Radiation effects and mitotic recombination in diploids of Aspergillus nidulans.

NTENSE gamma-radiation of conidia has been found to produce a very high I frequenc y of abnormal, sectoring colonies in heterozygous diploids of Aspergillus nidulans (=FER 1960). To obtain information about the nature of the radiation effects which cause the formation of colonies with many large segregant sectors, a detailed analysis of a relatively small number of such sectoring colonies has been carried out. Sectoring colonies with abnormally slowly growing centers have been obtained after treatment with X rays or ultraviolet light in heterozygous diploids of several species of fungi (e.g., in Penicillium chrysogenum by MORPURGO and SERMONTI 1959). Similar sectoring colonies were produced after treatment with various chemicals, for example nitrogen mustard ( MORPURGO and SERMONTI 1959), formalin (FRATELLO, MORPURGO and SERMONTI 1960) and para-fluoro-phenylalanine ( MORPURGO 196 1 ) , and substances reducing these effects have been described (SERMONTI and MORPURGO 1959, BALLIO and SERMONTI 1961). The question arises whether processes different from the spontaneous ones lead to the formation of these induced mitotic segregants or whether such treatments increase the frequency of the spontaneously occurring processes of mitotic recombination. The only regularly observed type of spontaneous segregants which show similar abnormal growth and large segregant sectors are aneuploids resulting from mitotic nondisjunction in diploids (=FER 1961 ) . Such spontaneously occurring aneuploids show surprisingly regular growth patterns and characteristic, distinguishable phenotypes depending on the linkage group to which the extra chromosome corresponds. From the beginning it became apparent, that the drastic treatments with y-rays (50-60 kr) produced an extremely large variety of segregation patterns very different from the ones observed in aneuploids. It seemed, therefore, unlikely that a radiation-induced increase of mitotic nondisjunction could have produced the many abnormal sectoring colonies. In accordance with the known radiation effects on chromosomes of higher organisms the hypothesis is advanced that the colonies with induced sectoring originate from uninucleate spores with chromosomes which have been damaged, often broken, by the effects of ionizing radiation. It assumes that certain deficiencies resulting from chromosome breakage have a semidominant lethal effect; that is, heterozygosity for chromosome deletions would cause slow growth and poor conidiation, as is found in the center of the sectoring colonies produced by