From: Andy Soos, ENN
Published October 25, 2012 09:52 AM
Bacteria Evolution
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”�), 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 µm 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.
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