Phylogenetic spectrum of fungi that are pathogenic to humans.
Recent phylogenetic studies based on ribosomal RNA sequences have confirmed that the organisms traditionally treated as fungi include those that have evolved from several different lines (multiphyletic organisms), as has been suspected. Even organisms causing disease in humans represent at least two evolutional lines. Pythium insidiosum and Prototheca species are both believed to have evolved from one line, while the rest of the pathogens have evolved from another line. P. insidiosum is more closely related to red algae and diatoms than to fungi. Prototheca species, as has been previously postulated, are closer to blue-green algae and plants than to fungi. Pythiosis and protothecosis, however, will still be dealt with by medical mycologists because of the morphological and in vivo staining characteristics of the causative organisms. Molecular genetic studies have revealed that Pneumocystis carinii can best be categorized as a fungus, although questions regarding its fungal status may remain unanswered until additional information becomes available on its life cycle, nuclear division, cell-wall chemistry, nutritional uptake pattern, and lysine biosynthetic pathway as well as the ultrastructural characteristics of its cellular components such as the Golgi complex. The phylogeny of the agents of lobomycosis and rhinosporidiosis, although they are treated as fungi, remains unknown. Although there is no in vitro culture system for Loboa loboi and Rhinosporidium seeberi at present, a molecular approach would allow us to reveal their phylogenetic relationship, and we can hope that such attempts are forthcoming.