Print PageMosses are dependent upon the release of motile gametes into the environment for sexual reproduction to occur. It is generally accepted that without specialized structures for sperm dispersal, the spermatozoa are dependent upon rainfall for transport to the receptive archegonia where fertilization of the egg ensues, resulting in the formation of a diploid sporophyte. It is assumed that limitations on sperm dispersal are key factors influencing moss reproduction. As spermatozoa are released into the environment, they potentially encounter significant abiotic stress before reaching the archegonia. Surprisingly few studies have examined the roles of biotic and abiotic influences on sexual reproduction in bryophytes despite their ancient origins. Here, we investigated the effects of environmental stress on spermatozoa of three moss species (Bryum argenteum, Campylopus introflexus, and Ceratodon purpureus), by means of sperm extracted from mature male gametocia. We used sperm motility to determine sperm survival after all treatments. We also conducted experiments testing the relative importance of rainfall on sporophyte formation in simulated bryospheres.
Results/Conclusions
We found that spermatozoa from all three species are tolerant to desiccation for extended periods of time, that the addition of sugars during dry-down increases this tolerance to water stress, and that when hydrated in rainwater, moss spermatozoa are remarkably long-lived. Combined with our data on the interaction of water and microarthropods as sperm dispersal agents, these data provide further evidence for the complexity of sexual reproduction in mosses. We discuss our results with respect to the exciting implications of stress-tolerant motile gametes in a 450 million-year-old mating system, and the evolution of stress tolerant gametes in land plants.
