Spore morphology and evolution in Isoetes (Isoetales)
Isoetes (Isoetaceae, Isoetales) is a cosmopolitan genus with an ancient and diverse evolutionary history that presumably peaked in the Paleozoic. The age of the living clade has never been satisfactorily clarified. With little morphological (and molecular) divergence among species, megaspore morphology may provide valuable information on relationships in the group, not least among extant and extinct forms. We study megaspore ornamentation and surface texture/structure in 74 phylogenetically placed samples representing 59 species of Isoetes using SEM, and we discuss evolutionary implications of the results. Ornamentation (classified into 12 categories) and surface structure/texture (10 categories) were mapped onto a phylogeny constructed based on molecular data from the same samples whenever possible. All megaspores of the family are trilete with an outermost siliceous coating. There is ample micromorphological variation, of which some appears clade specific. The megaspores of Isoetes reticulata, discovered from the late Oligocene to early Miocene of Tasmania, share remarkable similarities with those of extant species in the Australasian clade, in particular Isoetes neoguineensis and partly also Isoetes japonica. We argue that this fossil could be used to calibrate the age of the Australasian crown group (clade D) to absolute time (c. 20-25 Ma) in analyses of divergence times of clades based on molecular data.