Collaborative Research: Using Phylogeny to Investigate the History of Morphological Evolution in Heterosporous Ferns

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Throughout the evolutionary history of land plants there have been repeated invasions of aquatic environments by terrestrial plants from distantly related lineages. In order to survive these very different physical conditions, these plants have been selected to modify their vegetative, reproductive, and dispersal systems. Few living ferns are aquatic; most are terrestrial and homosporous (bearing a single spore type). The exceptions are the heterosporous ferns, the semi-aquatic Marsileaceae and aquatic Salviniaceae, which belong to a single evolutionary lineage. The five extant genera of heterosporous ferns have been little studied, but the fossil record of their ornamented spores is relatively good for these ferns, showing a Cretaceous diversification contemporary with the rise of the flowering plants. In a collaboration between fern specialists Kathleen Pryer and Harald Schneider at Duke University and paleobotanist Richard Lupia at University of Oklahoma, the researchers will integrate morphological and molecular studies on the rates and sequences of evolution of all the known species of water ferns, to understand the history of adaptations to the aquatic environment. A comprehensive phylogeny of all the species will be constructed from DNA sequence comparisons, integrated with morphological studies. Spore ultrastructural characters will be studied with the electron microscope to investigate developmental features of spore wall construction and ornamentation, in order to interpret and integrate fossil spores known for the group. Numerous morphological trends have been proposed on the basis of the relative order of appearance in the fossil record, and the explicit phylogeny constructed for the group will allow testing of these ideas about character evolution in water ferns. The investigators bring complementary skills to the study from fields of morphology, DNA sequence analysis, phylogenetic methods, paleobotany, and quantitative analyses. The study provides an ideal opportunity to test hypotheses about the adaptive radiation of the modern fern flora from the Cretaceous to the Recent, the colonization of aquatic habitats, and genetic models of developmental change.