Systematics of the Genus Cryptococcus and Its Type Species C. neoformans

This chapter focuses on the genetic diversity of Cryptococcus species as assessed by sequence analysis of DNA. In 1935, a comprehensive study was conducted with numerous yeasts, which included 22 strains of pathogenic and nonpathogenic Cryptococci isolated from humans. Prior to the discovery of the genus Filobasidiella, the teleomorph of Cryptococcus neoformans and C. gattii, yeast taxonomists had speculated that C. neoformans belonged to the Basidiomycota for the following reasons. First, it is the only basidiomycetous genus that contains human pathogens that produce life-threatening infection of the central nervous system in immunocompromised as well as immunocompetent patients. Second, Filobasidiella is the only genus in the phylum Basidiomycota that produces long chains of basidiospores on the apex of the holobasidia by repetitive basipetal budding. Third, the genus contains two groups of species, one with and one without ontogenetic yeast stages. Due to taxonomic confusion about the status of the type species of the genus Cryptococcus, as well as the widely recognized importance of C. neoformans and C. gattii as human pathogens, the genus was neotypified by C. neoformans. This revision implies that if one uses a modern phylogenetic genus concept, only those anamorphic species that belong to the Filobasidiella clade should be classified as Cryptococcus species, e.g., C. neoformans, C. gattii, and C. amylolentus. Among the Tremellales, species classified as Cryptococcus occur intermingled with species of diverse genera such as Bullera, Tremella, Papiliotrema, Auriculibuller, Trimorphomyces, Kwoniella, Fibulobasidium, and Bulleribasidium.