Chlamydomonas reinhardtii: no longer single (Image: AMI Images/SPL)

A single-celled alga has evolved a crude form of multicellularity in the lab – a configuration it never adopts in nature – giving researchers a chance to replay one of life’s most important evolutionary leaps in real time.

This is the second time researchers have coaxed a single-celled organism into becoming multicellular – two years ago, the same was done with brewers yeast. But the alga is an entirely different organism, and comparing the two could explain how the transition to multicellular life happened a billion years ago.

Multicellularity has evolved at least 20 times since life first began, but no organisms have made the leap in the past 200 million years, so the process is difficult to study. To replicate the step in the lab, Will Ratcliff and Michael Travisano, evolutionary biologists at the University of Minnesota in St Paul, and their colleagues grew 10 cultures of a single-celled alga, Chlamydomonas reinhardtii. Every three days, they centrifuged each culture gently and used the bottom tenth to found the next generation. Since clusters of cells settle faster than single ones, this meant that they effectively selected for algal cells that had a tendency to clump together.


Video: Life cycle of newly multicellular algae

Sure enough, after about 50 transfers, algal cells in one of the 10 cultures began to form clusters. To the researchers’ surprise, these clusters – the first step towards true multicellularity – seemed to pass through a coordinated life cycle. Cells stuck together for hours while they settled, then quickly broke apart into single cells again each of which then divided to form new multicellular colonies (see video, above).

No multicellular ancestor

Ratcliff and his colleagues used a similar technique to evolve multicellularity from a single cell of yeast. However, critics noted that although modern yeasts are single-celled, they have descended from a multicellular ancestor, so the yeast may have merely been exhibiting an ancestral hangover. Chlamydomonas, on the other hand, has always been unicellular.

Another difference between the two organisms is that they become multicellular in different ways. Individual yeast cells remain attached to one another after cell division to form multicellular “snowflakes” that reproduce by breaking off arms. The algal cells, in contrast, divide fully but the cells remain embedded in a jelly-like sheath. This multicellular mass later releases individual cells to reproduce.

“Those differences matter,” says Travisano. Further study of the differences could shed light on why multicellularity has developed differently in the various lineages of life, he says.

The researchers’ two successes suggest that multicellularity itself is not a difficult evolutionary hurdle. “It really seems to be a simple step,” says Cristian Solari of the University of Buenos Aires, Argentina, who studies the evolution of multicellularity in Volvox, a related alga. “What’s difficult is to have complex multicellularity, where you have tissues or organ systems.”

Journal reference: Nature Communications, DOI: 10.1038/ncomms3742.