The ancestors of modern bacteria were single-celled microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, all organisms were microscopic, and bacteria and archaea were the dominant forms of life. Bacteria have a bad rap as agents of disease, but scientists are increasingly discovering their many benefits, such as maintaining a healthy gut. A new study now suggests that bacteria may also have helped kick off one of the key events in evolution: the leap from one-celled organisms to many-celled organisms, a development that eventually led to all larger multicelled animals, including humans.
Published this month in the inaugural edition of the new online journal eLife, the study by University of California, Berkeley, and Harvard Medical School scientists involves choanoflagellates (aka “choanos”Ł), the closest living relatives of animals.
These microscopic, one-celled organisms sport a long tail or flagellum, tentacles for grabbing food and are members of the ocean’s plankton community. As our closest living relative, choanos offer critical insights into the biology of their last common ancestor with animals, a unicellular or colonial organism that lived and died over 650 million years ago.
The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes. As the name suggests, choanoflagellates (collared flagellates) have a distinctive cell morphology characterized by an ovoid or spherical cell body 3—10 ┬Ám in diameter with a single apical flagellum surrounded by a collar of 30—40 microvilli.
"Choanoflagellates evolved not long before the origin of animals and may help reveal how animals first evolved," said senior author Nicole King, UC Berkeley associate professor of molecular and cell biology.
Since first starting to study choanoflagellates as a post-doc, King has been trying to figure out why some choanoflagellates live their lives as single cells, while others form colonies. After years of dead ends, King and undergraduate researcher Richard Zuzow discovered accidentally that a previously unknown species of bacteria stimulates one choanoflagellate, Salpingoeca rosetta, to form colonies. Because bacteria were abundant in the oceans when animals first evolved, the finding that bacteria influence choano colony formation means it is plausible that bacteria also helped to stimulate multicellularity in the ancestors of animals.
"I would be surprised if bacteria did not influence animal origins, since most animals rely on signals from bacteria for some part of their biology," King said. "The interaction between bacteria and choanos that we discovered is interesting for evolutionary reasons, for understanding how bacteria interact with other organisms in the oceans, and potentially for discovering mechanisms by which our commensal bacteria are signaling to us."
No one is sure why choanoflagellates form colonies, said one of the study’s lead authors, UC Berkeley postdoctoral fellow Rosanna Alegado. It may be an effective way of exploiting an abundant food source: instead of individual choanoflagellates rocketing around in search of bacteria to eat, they can form an efficient bacteria-eating Death Star that sits in the middle of its food source and chows down.
Whatever the reasons, colonies of unicellular organisms may have led the way to more permanent multicellular conglomerations, and eventually organisms comprised of different cell types specialized for specific functions.
Surprisingly, when Zuzow tried to isolate the colony-forming choanoflagellate by adding antibiotics to the culture dish to kill off residual bacteria, strange things happened, said King.
"When he treated the culture with one cocktail of antibiotics, he saw a bloom of rosette colony formation," she said, referring to the rose petal-shaped colonies that were floating in the culture media. "When he treated with a different cocktail of antibiotics, that got rid of colony formation altogether."
That observation led Zuzow and Alegado to investigate further and discover that only one specific bacterial species in the culture was stimulating colony formation. When other bacteria outnumbered it, or when antibiotics wiped it out, colony formation stopped. Alegado identified the colony-inducing bacteria as the new species, Algoriphagus machipongonensis. While she found that other bacteria in the Algoriphagus genus can also stimulate colony formation, other bacteria like E. coli, common in the human gut, cannot.
Working with Jon Clardy of Harvard Medical School, a natural products chemist, the two labs identified a molecule — a fatty acid combined with a lipid that they called RIF-1 — that sits on the surface of bacteria and is the colony development cue produced by the bacteria.
For further information see Bacteria Evolution.
Bacteria image via Wikipedia.