# Cryptococcus: History, Epidemiology and Immune Evasion
## 5 Structure and Virulence Factors  
Although estimates indicate the genetic separation of ancestors of the genus _Cryptococcus_ at about 30 to 40 million years ago [48], evidence suggests that about 80 to 100 million years ago, ancestors belonging to the species complex of the genus _Cryptococcus_ were physically separated by the drift of the supercontinent Pangea into two continental masses in the region that today corresponds to South America and Africa. This event would have made possible the accumulation of genetic determinants in the genus _Cryptococcus_ in different environments that, in the future, would lead to differentiation in the contemporary species _C. neoformans_ and _C. gattii_[49; 50; 51]. The role that birds currently play in the aerial dispersal of _Cryptococcus_ spp. between different regions is closely linked with the emergence of bird species in the early Cretaceous and the dispersal of the _Cryptococcus_ genus between continents in the same geological period [49; 52]. However, despite the evolutionary distance between the current species of the genus _Cryptococcus_, the structural determinants that are shared between these species are still well preserved, especially between the pathogenic _C. neoformans_ and _C. gatti_[27]. Among these components, the cell wall of _Cryptococcus_ spp. itself. has molecules that help the fungus defend against host defenses and environmental stress, in addition to intrinsic characteristics of the yeast, such as resistance to temperatures of 37 \({}^{\circ}\)C [53]. A protein that is required for this thermal resistance is a serine/threonine phosphatase called calmodulin, responsible for modulating the yeast response to environmental stresses. Given that calmodulin mutations adversely affect _Cryptococcus_ spp. resistance to the physiological temperature of mammals, it is fundamental in enabling its successful growth in the host [54; 55]. Other proteins involved in _C. gatti_ thermotolerance are superoxide dismutase (SOD2p) [56] and trehalose-6-phosphate synthase (TPS1p and TPS2p) [57]. Another important virulence factor in the fungus is the production of melanin [3], present in the cell wall of the fungus and associated with: resistance to oxidative stress [58; 59; 60], resistance to antimicrobial peptides and antifungal components [60; 61; 62] and tolerance to environmental radiation, with the example of some _Cryptococcus_ variants found surviving in environments with high levels of ionizing radiation, such as on the walls of the reactor of the Chernobyl nuclear disaster-UA [63; 64]. Regarding melanin synthesis, lacase (Lac1), a phenol oxidase present in _Cryptococcus_ spp., is responsible for this role, depositing this molecule in the yeast cell wall [65; 66]. It has already been described that the deposition of melanin in the cell wall of _Cryptococcus_ spp. depends on the composition and flexibility of the cell wall; as the _C. gatti_ variant R265 has a higher chitosan composition than the _C. neoformans_ variant H99, this ends up contributing to a more homogeneous distribution of melanin in the cell wall of C. gattii [67]. Chitosan is a deacetylated form of chitin and its presence in the cell wall of _Cryptococcus_ spp. provides structure and stability to the other molecules in the cell wall [68]. In addition, differences in the amount of chitosan between different species of _Cryptococcus_ have already been seen, with possible implications concerning the the virulence gap between _C. gatti_ variant R265 and _C. neoformans_, since _C. gatti_ has two- to three-times more chitosan in its cell wall [69]. Other virulence factors have already been described, such as the production of hexitol D-mannitol that helps in the survival of _Cryptococcus_ spp.