Dimorphism in Fungal Pathogens of Mammals, Plants, and Insects

The capacity for pathogenic fungi to change morphology during their lifecycle is widespread. However, relatively few fungi are considered dimorphic, which refers to the ability to switch between two morphologies, yeast and hyphae (Table 1). These pathogens can be roughly subdivided into thermal (morphologic switch induced by temperature) and non-thermal dimorphic fungi (Table 1). Worldwide, the thermally dimorphic fungi cause several million human infections each year. In the United States, Histoplasma capsulatum and Coccidioides spp. are estimated to infect 500,000 and 150,000 persons annually, respectively [1,2]. The incidence of coccidioidomycosis has recently increased and its endemic range has extended beyond the Southwest to include eastern Washington state [3]. Although the thermally dimorphic fungi typically infect healthy hosts, these pathogens account for 5.3% of fungal infections in solid organ transplant recipients [4]. Immunosuppressed patients are at risk for respiratory failure and extrapulmonary dissemination [4]. Phytopathogenic dimorphic fungi have had a major impact on urban landscapes and agriculture. Ophiostoma ulmi, which caused the first Dutch elm disease epidemic, has been replaced by a more virulent species, Ophiostoma novo-ulmi, which has destroyed millions of elm trees in the US and Europe [5]. Taphrina deformans, the etiologic agent of peach leaf curl, results in economic losses of $2.5–3 million in the US [6]. Although Ustilago maydis, which causes corn smut, is not a major agricultural threat, the galls (i.e., huitlacoche) caused by infection are eaten as a delicacy [7]. Entomopathogenic fungi have been utilized to control insects harmful to agriculture and to study how pathogens control host behavior [8,9].