Fertilization syndromes in bryophytes: Parallels to pollination syndromes in vascular plants.
Nils Cronberg
dept. Ecology, Section of Plant Ecology and Systematics, Lund University, Sweden
As a heritage from their algal ancestors, bryophytes (mosses, liverworts and hornworts) and early-divergent tracheophytes (lycophytes and pteridophytes) have motile sperm that need a water surface for transportation. This is considered to be a major evolutionary constraint in terrestrial environments. Thus, any character that improves fertilization efficiency would be strongly selected. In fact, several suites of characters that promote cross-fertilization, comparable to pollination syndromes in vascular plants, occur in various bryophytes. Splash-cup mechanisms take advantage of the force of falling water drops to increase the dispersal distance of sperm, which are hitchhiking with the split droplets. Phyllodioicy involves dwarf-sized males, which germinate and grow on normal-sized female plants, adjacent to female reproductive structures, thereby facilitating out-crossing at minimal sperm dispersal distances. Mechanisms that discharge airborne sperm are known from some liverworts. We have recently proved that microarthropods (springtails and mites) can assist in the transfer of sperm of the moss Bryum argenteum and thus mediate fertilization. We also have demonstrated that microarthopods are specifically attracted to fertile moss shoots. From these experiments we concluded that an animal-mediated fertilization syndrome similar to pollination in flowering plants occur in mosses. Bryophytes and microarthropods are evolutionarily much older than flowering plants and this fertilization syndrome is therefore potentially a precursor to pollination syndromes in flowering plants.
