The biosystematics of pteridophytes: aspects of morphology and reproductive biology of some epiphytic ferns
Characteristics of fern gametophytes relate to their habitats. Epiphytic habitats are generally covered by bryophyte mats, making a challenging environment for sporophyte production by resident fern gametophytes. Gametophyte morphology and reproductive biology of several epiphytic ferns, including seven species of Polypodiaceae (Campyloneurum angustifolium, C. phyllitidis, Lepisorus thunbergianus, Microgramma heterophylla, Phlebodium aureum, Phymatosorus scolopendria, Polypodium pellucidum) and six of Elaphoglossum (E. alatum, E. callifolium, E. crassifolium, E. marginatum, E. paleaceum, E. yoshinage), were studied. All of these species produce perennial gametophytes with clone-forming growth habits;All Polypodiaceae except P. aureum were induced to produce antheridia precociously by their own antheridiogen and by that of Pteridium aquilinum (APt). Phlebodium aureum responded to APt and promoted antheridium formation in Onoclea sensibilis but did not respond to its own antheridiogen. Neither antheridiogen production nor response was found in Elaphoglossum tested (E. callifolium and E. crassifolium);Spores of all species except P. aureum and the two tested Elaphoglossum were induced to germinate in darkness by antheridiogen. Antheridiogen may induce germination of spores which are deep in bryophytes mats where light is insufficient for spore germination, then induce the resulting gametophytes to produce antheridia, thus promoting outcrossing;Genetic load indicates that L. thunbergianus, M. heterophylla, P. pellucidum, E. alatum, E. callifolium, E. crassifolium and one source of C. angustifolium produce sporophytes mainly through intergametophytic mating whereas P. aureum, P. scolopendria and another source of C. angustifolium undergo intragametophytic selfing. Reproduction in C. phyllitidis is both intergametophytic and intragametophytic. Both genetic load and isozyme data revealed that diploids (C. angustifolium, P. pellucidum, E. alatum and E. crassifolium) reproduced by intergametophytic mating whereas polyploids reproduced by either intragametophytic selfing or intergametophytic mating;Genetic load, antheridiogen, clone-formation and perennial habit are complexly interwoven determinants of reproductive mode. Genetic load both results from and maintains outcrossing. Antheridiogen promotes male expression and thus outcrossing. The long-lived and clone-forming habit increases the gametophytes' life span and space occupied, thus increasing opportunity for gametophyte interaction and outcrossing. For the primarily inbreeding polyploids, antheridiogen may increase the opportunity for occasional intergametophytic mating and partially compensate for low genetic diversity, or antheridiogen activity may be a vestige inherited from diploid ancestors. In Elaphoglossum, due to absence of antheridiogen, clone-formation and long-lived habit may be of primary importance in promoting intergametophytic mating.