The genus Isoëtes L. (Lycophyta, Isoëtaceae): Synthesis of karyological data

A synthesis of relevant karyological data regarding the genus Isoëtes is given, with an up-to-date check list of chromosome numbers including 152 records referring to 67 taxa (62 species and 5 infraspecific taxa) and 16 hybrids. Total of different chromosome counts (considering two or more cytotypes for certain taxa, excluding hybrids) is 80. Probable base number is x = 11, even though x = 10 and x = 12 have been proposed for certain species. On the basis of data here summarized, the mean percentage of polyploidy is 61.2%; 31 taxa are diploids (38.8%), 28 are tetra-1ploids (35.0%), 8 are hexaploids (10.0%), with a few species each which are tri-, penta-, octo-, deca- and dodeca-ploids. These data are briefly discussed in the frame of the evolution of the genus: palaeopolyploidy did not play a primary role, autopolyploidy seems rare, allopolyploidy is an active mechanism of speciation especially in aquatic species. Correlation between size of cells (spores and stomata) and ploidy level is also discussed.