Dispersal and colonization in heterosporous lycophytes: palynological and biogeographical notes on the genus Isoetes in the Mediterranean region

After a short review of the available information on dispersal in the heterosporous genus Isoetes, which has been the subject of some mainly anecdotal or occasional observations, a case study in the Mediterranean area is presented. In the geographical context given by the island of Sicily with its satellite islets and archipelagos, the distribution of terrestrial and amphibian Isoetes species was analysed to verify if any of them occurred in the oceanic (‘thalassogenous’) islets, never connected to the mainland. The only species revealed to be able to cross the geographical and ecological barrier represented by the sea and colonize some volcanic islands was Isoetes durieui Bory, a terrestrial polyploid species. On the basis of biological, ecological and morphological observations, including scanning electron microscope observations of the spores, the present work suggests that in this species – as in other terrestrial Isoetes species – animals could have a fundamental role in dispersal and dispersion, and synaptospory could be an important mechanism in dispersal events; in addition, polyploidy can be an important selective advantage for colonization events also in this group of vascular plants.