Submarine pollination in seagrasses

Abstract

THERE is great interest in water plants and their adaptation to the aquatic environment. Seagrasses, in the family Cymodoceaceae, are the only wholly marine group of flowering plants which carry out their entire life cycle in the sea¹. The male and female flowers are borne on different plants. Consequently, their pollination mechanism presents intriguing problems, for terrestrial angiosperms, from which seagrasses presumably arose^2,3, shed dry pollen. This becomes hydrated only after alighting on the female stigma, where recognition events determine acceptance or rejection of the pollen⁴. These events seem to involve interactions between surface proteins or glycoproteins borne by both pollen and stigma⁵. The pollen proteins are released on to the stigma during normal pollination^6,7, but are lost from the surface whenever pollen is wetted⁸. The system found in terrestrial species could thus hardly operate with submarine pollination. We have found that adaptations have occurred in both pollen and stigma of the seagrasses, accommodating the pollination system to the marine environment. Changes in the shape and form of the pollens, together with the loss of the outer wall layer, make it possible for the pollen to be carried in water currents as long, rope-like masses. The receptive stigma cells secrete a proteinaceous surface layer that is not dispersed in seawater, providing a suitable medium for trapping the pollen during submarine pollination.