You know that from the little bees and the little flowers: if some of them don't hum by, the others won't have any offspring. Apparently, this system of cross-pollination does not only exist on land: in the sea, crustaceans transport male germ cells to the female reproductive organs while migrating on red algae.
This is shown by the study by an international research team in the journal "Science". According to the researchers, it is even conceivable that this type of reproduction, which is mainly known from flowering plants, originated in the sea – long before the first plants grew on land.
Basically, one assumes that the pollination of land plants with the help of animals arose a good 140 million years ago, write the researchers led by Emma Lavaut from the Sorbonne Université in Paris. However, many questions related to the origin and development of this reproductive strategy remain unanswered.
Until 2012, it was believed that animal-mediated fertilization did not exist in the sea. Then researchers showed that in crustaceans and worms, after visiting underwater plants, pollen sticks to the body. They suspected that the animals might play a role in the pollination of aquatic plants.
Lavaut's team has now found further evidence of this in laboratory experiments with the red algae Gracilaria gracilis and the Baltic sea lice (Idotea balthica), a small crustacean. In red algae, fertilization occurs when male germ cells released into the environment meet the female sex organ, a filamentous process called trichogyn.
Since the male gametes cannot move by themselves, it is assumed that the male gametes are spread with the water current. From the merging of the germ cells, a structure called cystocarp emerges on the female algae.
The researchers first placed female algae some distance away from the males in a seawater aquarium, one with and one without crabs in the tank. They later counted the cystocarps on the plants as a measure of reproductive success. In fact, their numbers in the tanks with crayfish were about 20 percent higher than in the cancer-free tanks.
In a second experiment, the researchers put unfertilized female red algae back into the seawater tanks with or without crabs. The crabs had previously been brought together with male red algae. The result: fertilization only occurred when the crabs were present.
Microscope images showed that male germ cells of the red algae adhered to the bodies of the crabs – and that these can thus be transported from plant to plant. "Our results show for the first time that such biotic interactions can dramatically increase the probability of fertilization in seaweed," the researchers write.
However, the crabs may also benefit from the relationship: They feed on small diatoms that grow on the red algae and also find protection on the algae. The principle of animal-mediated fertilization in algae may have emerged around 650 million years ago, long before the first land plants. Perhaps the principle of fertilization also developed independently in the sea and on land.
