The taxonomy, status and origin of <i>Isoetes</i> × <i>brochonii</i> and <i>I. creussensis</i> (Isoetaceae), two Pyrenean endemic taxa

Isoetes brochonii has possibly been subject to more taxonomic designations over the last century than any other European quillwort and most commonly has been put into synonymy with the widespread I. lacustris. Investigations of herbarium and living material in the present study, however, indicate it to be a sterile (aborted spored) hybrid, I. ×brochonii (pro sp.), confined to high elevation lakes of the Pyrenees mountains in France, Spain and Andorra. This represents the first uniquely European Isoetes hybrid to be described. Putative parents are diploid I. echinospora and dodecaploid I. creussensis, probably a hybridogenic Pyrenean endemic. The speciation of I. creussensis may well have occurred in Pyrenean refugia during Quaternary glacial-interglacial fluctuations. We found no convincing evidence for the occurrences of decaploid I. lacustris in the Pyrenees where it appears to have been confused with I. creussensis. A comparative table of the distinguishing morphological characteristics of I. creussensis and I. lacustris is provided, with megaspore ornamentation and microspore size indicated as being particularly important indicators.