SCHNEIDER, HARALD* AND KATHLEEN M. PRYER. University Herbarium, University of California, Berkeley, CA 94720;, Department of Botany, Field Museum of Natural History, Chicago, IL 60605. - Spore morphology of heterosporous ferns and its possible implications for understanding the evolution of the seed habit.
Heterospory has originated independently several times in vascular
plant evolution, but seeds are known only from one living lineage.
Although fossilized remains of seed plants from the Late Devonian and
Early Carboniferous (ca. 340 mya) have provided exceptional insights
into understanding the seed habit, events leading to its origin are
still unclear. Monomegasporangy (single megaspore/megasporangium) is
assumed to be one of the key innovations in the evolution of seed
plants. This character state is found in only one other living group
of vascular plants, the heterosporous ferns, which include two
families with a fossil record dating back to the Early Cretaceous,
Marsileaceae and Salviniaceae. Recent phylogenetic studies have
demonstrated convincingly that heterosporous ferns are a monophyletic
group nested within leptosporangiate ferns. Heterospory and
monomegasporangy, therefore, evolved independently and at different
times in the geological record in ferns and seed plants.
Heterosporous ferns have an aquatic lifestyle, similar to that of
Paleozoic seed plants. Comparative spore studies reveal remarkable
similarities among the spores of heterosporous ferns, in particular,
the presence of a gula, a modified perine structure above the
aperture. The gula comprises inner and outer portions, which together
enclose a chamber (= sperm lake). Each of the five extant genera
(Azolla, Marsilea, Pilularia,
Regnellidium, Salvinia) have their own characteristic
gula, reflecting differences in ecology (e.g., amphibious versus
floating growth forms). Additional differences are also found in the
perine ultrastructure. Structures similar to a gula - lagenostomes -
are also found in the megaspores of Paleozoic seed plants. This leads
to the hypothesis that heterosporous ferns have evolved analogous
megaspore structures to seed plants and these can be used as a model
to better understand the biological constraints in which the seed
habit evolved.
Key words: gula, heterospory, Marsileaceae, monomegasporangy, Salviniaceae, seed habit