Bacteria produce a molecule that stimulates sexual reproduction in the closest living relatives of animals, according to researchers at UC Berkeley and Harvard Medical School.
Choanoflagellates are single-celled organisms that are often referred to as the last living relatives of animals. These microscopic organisms are of significant interest to scientists as they provide insight into the evolution of multicellular animals, such as humans, from their unicellular ancestors.
Although single-celled, choanoflagellates can form multicellular structures called rosettes, and it is these structures that Arielle Woznica, lead author of the study, was initially investigating. She and her colleagues wanted to understand how different bacteria affect the formation of these multicellular rosettes and decided to use Vibrio fischeri as a control bacteria since it is known not to induce multicellularity.
Strikingly, the scientists soon discovered that in the presence of this bacteria, the choanoflagellates displayed a frenzied swarming behaviour similar to that found in mating amoeba. Indeed, further investigation showed that the choanoflagellates were undergoing sexual reproduction. This was a startling discovery as, although choanoflagellates can reproduce sexually under starvation conditions, this type of swarming behaviour had never been seen in these organisms before.
An important question the team then had to address was whether it is the presence of the bacteria or a molecule produced by the bacteria that triggers sexual reproduction. The results, published in Cell, show that when choanoflagellates are placed in water which previously contained bacteria, they begin to swarm and sexually reproduce, demonstrating that it is not the presence of the bacteria but a substance they produce that causes this behaviour. They then isolated the molecule responsible and, appropriately, called it EroS: Extracellular regulator of Sex. EroS is an enzyme, which breaks up long sugar molecules enabling the bacteria to feed. However, similar to its mythological Greek namesake, EroS also triggers sexual attraction.
Importantly, this sexual behaviour is triggered in choanoflagellates by concentrations of EroS likely to be found in nature suggesting that bacteria may influence their mating behaviour in the wild.
The researchers now want to figure out exactly how EroS is stimulating sexual reproduction in these organisms. Interestingly, molecular analysis shows that ERoS is similar to an enzyme found in the head of human sperm, which weakens the egg structure upon contact allowing fertilisation to occur. EroS may have a similar function in choanoflagellates, weakening their outer membranes and making the fusion events of sexual reproduction easier. Another possibility is that EroS is producing a signal which triggers sexual reproduction in these organisms.
Bacteria have interacted with animals for millions of years and scientists are still unravelling exactly how this has affected our evolution. These results provide the first example of bacteria triggering sexual reproduction in an organism closely related to animals and researcher on the study, Jon Clardy, believes that this ‘could be the beginning of finding out yet another way in which bacteria have affected the evolution of animals’.
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