Format: ORAL

Eva Larsn1 Catarina Rydin1

1 Department of Ecology, Environment and Plant Sciences, Stockholm University, Sweden

Isoetes L. (Isoetaceae, Isoetales) is a cosmopolitan genus of heterosporous lycopods with an ancient and diverse evolutionary history that presumably reached its zenith in the Paleozoic. The genus belongs to the order Isoetales, which appeared in the Late Devonian and had its greatest diversity and ecological dominance in the late Carboniferous when arborescent lepidodendrids were widespread in the coal swamps of Euramerica. But living alongside and outliving their tree-shaped relatives were also several genera of smaller and mostly unbranched isoetaleans that were common in the Triassic and continued to diversify throughout the Mesozoic and Cenozoic. The age of the living clade has however never been satisfactorily clarified and no fossils have been phylogenetically placed within the extant clade. With little morphological (and molecular) divergence among species, megaspore morphology may be one of the best hopes for resolution of relationships in the group as well as relationships among extant and extinct forms. We have studied the megaspore morphology, ornamentation and surface texture/structure, in 74 phylogenetically placed samples representing 59 species of Isoetes, and discuss evolutionary implications of the results. The samples were studied with scanning electron microscopy, and ornamentation and surface structure/texture were mapped onto a phylogeny constructed based on molecular data from the same samples whenever possible. While there are general features common to all megaspores of the family (trilete with an outermost siliceous coating) there is in addition ample morphological variation, of which some appears clade-specific.