Reproductive biology of isolated fern gametophytes

Long distance dispersal of ferns is dependent on sexual reproduction via isolated spores. A single, haploid spore of an homosporous fern can produce a gametophyte which is potentially capable of producing both male and female gametes and a new sporophyte generation through intragametophytic selfing. Previous research suggests that reproductive potential of isolated spores is subject to the following limitations: (1) isolated spores may exhibit retarded germination, irregular development, and abnormal morphology, (2) isolated gametophytes of some species may tend to develop as females and remain unisexual, and (3) presence of genetic load (lethal and deleterious genes) may prevent the development of totally homozygous sporophytes produced by intragametophytic selfing. This investigation tested reproductive potential of 6,225 isolated spores from 164 plants of 14 fern species Athyrium angustum (Willd.) Presl, Cryptogramma stelleri (S. G. Gmel.) Prantl, Cystopteris bulbifera (L.) Bernh., Cystopteris tenuis (Michx.) Desv., Dryopteris cristata (L.) Gray, Dryopteris goldiana (Hook.) Gray, Dryopteris marginalis (L.) Gray, Dryopteris spinulosa (O. F. Muell.) Watt, Matteuccia struthiopteris (L.) Tod., Polypodium virginianum L., Thelypteris noveboracensis (L.) Nieuwl., Thelypteris palustris (L.) Schott, Thelypteris simulata (Davenp.) Nieuwl., and Woodsia obtusa (Spreng.) Torr. For 10 species analyzed in detail after 16 weeks of culture, germination-survival ranged from 26 to 94%; extent of morphological aberration ranged from 0 to 100%; neuter gametophytes ranged from 0 to 42%, males range from 0 to 21%, females ranged from 0 to 77%, bisexuals ranged from 17 to 95%; selfing success ranged from 2 to 100%; genetic load ranged from 0 to 98%; isolate potential, the ability of isolated spores or gametophytes to produce a sporophyte, ranged from 1 to 80% for mature gametophytes and from 1 to 66% for spores sown. Correlation analyses were performed using 16 variables of reproductive biology of the 10 species. Genetic load and bisexuality were not correlated; maleness was correlated with abnormal morphology; germination-survival was negatively correlated with percent neuter gametophytes. Low isolate potential for disjunct or peripheral populations supports phytogeographic hypotheses that these populations are post-Pleistocene relicts rather than the result of recent, long-distance dispersal. High isolate potential is related to active sexual reproduction within local opulations. Variation among species in isolate potential and other reproductive factors may significantly influence the dispersability of fern species.