Microsporidia: a journey through radical taxonomical revisions

Microsporidia are obligate intracellular parasites of medical and commercial importance, characterized by a severe reduction, or even absence, of cellular components typical of eukaryotes such as mitochondria, Golgi apparatus and flagella. This simplistic cellular organization has made it difficult to infer the evolutionary relationship of Microsporidia to other eukaryotes, because they lack many characters historically used to make such comparisons. Eventually, it was suggested that this simplicity might be due to Microsporidia representing a very early eukaryotic lineage that evolved prior to the origin of many typically eukaryotic features, in particular the mitochondrion. This hypothesis was supported by the first biochemical and molecular studies of the group. In the last decade, however, contrasting evidence has emerged, mostly from molecular sequences, that show Microsporidia are related to fungi, and it is now widely acknowledged that features previously recognized as primitive are instead highly derived adaptations to their obligate parasitic lifestyle. The various sharply differing views on microsporidian evolution resulted in several radical reappraisals of their taxonomy. Here we will chronologically review the causes and consequences for these taxonomic revisions, with a special emphasis on why the unique cellular and genomic features of Microsporidia lured scientists towards the wrong direction for so long.