X-ray irradiation of Coccidioides immitis arthrospores: survival curves and avirulent mutants isolated

There are 2 morphologically (listinet phases in the life history of the pathogenic fungus, Coccidioides immitis. The in vivo phase consists of spherules which resemble phycomycetous spo-rangia in that they form endospores by the process of progressive cleavage, and the in vitro phase consists of a septate mycelium which produces spores as the culture ages. These spores have been designated conidia (Emmons, 1942) and oidia (Baker et al., 1943) but are generally referred to in the literature today as arthrospores (Smith, 1955; O'Hern and Henry, 1956). They have been variously reported to contain 1 or 2 nuclei (Baker et al., 1943), and to be multinu-cleate (O'Hern, 1956). The ploidy of the nuclei of neither stage is known. The nutritional needs of the saprophytic phase are easily satisfied. No vitamins, amino acids, or special growth factors are required, and growth occurs on a wide variety of carbon and nitrogen sources (Baker and Smith, 1942). Although natural isolates of low virulence are known (Smith, 1951-1952; Pappagianis, 1955; Friedman et al., 1956b; Friedman and Smith, 1957), there have been no reports of artificially induced avirulent mutants. Two such mutants, both of which require riboflavin for growth, have been isolated following irradiation of arthro-spores with X-rays. This communication describes the altered characteristics of these strains. A knowledge of the X-ray survival charac-teristics of arthrospores was a necessary pre-requisite to mutation studies with C. immitis. Since no studies of this sort have been reported in the literature, a series of survival curve deter-minations was carried out. 1 This investigation was sponsored by the Offic