EVERAL cases of the following type have been reported in studies of micro-S organisms in recent years: two or more mutants of independent origin have similar phenotypes and appear to be allelic to each other or very closely linked; crosses between the mutants yield a small proportion of nonmutant progeny, but the event producing these recombinants does not have the characteristics of a crossover. Such a case, involving two cysteine-requiring mutants of Neurospora crassa, is reported here.

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... S Crosses were made at 25°C on synthetic crossing medium (WESTERGAARD and MITCHELL 1947) supplemented with the growth factors required by the protoperithecial parent (250 mg/liter of cysteine and in some cases 50 mg/liter of adenine or of lysine). To isolate cysteine-independent progeny, ascospores were picked up from the wall of the cross tube in loopfuls of water and spread on petri plates of Fries minimal medium (BEADLE and TATUM 1945) without cysteine, but supplemented to be nonselective for linked markers; most plates contained less than 500 ripe spores. Plates were heat shocked and then incubated at 25°C. Starting 15 hours after heat shock, plates were examined at intervals for the next 24 hours, and all cysteine-independent colonies were transferred to slants of complete medium to classify for the linked markers. Total numbers of germinated spores were determined by direct counts on the same plates. Forty-two of the cysteine-independent strains were crossed to wild type to confirm that their genotypes corresponded to the vegetative phenotypes. Analyses of asci from these crosses revealed in every case that vegetative classification had been correct. This was a check against misclassification of linked markers and also against the possibility of the cys+ condition arising from a pseudo-wild type (MITCHELL, PITTENGER and MITCHELL 1952) rather than a true cys+ genetic region. EXPERIMENTAL RESULTS Strains 80702 and 48401 are single gene cysteine-requiring mutants of Neurospora. They differ in nutritional responses at 25°C. 80702 (hereafter referred to as cys-t) will grow on minimal medium supplemented with either thiosulfate o r cysteine, while 48401 (cys-c) will grow on cysteine but not on thiosulfate.