What's the News: We walking, talking agglomerations of cells have always thought of multicellular life as a profound jump in evolution. The first organisms were just single cells, but at some point, they began to work together for the good of the whole, divvying up tasks like nutrient transport and cellular messaging. Eventually, these colonies became the complex multicellular life that we know and love. But maybe being multicellular isn't as difficult to achieve as we thought. Scientists presenting
at the Society for the Study of Evolution
conference have, over just a couple months, gotten single-celled yeast to grow into colonies that function as multicellular organisms. How the Heck:
First, to get populations of yeast that would be naturally inclined to stick together, the biologists made it hard for lone cells to survive. They suspended cells in tubes of liquid and then spun them in centrifuges, which caused clumped cells to sink to the bottom, while lighter, singleton cells stayed afloat. While floating cells were discarded, the sticky cells underwent the selection process again and again. The team developed 10 separate strains of sticky cells this way, which they spread on Petri dishes and watched grow.
Within 60 days, the cells had developed snowflake-shaped colonies whose reproduction resembled that of a multicellular organism: when a snowflake got large, part of it broke off and formed a new, smaller snowflake.
Furthermore, there were tantalizing signs of division of labor in the form of cell death, or apoptosis. When a snowflake grew large, some of its cells killed themselves to make a weak point where a smaller snowflake could break off. While part of a larger organism often sacrifices itself for the good of the whole (such cell death is a major theme of multicellular development, from fruit flies to humans), this isn't something single cells generally do.
The researchers showed that the size at which snowflakes started fragmenting this way changed according to how much evolutionary pressure they put on the colonies, indicating that evolution was occurring on the multicellular level. Selection, in other words, was having an effect on the whole snowflake, not just on individual cells.
The sticky cells in each of these snowflakes were genetically identical to each other---each cell had budded off from a mother cell, but had stuck around rather than setting off on its own. Since each colony was a mass of identical cells, it made evolutionary sense that they would cooperate for the good of the group.
What's the Context:
Multicellular life has evolved at least 20 times in the past---the first time may have been more than 2 billion years ago---and there are likely several different ways of achieving this state.
Understanding how it arose is one of the central problems in biology, and this work points to certain characteristics, like cell death, that might have been important in the early stages of multicellular life. Last year, geneticists working on a sponge genome identified more than a thousand genes that appear to make up the toolkit required for the jump, including genes for apoptosis, cell signaling, and telling self from other.
Not So Fast: While the speed with which these yeast colonies developed multicellular characteristics is pretty exciting, in some ways it's not that surprising that yeast are capable of banding together. Though today's yeast are single-celled organisms, they were multicellular in the past, millions of years ago, says Neil Blackstone
, an evolutionary biologist at Northern Illinois University in DeKalb (via New Scientist
). "I bet that yeast, having once been multicellular, never lost it completely. I don't think if you took something that had never been multicellular you would get it so quickly," he says. The Future Holds: To address that criticism, the researchers plan to try similar experiments with an algae species that has never been multicellular. And they will keep watching the evolution of the snowflakes, as well, to see what other tricks they have up their sleeves, as they work to have the findings published in a journal. (via New Scientist
)
Image credit: Wikimedia Commons
