Pollen exine – the sporopollenin enigma and the physics of pattern

Summary

Angiosperm pollen represents the pinnacle of an evolutionary progression towards gametophyte miniaturisation that probably began with the development of a toughened wall surrounding the zygote of charphycean algae, the progenitor of the land plants, some 400 million years ago (Delwiche, Graham & Thomson, 1989). The combination of acute vulnerability and central importance in the life-cycle of sexually reproducing plants inherent in the usually brief life-cycle of the male spore has driven the assembly of various adaptive features that endow its protective capsule with remarkable properties. Foremost among these is the unparalleled combination of physical strength, chemical inertness and resistance to biological attack of the outer wall, or exine, due principally to its major structural component, sporopollenin. The evolutionary history of sporopollenin, and that of the spores of land plants, is indivisible (see Chaloner, 1976). Fossil green algae dating back to the Devonian period have been shown to contain sporopollenin (Wall, 1962) and there are reports that sporopollenin also occurs in fungi (see Shaw, 1971) indicating an origin predating the appearance of plants. In an article that fuelled the debate about the existence of extra-terrestrial life, Brooks and Shaw (1969) also reported the presence of sporopollenin in meteorites and suggested that the origin of sporopollenin could even predate life on earth.

However, the exine is often much more than a protective shield. In angiosperms, for example, the pollen wall of many species has become modified to carry specific self-incompatibility proteins that promote outbreeding.