Characterization and Genetic Analysis of SS-18, a new Osmotic-Sensitive Mutant of \u3cem\u3eNeurospora crassa\u3c/em\u3e

Osmotic-sensitive mutants of the fungus Neurospora crassa fail to grow on medium of increased osmotic pressure. The usual test is their failure to grow on medium with 4% NaCI or with 6% NaCI. These mutants have an accompanying altered morphology resulting in short, cropped aerial hyphae that exude pigmented cytoplasm. A faulty cell wall is thought to be responsible for the phenotype of these mutants. An osmotic-sensitive mutant SS-18 whose morphology resembled the morphology of wild type and differed from that of the typical earlier described osmotic-sensitive mutants was isolated at the Georgia Southern University Laboratory following UV irradiation of the wild type strain. The two backcrosses of SS-18 to wild type 74 each resulted in a 1:1 segregation of wild type to mutant progeny (34:29 and 43:28). These results indicated that the osmotic sensitivity observed was due to a single gene under Mendelian control. Since SS-18 resembled SS-462, another osmotic sensitive mutant isolated in our laboratory, a complementation test between the two mutant was performed. Results of this test ruled out allelism of the two mutants. A cross of SS-18 was made to the multi-marker tester strain alcoy. The results indicated that SS-18 was linked to ylo and Linkage Group III or Linkage Group VI. Complementation tests indicated that SS-18 was not an allele of os-8 in LG III or of os-9 in LGVI. Therefore, SS-18 represents a new locus for osmotic sensitivity. Since some osmotic sensitive mutants produce few or no conidia, SS-18 was compared to wild type and to os-5 for abundance of conidia. os-1 did not produce sufficient conidia to be included in this study. SS-18 was found to have 3.5 x105 conidia/ml, wild type 3 x105 conidia/ml, and os-5 76 x 105 conidia/ml. The number of conidia produced by SS-18 was significantly different (p\u3c.05) from the number produced by wild type 74 and by os-5. Sensitivity of SS-18 to minimal media containing various osmotica was compared to that of os-1 and os-5. Wild type 74 served as the control. The results disclosed that SS-18 had no growth (clean) on medium of 1.37 osmolality (6%) NaCI, but did not show sensitivity on medium containing KCI or glucose as the osmotica. SS-18 differed from os-1 which exhibited a greater degree of sensitivity to media containing NaCI, and was also sensitive to media with KCI or glucose as the osmotica; os-5 was leaky on media with NaCI and was sensitive to 1.71 osmoles of glucose This suggests that some mechanism other than a faulty cell wall might be responsible for osmotic sensitivity observed in SS-18