We examined the reproductive strategy of two Amazonian quillworts (Isoëtes cangae and Isoëtes serracarajensis), endemic and threatened species of canga ecosystems. Sexual propagation was examined by in vitro fertilization assays, while asexual propagation was examined by tiller emission. Isoëtes cangae is an outcrossing species that reproduces exclusively by spore germination and is able to propagate by self- and cross-fertilization. Isoëtes serracarajensis reproduces asexually by emitting tillers from the plant corm, despite producing male and female sporangia. These distinct reproductive strategies in the different species may be linked to their contrasting habitats. Isoëtes cangae inhabit a permanent oligotrophic lake with mild environmental changes, while I. serracarajensis are found in temporary ponds facing severe seasonal drought, where asexual propagation may represent an adaptive advantage to the short growth period during access to water. We also observed different relationships between plant growth and reproductive traits between the species, despite their common production of sporophytes with high survival rates. Together, these results are of paramount importance for establishing conservation plans for both species considering the advantages of sexual propagation to maintain the genetic diversity of I. cangae and the diligent management required to do the same with asexually propagated I. serracarajensis.
