Significance of Form in Fern Gametophytes: Clonal, Gemmiferous Gametophytes of Callistopteris baueriana (Hymenophyllaceae)

Gametophytes of Callistopteris baueriana (Endl.) Copeland exhibit an unreported and unique morphology for the Hymenophyllaceae, combining characters that traditionally have been thought to be specific to either Hymenophyllum s. 1. or Trichomanes s. 1. We describe here the mature gametophyte morphology of C. baueriana from Hawaii. To explore gametophyte character evolution in the Hymenophyllaceae, gametophyte characters of selected genera within the family were analyzed phylogenetically. In Hawaii, gametophytes of C. baueriana were associated with sporophyte production or grew independently, often in extensive, dense mats. Well-developed sporophytes occurred at high elevations. At drier midelevations, sporophytes were dwarfed. Sporophytes were absent at the lowest elevations and gametophytes grew independently in large bryophyte-like mats. The sporophytes of C. baueriana seemed unable to survive and reproduce in dry conditions, whereas gametophytes survived and maintained their presence by vegetative growth and the production of gemmae. The gametophytes had a branched, ribbon-like thallus and were long-lived and clone-forming. Generally, a meristematic branch grew along the substrate while subordinate branches grew perpendicularly to the substrate and terminated in the production of gemmae. The gemmae were largely uniseriate, spindle-shaped, and branched or unbranched. Each sat between apical protuberances of a bulbous gemmifer. At maturity the gemma abscised from the gemmifer, possibly with the assistance of pressure from the apical protuberances and swollen dorsal side of the gemmifer. Antheridia, produced marginally on small thalli or on gemmae, were composed of at least five cells, and averaged 76 μm in diameter. Archegonia were formed ventrally on lateral pads and had necks composed of four tiers of cells. Parsimony analysis of gametophyte characters of selected genera within the Hymenophyllaceae resulted in a single, most parsimonious tree, with Callistopteris sister to Trichomanes s. s. and Vandenboschia, the two most derived taxa in the analysis. Mecodium was sister to the clade containing Callistopteris, Trichomanes s. s. and Vandenboschia. Hymenophyllum s. s. was the basal-most taxon. The tree was consistent with hypotheses of gametophyte character evolution within the Hymenophyllaceae, proposing that gametophyte growth habit composed partly or wholly of filaments, one-dimensional gemmae, elongate, flask-shaped gemmifers, archegonia with necks of four tiers of cells, and small antheridia of five jacket cells are derived characters within the family. A thalloid growth habit, two-dimensional gemmae, bulbous gemmifers, archegonial necks with six to 10 tiers of cells, and large complex antheridia represent ancestral characters.