of five experimental populations of the single-celled creatures evolved into multicellular life.
"Here we show that de novo origins of simple multicellularity can evolve in response to predation," the team write in their paper.
Fifty weeks is a relative blink of an eye on the evolutionary scale. For the algae it was a little longer - 750 generations. But that's still quite impressive when you think that they evolved entirely new life cycles.
Being able to witness something like this is not only absolutely mind-blowing, but it also suggests that predation could have played some kind of role in at least part of the evolution of multicellularity.
Not only that, but the resulting multicellular organisms were all incredibly varied. Just like you'd expect in natural evolution.
"Considerable variation exists in the evolved multicellular life cycles, with both cell number and propagule size varying among isolates," the team write in their paper.
"Survival assays show that evolved multicellular traits provide effective protection against predation."
The research has been published in Scientific Reports and the full paper is freely available.
Evolutionary Biologists Make Multicellular Life Evolve in the Lab
The evolutionary transition between single-celled organisms and multicellular life as we know it took several billion years to occur in nature, but under artificial pressure, evolutionary biologists have been able to make it happen in 60 days.
Single-celled yeast became multicellular creatures, a crucial step for life’s progression, from algae and bacteria to more complex forms of life. While this doesn’t duplicate what happened in the prehistoric transitions, it could help evolutionary biologists reveal the principles that guided them.
In the new study, published on January 17th in the Proceedings of the National Academy of Sciences, researchers led by evolutionary biologists Michael Travisano and William Ratcliff from the University of Minnesota grew brewer’s yeasts in flasks of nutrient-rich broth.
The experimental protocol was quite simple: once per day, they shook the flasks, removed the yeast that settled at the bottom and used it to start new cultures. Free-floating yeast was left behind, while yeast gathered in heavy, falling clumps survived to reproduce. Within a few weeks, the individual yeast cells were still single-celled, but clumped together. After two months, they came to a permanent arrangement. Each strain evolved to become multicellular, displaying all of the characteristics of complex forms of life.
The cells cooperate, and this cooperation benefits all of the cells involved. They gave a single-celled organism reason to become multicellular, and proved that this was what happened experimentally. Rapid evolution could occur in nature, it just needs to be discovered.
The multicellularity was engineered via artificial selection, but there is no reason why this couldn’t have happened in nature.
Are there single-celled organisms that have evolved from multi-cellular ones? - Biology
The transition from one-celled microbes to multicellularity was a huge step in the evolution of life on this planet, but as daunting as this evolutionary step seems, it didn't happen just once. Today's plants, fungi, animals, and various types of algae are all descendants of separate transitions to multicellular life.
All of these transitions from a single-cell lifestyle to multicellularity occurred in the very distant past, so how can we learn anything about them? It turns out that it is not hard to find living, modern examples that closely parallel the momentous evolutionary transitions that led to animals, plants, and fungi. Right now on earth there are primitive multicellular organisms that, in many ways, resemble the first multicellular creatures that existed a billion years ago. Researchers are using these organisms to understand what kinds of genetic changes are needed to turn a single-celled organism into a multicellular one.
A group at the University of Arizona has published a study of of one group the these amazing organisms, the volvocine green algae. What's amazing about this group of algae is that you can find a range of multicellular sophistication in closely relate algae species. There are species that form simple sets of four identical cells stuck together, other that form balls of 32-64 not quite identical cells with some specialized functions, up to full-blown multicellular organisms with 50,000 highly specialized cells, including reproductive germ cells. The evolution of multicellularity is not an irrecoverable event from an unimaginably distant past it is something we can observe, manipulate, and test in the lab today.
With the availability of so many different types of green algae at varying levels of multicellular sophistication, the U. of Arizona researchers were able to put a timeline on the evolution of specific features of multicellular algae. They did this by calibrating DNA differences between species with absolutely dated fossils: DNA provides a relative time scale, since the more DNA differences there are between species, the longer it's been since their lineages diverged and this relative time scale can be matched up against dated fossils that show when new major types of multicellular algae began to appear.
Here is part of the time line the researchers came up with:
223 million years ago, a species of single-celled green algae began forming aggregates of cells stuck together by a glue of secreted proteins and sugars (and we can see species which do this today).
200 million years ago, the rate of cell division began to be controlled genetically. Unlike single-celled organisms, which reproduce whenever the surrounding environment is right, the new multicellular algae began controlling exactly how many daughter cells they produce. This is a critical step towards establishing a multi-cellular body-plan with genetically controlled dimensions.
3) Roughly 10 million years later, the cells of some multicellular algae species began to orient their whip-like flagella in the same direction, so that all of the flagella would work together to control the swimming direction of the organism.
100 million years ago, some of the algae species had established separate reproductive germ cells, and ever since then, various volvocine algae species have developed more cells with highly specialized functions.
One feature of this time scale is that the major innovations occur sporadically. The researchers suggested that these major events coincided with the inventions of new ways for resolving conflicts among individual cells in the organism: in other words, formerly independent cells had to learn how to be civilized. Single-celled microbes function very well as individuals. Some of that individuality has to be given up for the greater good when cells hitch their evolutionary fates together as one multicellular organism. A key example of conflict resolution is the evolution of genetic limits on cell division: to have a coherent, multicellular body plan, individual cells can't just divide with abandon, the way bacteria do. When cells escape these genetic controls on division in humans, you get cancer.
The evolution of multicellular organisms is a major evolutionary step. In our history (the history of animals), how that step happened is lost somewhere in deep history. Nevertheless, the evolution of multicellularity has happened over and over again, and in the case of the volvocine algae, we can study this key evolutionary step in the lab.
Join me tomorrow, here at Adaptive Complexity, for day 19 of 30 Days of Evolution Blogging Evolution as a science is alive and well. Each day I will blog about a paper related to evolution published in 2009.
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Front Page image of Volvox aureus by Dr. Ralf Wagner, courtesy the Wikimedia Commons, published under the GNU Free Documentation License.
Welcome to Adaptive Complexity, where I write about genomics, systems biology, evolution, and the connection between science and literature,
The Institute for Creation Research
Recent headlines claim, &ldquoScientists Have Witnessed a Single-Celled Algae Evolve Into a Multicellular Organism.&rdquo 1 In reality, the experiment showed that nothing more than a crude clumping together of individual cells had occurred. A new multicellular organism was not created, nor was any real evolution observed.
One of the major hurdles in the grand story of molecules to man evolution is how life first transitioned from unicellular to multicellular organisms. Plants and animals are complex systems of interlocking cells that form tissues, structures and whole bodies. How could creatures like bacteria or algae make the grand evolutionary hurdle into complex multicellular creatures? There is no evidence of this ever occurring in the fossil record and we don&rsquot see this sort of thing happening now.
Despite the futility of the evolutionary paradigm to explain real-world data, scientists who reject God will latch onto virtually any natural phenomena and then put some strange twist on it to support their paradigm. Such is the case with a new study involving a unicellular type of algae called Chlamydomonas reinhardtii. This type of creature is typically found as a free swimmer in either fresh or salt water with the help of two flagellum (whip-like tails). However, it&rsquos also well known for its ability to form a gelatinous coat and then clump together with other algae cells to form small clusters of cells called palmelloids. 2 This clumping behavior is an adaptive response that arises as a result of its interaction with its environment.
In 2006, scientists discovered that C. reinhardtii would form these clusters when cultured with rotifer&mdashanother microscopic creature that liked to eat them. 2 The clustering together would help them avoid being eaten. Now, in this current study the same phenomena has been observed in a somewhat more elaborate experiment. 3 In this new study, an inbred strain of C. reinhardtii was crossed with other genetically diverse types to create new populations with a large amount of genetic variability. Then the researchers exposed isolates taken from these crosses to a single-celled predator called a paramecium. In two of the five isolates, the algae seemed to permanently express the tendency to cluster together. In the genetically diverse populations exposed to the predator, this never happened.
Not only did the researchers make the extravagant claim that they had observed the evolution of multicellularity, but they also claimed that the &ldquoselection pressure&rdquo of a predator&rsquos presence caused the alleged evolutionary process. But a newly evolved multicellular creature was never observed&mdashjust globs of algae documented previously as in other studies. The fact that some isolates expressed the trait permanently likely meant that a loss of information had occurred. Perhaps a mutation of a gene occurred in the adaptive response pathway enabling them to cycle back and forth between clumping and free-living. In the genetically diverse populations, this never happened. Typically, mutations like this do not allow such creatures to survive in the wild because they are handicapped and can&rsquot flexibly adapt.
In a previous study, the researchers had wisely noted that the ability of the algae to dynamically adapt their size and clumping traits to their environment was evidence that they could &ldquotrack environmental changes and respond appropriately.&rdquo 2 This new study is yet just another example of evidence for what ICR scientist Dr. Randy Guliuzza has documented called &ldquocontinuous environmental tracking&rdquo&mdasha hallmark of built-in engineered adaptability. 4 Research like this ought to be giving glory to the Creator that engineered the pre-programmed adaptability of these creatures, not the illogic of evolutionary myth.
From single cells to multicellular life
All multicellular creatures are descended from single-celled organisms. The leap from unicellularity to multicellularity is possible only if the originally independent cells collaborate. So-called cheating cells that exploit the cooperation of others are considered a major obstacle. Scientists at the Max Planck Institute for Evolutionary Biology in Plön, Germany, together with researchers from New Zealand and the USA, have observed in real time the evolution of simple self-reproducing groups of cells from previously individual cells. The nascent organisms are comprised of a single tissue dedicated to acquiring oxygen, but this tissue also generates cells that are the seeds of future generations: a reproductive division of labour. Intriguingly, the cells that serve as a germ line were derived from cheating cells whose destructive effects were tamed by integration into a life cycle that allowed groups to reproduce. The life cycle turned out to be a spectacular gift to evolution. Rather than working directly on cells, evolution was able to work on a developmental programme that eventually merged cells into a single organism. When this happened groups began to prosper with the once free-living cells coming to work for the good of the whole.
Diversity among nascent multicellular collectives: In such dishes containing various strains of Pseudomonas fluorescens scientists have observed in real time the evolution of simple self-reproducing groups of cells from previously individual cells.
Single bacterial cells of Pseudomonas fluorescens usually live independently of each other. However, some mutations allow cells to produce adhesive glues that cause cells to remain stuck together after cell division. Under appropriate ecological conditions, the cellular assemblies can be favoured by natural selection, despite a cost to individual cells that produce the glues. When Pseudomonas fluorescens is grown in unshaken test tubes the cellular collectives prosper because they form mats at the surface of liquids where the cells gain access to oxygen that is otherwise – in the liquid – unavailable.
Given both costs associated with production of adhesive substances and benefits that accrue to the collective, natural selection is expected to favour types that no longer produce costly glues, but take advantage of the mat to support their own rapid growth. Such types are often referred to as cheats because they take advantage of the community effort while paying none of the costs. Cheats arise in the authors’ experimental populations and bring about collapse of the mats. The mats fail when cheats prosper: cheats obtain an abundance of oxygen, but contribute no glue to keep the mat from disintegrating – the mats eventually break and fall to the bottom where they are starved of oxygen.
Paul Rainey, who led the study at the New Zealand Institute for Advanced Study and the Max Planck Institute for Evolutionary Biology, explains: “Simple cooperating groups – like the mats that interest us – stand as one possible origin of multicellular life, but no sooner do the mats arise, than they fail: the same process that ensures their success – natural selection – , ensures their demise.” But even more problematic is that groups, once extant, must have some means of reproducing themselves, else they are of little evolutionary consequence.
Pondering this problem led Rainey to an ingenious solution. What if cheats could act as seeds – a germ line – for the next set of mats: while cheats destroy the mats, what about the possibility that they might also stand as their saviour? “It’s just a matter of perspective”, argues Rainey. The idea is beautifully simple, but counter-intuitive. Nonetheless, it offers potential solutions to profound problems such as the origins of reproduction, the soma / germ distinction – even the origin of development itself.
In their experiments the researchers compared how two different life cycles affected group (mat) evolution. In the first, the mats were allowed to reproduce via a two-phase life cycle in which mats gave rise to mat offspring via cheater cells that functioned as a kind of germ line. In the second, cheats were purged and mats reproduced by fragmentation. “The viability of the resulting bacterial mats, that is, their biological fitness, improved under both scenarios, provided we allowed mats to compete with each other,” explains Katrin Hammerschmidt of the New Zealand Institute for Advanced Study.
Surprisingly however, the researchers found that when cheats were part of the life cycle, the fitness of cellular collectives decoupled from that of the individual cells: that is, the most fit mats consisted of cells with relatively low individual fitness. “The selfish interests of individual cells in these collectives appear to have been conquered by natural selection working at the level of mats: individual cells ended up working for the common good. The resulting mats were thus more than a casual association of multiple cells. Instead, they developed into a new kind of biological entity – a multicellular organism whose fitness can no longer be explained by the fitness of the individual cells that comprise the collective” says Rainey.
Model of multicellular evolution overturns classic theory
Cells can evolve specialised functions under a much broader range of conditions than previously thought, according to a study published today in eLife.
The findings, originally posted on bioRxiv*, provide new insight about natural selection, and help us understand how and why common multicellular life has evolved so many times on Earth.
Life on Earth has been transformed by the evolution of multicellular life forms. Multicellularity allowed organisms to develop specialised cells to carry out certain functions, such as being nerve cells, skin cells or muscle cells. It has long been assumed that this specialisation of cells will only occur when there are benefits. For example, if by specialising, cells can invest in two products A and B, then evolution will only favour specialisation if the total output of both A and B is greater than that produced by a generalist cell. However, to date, there is little evidence to support this concept.
“Rather than each cell producing what it needs, specialised cells need to be able to trade with each other. Previous work suggests that this only happens as long as the overall group’s productivity keeps increasing,” explains lead author David Yanni, PhD student at Georgia Institute of Technology, Atlanta, US. “Understanding the evolution of cell-to-cell trade requires us to know the extent of social interactions between cells, and this is dictated by the structure of the networks between them.”
To study this further, the team used network theory to develop a mathematical model that allowed them to explore how different cell network characteristics affect the evolution of specialisation. They separated out two key measurements of cell group fitness – viability (the cells’ ability to survive) and fecundity (the cells’ ability to reproduce). This is similar to how multicellular organisms divide labour in real life – germ cells carry out reproduction and somatic cells work to ensure the organism survives.
In the model, cells can share some of the outputs of their investment in viability with other cells, but they cannot share outputs of efforts in reproduction. So, within a multicellular group, each cell’s viability is the return on its own investment and that of others in the group, and gives an indication of the group’s fitness.
By studying how the different network structures affected the group fitness, the team came to a surprising conclusion: they found that cell specialisation can be favoured even if this reduces the group’s total productivity. In order to specialise, cells in the network must be sparsely connected, and they cannot share all the products of their labour equally. These match the conditions that are common in the early evolution of multicellular organisms – where cells naturally share viability and reproduction tasks differently, often to the detriment of other cells in the group.
“Our results suggest that the evolution of complex multicellularity, indicated by the evolution of specialised cells, is simpler than previously thought, but only if a few certain criteria are met,” concludes senior author Peter Yunker, Assistant Professor at Georgia Institute of Technology, Atlanta, US. “This contrasts directly to the prevailing view that increasing returns are required for natural selection to favour increased specialisation.”
Is there any evidence that some single-celled organisms evolved *from* multi-cellular organisms?
I think it is assumed that life started out with microorganisms which combined into more complex organisms. But I have read that amoebas have complex genomes by certain metrics -- would this "extra" information potentially be the result of them have once been part of something more complex and perhaps such organisms could be made to differentiate into more specialized cells?
A commonality among all I know are that they are parasitic descendants of the host species -- that is, infectious cancers that became transmissable parasites.
First is Canine transmissible venereal sarcoma. They are cancerous cells that became a sexually transmitted disease several thousand years ago. Over the years they have lost quite a bit of their canine genetic material, being single-celled organisms instead of wolves/dogs. So, they are no longer genetically canine (unlike a typical cancer), but rather a different species.
For a different type of example that's not quite single-celled is a ridiculously reduced jellyfish that causes whirling disease in trout.
Any evolution paradigm (Darwinism, emergent evolution, extended synthesis, etc.) presumes that new genes will constantly form as organisms evolve. Yet frequently cited examples of new gene evolution are actually the loss of pre-existing genetic activity.23 Instead, the formation of new genes remains largely undocumented.24
This is a significant problem, and one almost always overlooked by the evolutionary community. Regardless of what historical reconstructions and circumstantial evidence is put forward, without a plausible genetic mechanism any evolutionary scenario has little credibility. They are literally just a story.