By Elizabeth Pennisi
Evolutionary biologist Thibaut Brunet studied single-celled organisms called choanoflagellates when he noticed something strange: the microbes are usually rigid, but when trapped in a cramped space, they began to move like The blob (see video above). In his lab at the University of California (UC), Berkeley, he watched their external whip-like flagella disappear; parts of their bodies began to emit and formed bubbles called leaves; and they could push into new spaces, like jelly pushing through a maze.
Because choanoflagellates are relatives of animals, the finding suggests complex movements that first developed in the ancestors of both groups. It also supports the idea that animals evolved from an ancestor that looks like choanoflagellates, says Maja Adamska, an evolutionary developmental biologist at the Australian National University who was not involved in the work. “The finding is so clear – it just makes you wonder why no one has looked before.”
After Brunet makes his first observation, he, UC Berkeley evolutionary biologist Nicole King, and their colleagues choose through more workouts. They used various ways to limit the ‘choano’, as Brunet gave them the nickname, including placing them in rooms with narrow and wide areas. Each time, the microbes became stains that escaped to escape, the team reported this week eLife. The choano can even easily switch between crawling and swimming to get out of a tight pressure in their aqueous environment.
These two behaviors are reminiscent of those seen in animal life today. Animals rely on two basic types of tissue organizations. One is a flat sheet of epithelial cells with an upside-down orientation – like the cell of a swimming choanoflagellate, which has a clear top and bottom (see video on the right). The other form is 3D and contains more free-form cells that crawl around during development and settle in specific places to become organs. The new work demonstrates that choano can be both types, and under tension switches from its usually stiff cell to the deformable one.
This ability to switch back and forth was perhaps critical when early animals began to explore new environments. Eventually, organisms developed the ability to form different types of cells in different parts of the body at the same time. It donated the scene to complex multicellular organisms, and eventually humans would emerge, Brunet and King suggest.
Researchers discussed what came first: the ability to develop into an organism with many cells, or the ability to produce different cell types. This newfound flexibility in choanoflagellates suggests that ‘this ability to alternate between cell states preceded multicellular,’ says King. By studying choanoflagellates, she and her colleagues hope to learn about the organism that gave rise to choanoflagellates and animals. “We see a much more nuanced and detailed view of the last common ancestor.”