WHITTIER, DEAN P. Department of Biology, Vanderbilt University, Nashville, TN 37235. - Gametophyte and young sporophyte development in the Ophioglossaceae.
Gametophytes of the Ophioglossaceae are tuberous, nonchlorophyllous,
nonphotosynthetic, and subterranean. They are parasitic on
mycorrhizal fungi which invade their tissues. It is difficult to
study these gametophytes in their underground habitat because they are
rarely found. Recent work has been done in axenic culture because it
offers the opportunity to investigate their development and
reproduction for extended periods of time. Depending on the species,
the spores germinate after two weeks to several months in the dark.
Usually, germination is inhibited by nitrate and promoted by ammonium.
The gametophytes remain in the 4-5 celled stage in culture if sugar
is not available. Sugar replaces the need for a fungus under these
conditions. Normal gametophyte morphology occurs in culture on a
nutrient medium containing minerals, including ammonium, and sugar
without the fungus. Mature gametophytes of this family can be
cylindrical, spherical, tongue-shaped, or bean-shaped and they develop
antheridia before becoming bisexual. The gametangia are adjacent to
each other which may contribute to the inbreeding known for
Botrychium. Although gametopyhtes of some species may mature
in less than a year after spores are released in nature or sown in
culture, it has been reported that other species may take 20 years to
form embryos. In culture, the fastest gametophyte maturation is about
8 months with Ophioglossum crotalophoroides. Normal embryo
development produces a root that emerges from the gametophyte prior to
the first leaf. In axenic culture, young sporophytes of Botrychium
jenmanii can form one root and three leaves 15 months after the
spores were sown. However, after fertilization in some species, a
delay in the emergence of a leaf above the soil of 5 or more years can
occur because several roots or rudimentary leaves are formed by the
young sporophytes.
Key words: development, ferns, gametophyte, Ophioglossaeae