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