The idotée, a real bee of the seas

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The idol, a real sea bee

Idotée is a small crustacean sailor who participates in the reproduction of red algae.

Did pollination originate on earth with bees or millions of years earlier at the bottom of the oceans? The discovery of the role of the idotée, a marine crustacean, in the fertilization of red algae has turned our knowledge of the interactions between animals and plants upside down.

It's been 25 years since ;we asked ourselves the question, notes with a smile Christophe Destombe, professor at Sorbonne University, seated in the library of the biological station of Roscoff, in Brittany, a region in the north-west of France.

Until now, most scientists ruled out animal involvement in the fertilization of algae. There was the dogma that in the marine environment, most fertilization was done by water movement, says Myriam Valero, a population geneticist and research director at CNRS, France's public science body. scientific research.

But the male gametes of red algae have an unusual feature: they do not have a flagellum, unlike sperm. They cannot therefore move in the water to reach the female gametes, which are immobile.

However, we noticed that with the red algae, there was always a small crustacean, which is called an idotée. We thought that the presence of these idotées perhaps played a role in reproduction, says Mr. Destombe.

When the two researchers create a genetics laboratory and evolutionary biology of algae in Roscoff, in the early 2000s, one of the ideas at the start was to test this hypothesis, explains Ms. Valero.

Working with living material, poorly known and whose reproductive organs are hidden, however, is an obstacle course.

During the first experiment, all the algae die. Three experiments on red algae in Chile fail again. It was horrible, what frustration!, remembers Marie-Laure Guillemin, professor at the Austral University of Chile, specialist in the evolution of algae.

At the end of 2019, a doctoral student, Emma Lavaut, was finally able to devote herself 100% to this question and refine the experimental protocol: male and female red algae, spaced 15 centimeters apart, were placed in aquariums, some with idotas, x27; others without.

In the aquarium with idotas, the latter move from one algae to another, displacing male gametes which stick to their shell. The researcher observes in this aquarium 20 times more cases of fertilization than in the aquarium from which the marine crustacean is absent.

The experiment is replicated to ensure that the fertilizations are not due to the simple movement of water caused by the idotas. The crustacean is then immersed for an hour with male algae before being taken out of the water and then immersed in an aquarium with only female algae. Again, fertilization is observed.

Each experiment is reproduced five times and leads to the same conclusion: the crustacean, 8 mm on average, plays a major role in the reproduction of algae, by transporting male gametes to female algae.

“I didn't expect there to be so many fertilizations, to be so blatant, clear and clean. »

— Emma Lavaut, doctoral student

At the end of July, the article on this sea pollinator appeared on the cover of the journal Science. Because this discovery in red algae, more than 800 million years old, suggests that the appearance of fertilization by animals could have occurred in the marine environment long before the colonization of the terrestrial environment 450 years ago. millions of years.

Species are even more interdependent than previously thought, notes Emma Lavaut. In addition to fertilizing them, Idotes feed on the bacteria present on the red algae, thus helping to clean them and promote their photosynthesis. Of the same color, they also find refuge there against predators.

For Myriam Valero, it is now necessary to look everywhere to check if this kind of interaction exists in other marine species.

This opens up new questions for us. There is no reason that there is only one species that uses this system, agrees Marie-Laure Guillemin.

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