Evolution from Single to Multi-Cell Clusters Replicated
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January 17, 2012

Evolution from Single to Multi-Cell Clusters Replicated

 

           1-biologistsre

 

More than 500 million years ago, single-celled organisms on Earth's surface began forming multi-cellular clusters that ultimately became plants and animals.  Just how that happened is a question that has eluded evolutionary biologists. Now, scientists have replicated that key step in the laboratory using common Brewer's yeast, a single-celled organism.

The yeast "evolved" into multi-cellular clusters that work together cooperatively, reproduce and adapt to their environment--in essence, they became precursors to life on Earth as it is today.The results are published in this week's issue of the journal Proceedings of the National Academy of Sciences (PNAS).

"The finding that the division-of-labor evolves so quickly and repeatedly in these 'snowflake' clusters is a big surprise," says George Gilchrist, acting deputy division director of the National Science Foundation's (NSF) Division of Environmental Biology, which funded the research.

"The first step toward multi-cellular complexity seems to be less of an evolutionary hurdle than theory would suggest," says Gilchrist. "This will stimulate a lot of important research questions."It all started two years ago with a casual comment over coffee that bridging the famous multi-cellularity gap would be "just about the coolest thing we could do," recalled Will Ratcliff and Michael Travisano, scientists at the University of Minnesota (UMN) and authors of the PNAS paper.

Then came the big surprise: it wasn't that difficult.Using yeast cells, culture media and a centrifuge, it only took the biologists one experiment conducted over about 60 days.

"I don't think anyone had ever tried it before," says Ratcliff. "There aren't many scientists doing experimental evolution, and they're trying to answer questions about evolution, not recreate it.

"The results have earned praise from evolutionary biologists around the world."To understand why the world is full of plants and animals, including humans, we need to know how one-celled organisms made the switch to living as a group, as multi-celled organisms," says Sam Scheiner, program director in NSF's Division of Environmental Biology.

"This study is the first to experimentally observe that transition," says Scheiner, "providing a look at an event that took place hundreds of millions of years ago."In essence, here's how the experiments worked:The scientists chose Brewer's yeast, or Saccharomyces cerevisiae, a species of yeast used since ancient times to make bread and beer because it is abundant in nature and grows easily.They added it to nutrient-rich culture media and allowed the cells to grow for a day in test tubes.Then they used a centrifuge to stratify the contents by weight.As the mixture settled, cell clusters landed on the bottom of the tubes faster because they are heavier. The biologists removed the clusters, transferred them to fresh media, and agitated them again.

Sixty cycles later, the clusters--now hundreds of cells--looked like spherical snowflakes.Analysis showed that the clusters were not just groups of random cells that adhered to each other, but related cells that remained attached following cell division.That was significant because it meant that they were genetically similar, which promotes cooperation. When the clusters reached a critical size, some cells died off in a process known as apoptosis to allow offspring to separate.The offspring reproduced only after they attained the size of their parents.

"A cluster alone isn't multi-cellular," Ratcliff says. "But when cells in a cluster cooperate, make sacrifices for the common good, and adapt to change, that's an evolutionary transition to multi-cellularity."In order for multi-cellular organisms to form, most cells need to sacrifice their ability to reproduce, an altruistic action that favors the whole but not the individual, Ratcliff says.For example, all cells in the human body are essentially a support system that allows sperm and eggs to pass DNA along to the next generation.Thus multi-cellularity is by its nature very cooperative."Some of the best competitors in nature are those that engage in cooperation, and our experiment bears that out," says Travisano.

Evolutionary biologists have estimated that multi-cellularity evolved independently in about 25 groups.Travisano and Ratcliff wonder why it didn't evolve more often since it's not that difficult to recreate in a lab. Considering that trillions of one-celled organisms lived on Earth for millions of years, it seems like it should have, Ratcliff says.That may be a question the biologists will answer in the future using the fossil record for thousands of generations of multi-cellular clusters, which are stored in a freezer in Travisano's lab.

Since the frozen samples contain multiple cell lines that independently became multi-cellular, the researchers can compare them to learn whether similar or different mechanisms and genes were responsible in each case, Travisano says.The next steps will be to look at the role of multi-cellularity in cancer, aging and other critical areas of biology.

"Multi-cellular yeast is a valuable resource for investigating a wide variety of medically and biologically important topics," Travisano says."Cancer was recently described as a fossil from the origin of multi-cellularity, which can be directly investigated with the yeast system."Similarly the origins of aging, development and the evolution of complex morphologies are open to direct experimental investigation that would otherwise be difficult or impossible."

Image at top of page is a multi-cellular 'snowflake' yeast image with a blue cell-wall stain and red dead-cell stain. 

More information: Ratcliff, W. C., Denison, R. F., Borrello, M. & Travisano, M. Experimental evolution of multicellularity, Proc. Natl Acad. Sci. http://dx.doi.org/ … s.1115323109 (2012).

The Daily Galaxy via National Science Foundation 

Image credit: Will Ratcliff and Mike Travisano

Comments

Is space an act of randomness or is it too, evolving into something with order to it?

Hey! This is NOT Evolution. The DNA of the yeast involved is in no way altered. By the way, what would happen if the yeast cells were removed from the container? It would all fall apart. The theory of Evolution has a long way to go to prove itself scientifically, and by that I mean repeatable experiments.

George,

And your alternative theory to evolution is what? god? jesus? allah? mohammed? Zeus? Bigfoot?

Your study takes an existing life form, recreates the same form, to watch it behave differently (because its environment and living conditions direct its behaviors) and you make a quantum leap conclusion that the organism is different. It is still a yeast cell. You have not shown the cell did not previously have the same predisposition in a similar life environment. You perpetuate a myth more as a self fulfilling prophesy. This does not present evidence that the cell's genome or genetic nature has changed, or that these cells removed to a different environment would continue to produce cells and cell behavior similar to the layered cells. Creatures that grow on a sea reef grow in layers as colonies, but that does not mean they are genetically changed as they continue to build up layers.
Evolution is a myth, not a science.

Actually yes, the organism *was* different after 60 generations and behaved differently under the *same* conditions given to generation #1. The fact that more and bigger clusters were being produced with each successive run of the experiment would require this. Generation #60 clustered in ways that #1 did not - and that this came with a *genetic* difference is made clear in the full write-up of the research. This is just one case of evolution in 'bottle experiments' and there are plenty of researchers out there doing this.
http://www.lifescientist.com.au/article/322599/evolution_bottle/

This was just one case of forced evolution by artificial selection of a single trait (clustering). It's the same basic mechanism that was carried out over historical time to produce kale, broccoli, cauliflower, kohlrabi and Brussels sprouts from the same original food crop (this is established historical/archeological fact). It's the same process that produces antibiotic-resistant strains of bacteria from susceptible ones (though in that case the selection is accidental) and it also works naturally in bacteria in the wild - and for any organism given enough time and selective pressure.

If there are any genetic differences between a parent generation and an offspring generation (from recombination and/or mutation, or just some parents having more offspring than others) and any selection pressure within each generation (from the weakest going to the wall through lack of resources, or being selected for in some other way) then evolution *must* occur to some extent, given many many generations, because the genes that don't produce good adaptations will get weeded out. This isn't a theory, it's just how living things would have to work over long time spans given how genetics works. Evolution = biology + time. (so the real question should be about how old you think the world is)

I fully appreciate that many people don't like that idea of evolution or find it offensive or blasphemous. But misrepresenting the science in ways that let you avoid believing in it is just denialism.

Mmm, so they proved that a community of organisms achieved survival, Macro level human community working in concert to achieve a goal, those who don't agree get excluded or jailed cause they don't believe in the direction society/community is headed or die do to old age. Now the question becomes is there a blueprint or set plan to how things operate if so who set the plan in motion. I guess a better thought might be we should persue finding out what orchestrated the grand plan that everything operates by. The old adage "Many hands make light work" is proved


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