SCHNEIDER, HARALD1*, KATHLEEN M. PRYER1, and RICHARD LUPIA2. 1Department of Biology, Duke University, Durham, NC 27708; 2Sam Noble Oklahoma Museum of Natural History and School of Geology and Geophysics, University of Oklahoma, 2401 Chatauqua Avenue, Norman, OK 73072. - A comparative analysis of structure and function of spores in extant heterosporous ferns (Salviniales).
Heterosporous ferns are a monophyletic lineage comprising five extant
genera that fall into two clades: Marsileaceae, including
Marsilea, Pilularia and Regnellidium; and
Salviniaceae, including Azolla and Salvinia. The
Marsileaceae have an amphibious habit, whereas the Salviniaceae are
exclusively floating aquatics. Observations from light and scanning
electron microscopy indicate that the megaspores of each of the
heterosporous fern genera have a uniquely modified outer spore wall
(perine) above the aperture, which is referred to as an acrolamella,
and they also differ substantially in the details of their perine
ultrastructure. The megaspores of Marsileaceae are dispersed freely
into water and possess an ephemeral gelatinous outer perine that acts
as a flexible floating apparatus. In Salviniaceae, the megaspores are
retained in the sporangium wall and have either a completely
(Salvinia) or partly (Azolla) alveolate outer perine,
which also functions in providing buoyancy. In Azolla, parts of
the indusium and the sporangium wall form, together with the megaspore
body, an extraordinary megaspore apparatus, making these spores the
most complex among living land plants. As with the megaspores, the
microspores of Marsileaceae are freely released in water and possess a
gelatinous perine layer, whereas, the microspores of Salviniaceae are
retained in massulae (Azolla), or within the microsporangium
(Salvinia). The evolution of mega- and microspore characters is
inferred using a phylogeny obtained from an independent analysis of
nucleotide sequence data. Because extant genera in both clades each
exhibit multiple unique perine structures, it is not possible to
definitively reconstruct spore wall ultrastructure characters in the
common ancestor of heterosporous ferns. To help resolve this dilemma,
approaches using comparative developmental data and the integration of
fossil heterosporous ferns will be outlined.
Key words: character evolution, heterospory, Marsileaceae, Salviniaceae, sexual reproduction, spore morphology