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Abstract:
Plants-insects relationship have a long co-evolution history that has been going on for at least 140 millions of years. Insect-pollination is the most widely distributed mode of fertilization in flowering plants. While it has been extensively studied on land, animal-pollination was only demonstrated recently in the marine environment, in a seagrass where the role of invertebrates species in pollen transport was highlighted. This finding contradicts the paradigm that animal mediated pollination is absent in the sea, opening up new avenues of inquiry in other phylums of the tree of life. This is particularly relevant in Rhodophyta since male gametes are non-motile and fertilization takes place onto a remote female organ. The non-flagellated sperms cannot swim to reach the receptive area of the female reproductive organ and depend entirely on passive transport in the water column, unless an additional active mechanism is involved in sperm transfer. Here, we studied biotic interactions between the red alga Gracilaria gracilis and the isopod Idotea balthica, often found in association in nature. Our results reveal that algal fertilization success was twenty times higher in the presence of idoteas in experimental conditions, when compared with a control without idoteas. Moreover, confocal microscopy analysis shows that idoteas transported spermatia that adhere to their bodies. Thus, idoteas significantly increase the reproductive success of algae by facilitating the diffusion of spermatia, indirectly via water movement due to swimming activity and gill respiration, but also by direct transport from males to females. The discovery of animal mediated fertilization in seaweed brings a new vision on how the mating system evolves in algae, in which the role of animal interactions has never been questioned. These unprecedented results change our view of the evolutionary history of plant-animal relationships. They suggest that the role of animals in plant fertilization in the marine environment may have evolved either in parallel or well before the colonization of the terrestrial environment.
