Mutants ofArthroderma benhamiae

Abstract

The object of these studies was to isolate conditionally lethal mutants of Arthroderma benhamiae whose vegetative growth was temperature sensitive (ts). Three sorts of mutants were derived from nitrosoguanidine treatment of the microconidia of A. benhamiae: (i) ts "pleomorphic", (ii) ts "non-pleomorphic", and (iii) slow growers (sg) which were not temperature sensitive. The growth of wild type A. benhamiae, as measured by colony diameters, was 6 mm/day at 37 degrees C (chosen as non-permissive for screening mutants) and 5 mm/day at 25 degrees C (chosen as permissive for screening mutants). Growth of mutants of the first sort was virtually stopped at 37 degrees C but their growth at 25 degrees C was more rapid than that of wild type. The colony texture of these mutants was downy and their mycelium sterile. They failed to mate with parents of opposite mating type. These mutants were considered to be ts "pleomorphic". The second sort of mutants were ts "non-pleomorphic". Two isolates of this kind were recovered. They grew normally at 25 degrees C but their growth at 37 degrees C was reduced 50--70% of that of wild type. The nature of the temperature sensitive defect has not been identified. The third sort of mutant occurred very frequently. These isolates were slow growers (sg) regardless of temperature of incubation and their response to the respiratory inhibitors antimycin and salicyl hydroxamic acid suggested a defect in mitochondrial ribosome assembly and deficiencies in cytochromes not unlike those observed in the poky mutants of Neurospora. Two of the sg-mutants (sg 2 and sg3) produced abundant cleistothecia with asci and ascospores when back crossed to parents of opposite mating type. The sg 5 mutant produced cleistothecia but no ascospores. The ergosterol content of the sg 2 mutant was nearly the same as that of wild type while the ergosterol content of sg 3 was somewhat reduced and that of sg 5 was markedly reduced as compared to that of wild type. Thus the ergosterol content seems to play some role in sexual reproduction of A. benhamiae. The pattern of sensitivity to amphotericin B also reflected differences in the sterol content of the mutants, i.e., the two mutants with some alteration in their ergosterol content, sg 3 and sg 5 were more resistant to the antibiotic. During the course of these studies a number of variants were observed that produced different sorts and degrees of pigmentation of the reverse of their colonies. One stable variant of this kind gave rise to colonies with a red reverse when incubated at 37 degrees C and a yellow reverse when incubated at 25 degrees C. When plates were shifted from one temperature to another the next wave of growth was the color specified by the temperature.