Did bacteria drive the evolution of multicellularity?

The bacterium Vibrio fischeri acts as an aphrodisiac on a species of protozoan choanoflagellates, according to research presented at the 2016 American Society for Cell Biology Meeting in San Francisco.

Choanoflagellates in a mating swarm. Image courtesy of Arielle Woznica, University of California Berkeley

When V. fischeri releases an enzyme called chonodroitin sulfate lyase in the vicinity of Salpinogoeca rosetta, the choanoflagellates quickly gather in mating swarms, entering into cellular fusion while copying and recombining their genetic material.

Choanoflagellates are eukaryotes that can live freely as single cells or form multicellular colonies. They are the closest living relatives to animals, and may hold the key to understanding how ancient microbial eukaryotes evolved the capacity for multicellular complexity, eventually giving rise to the full diversity of the Animal Kingdom.

The influence of bacteria on a diverse array of animals – from coral to mammals – is widely known. V. fischeri have already been observed interacting with an organism in another kingdom: the bacteria influence the development of bioluminescent organs in the Hawaiian bobtail squid, which it uses to confuse prey while it hunts at night.

Bioluminescent hawaiian bobtail squid. Image by David Slater

But this is the first time bacteria have been shown to influence mating in eukaryotes.

The researchers who made the discovery, Nicole King and Arielle Woznica of the University of California, Berkeley, explore the origins of multicellularity by looking at shared characteristics in choanoflagellates that are conserved in animals.

King and Woznica have previously described another interaction between S. rosetta and a different bacterium, Algoriphagus machipongonensis. That bacterium secretes three separate bioactive signals that regulate the choanoflagellates’ ability to form the rosette-shaped colonies for which they are named.

S. rosetta forms rosette-shaped multicelled colonies through serial cell division, in which daughter cells remain attached to one another. Image by Ryan Null.

Dr King says this suggests bacteria may have inadvertently contributed to the evolution of multicellularity in animals through similar molecular mechanisms to those observed in S. rosetta colony formation.

The evolution of unicellular eukaryotes to multicellular animals was a dramatic moment in the history of life on Earth. All species of animals arose from a common ancestor some 600 to 800 million years ago, and that ancestor was likely a flagellated eukaryote very similar to S. rosetta. It’s possible that the early ancestors of animals were able to switch back and forth between individual and multicellular colonies, and only later did multicellularity become genetically “fixed” in the population

S. rosetta represents a model organism that researchers can use to study the molecular mechanisms that allow bacteria to  influence the behavior of choanoflagellates, which may shed light on how these mechanisms could have influenced the evolution of multicellular organisms.






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