Pleomorphism in Some Ambrosia and Related Fungi

Abstract

The term pleomorphism has been used to denote many conditions and variations encountered in dermatophytes or other fungi imperfecti. In one sense it implies the condition of cultural instability in comparatively old cultures, manifested by patches of fluffy aerial mycelium producing fewer conidia, as for example, in Trichophyton mentagrophytes (Robin) Blanchard (9) or in other dermatophytes (12). This type of growth is believed to be a result of genetic mutation and, upon subculturing, this growth pattern is not known to give parent type growth. More commonly, and perhaps correctly, the term pleomorphism implies the phenomenon of dimorphism found in some dermatophytes such as: Histoplasma capsulatum Darling, sporotrichum schenckii Matruchot, Blastomyces dermatitidis Gilchrist & Stokes, Blastomyces brasiliensis (Splendore) Conant & Howell and Coccidioides immitis Rixford and Gilchrist (7). Pleomorphism is also known in some plant pathogens. Tyler and Parker (22) reported yeast-like growth in Ceratocystis uln;zi (Schwartz) Buis., and Bauer (6) in Aureobasidium pullulans (deBary) Arnaud. The aforementioned organisms have two characteristic reversible phases, a yeast phase and a mycelial phase. The cultural conditions or factors that bring about this reversal of growth phase were recently summarized by Cochrane (10). The yeast phase in dermatophytes usually occurs in the host while the mycelial phase appears on cultural media under suitable laboratory conditions. Almost all ambrosia fungi are dimorphic. The yeast-like phase (referred to as ambrosia) predominates in tunnels of ambrosia beetles during their active life. Ambrosia fungi upon culturing on common laboratory media at room temperature usually yield mycelial growth and seldom sporulate or yield ambrosial masses. Recent isolates of some of these fungi, e.g., Monilia ferruginea Math. Kaiiirik (19) and Tuberculariella ips Leach et. al. (18) sporulate readily and form typical sporodochia similar to those found in nature. In other cases they rarely sporulate (2) and do not yield ambrosial growth. Still others neither sporulate nor give any ambrosia in culture. Literature on general characteristics, habitat, morphology, and taxonomy of ambrosia fungi was recently re-