The idotée, the crustacean “pollinator of the seas”

published on Friday 07 October 2022 at 09:34

Was pollination born on land with bees or millions of years earlier, at the bottom of the ocean? The discovery of the role of the idotée, a marine crustacean, in the fertilization of red algae upsets the knowledge of the interactions between animals and plants.

“We have been asking ourselves the question for 25 years,” smiles Christophe Destombe, professor at Sorbonne University, sitting in the library of the Roscoff biological station (Finistère).

Until now, most scientists ruled out the participation of animals in the fertilization of algae. “There was a dogma that in the marine environment most of the fertilization was done by the movement of water”, explains Myriam Valero, population geneticist and director of research at the CNRS.

But the male gametes of red algae have an unusual peculiarity: they do not have a flagella, unlike sperm. Therefore, they cannot move in the water to reach the female gamete, which is immobile.

However, “we realized that with the red algae there was always a small crustacean, which is called an idol. We told ourselves that the presence of these idotées perhaps played a role in reproduction”, says Mr. Destombe.

When the two researchers set up an algal genetics and evolutionary biology lab at Roscoff in the early 2000s, “one of the initial ideas was to test this hypothesis,” explains Ms Valero.

But working with little-known living material whose reproductive organs are hidden is part of the obstacle course.

During the first experiment, all the algae die. Three experiments with red algae in Chile fail again. “It was horrible, what frustration!” recalls Marie-Laure Guillemin, a professor at the Universidad Austral de Chile, a specialist in the evolution of algae.

At the end of 2019, a doctoral student, Emma Lavaut, can finally dedicate herself 100% to this question and refine the experimental protocol: male and female red algae, 15 cm apart, are placed in aquariums, some with idots, others without.

– 20 times more fertilizations –

In the aquarium with idots, the latter move from one algae to another, displacing the male gametes that adhere to their shell. The researcher observes in this aquarium twenty times more cases of fertilization than in the aquarium where 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 idiots. The crustacean is then submerged for an hour with male algae before being removed from the water and then submerged in an aquarium with only female algae. Again, fertilization is observed.

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

“I didn’t expect there to be so much fertilization, that it would be so brazen, clear and clean,” acknowledges Emma Lavaut.

At the end of July, the article about this “pollinator of the seas” was on the cover of the prestigious magazine Science. Because this discovery in red algae, over 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 million years ago.

“Species are even more interdependent than we thought,” says Emma Lavaut. In addition to fertilizing them, the Idotes feed on the bacteria present in the red algae, thus helping to clean them and favoring their photosynthesis. Of the same color, they also find a refuge from predators there.

For Myriam Valero, it is now necessary to “look everywhere” to see if this type of interaction exists in other marine species.

“It opens up new questions for us. There is no reason for there to be a single species that uses this system”, approves Marie-Laure Guillemin.

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