Influence of chromosomal aberrations on meiotic and mitotic nondisjunction in Aspergillus nidulans.

ANALYSIS of all types of mitotic segregants from diploids of Aspergillus nidulans with non-selective methods has shown that among conidia aneuploids resulting from mitotic nondisjunction occur with a relatively high frequency of 1-2% (GFER 1961). It seemed interesting to find out whether meiotic nondisjunction is also frequent in this species. In haploid organisms the only products of meiotic nondisjunction viable enough to be isolated are aneuploids of the n 41 type. In Neurospora crassa, in which no diploids have yet been isolated, rare n 41 aneuploids have been obtained from many crosses as pseudo-wild types, heterozygous for complementing alleles (RTTENGER 1954). In Aspergillus aneuploid colonies of this type are visually distinguishable from haploid ones since they grow poorly and produce large haploid sectors by loss of one of the disomic homologues. In standard crosses, and under standard conditions, colony morphology is very uniform and “abnormal” colonies have been found only rarely. This has made it possible to detect a small, but higher than normal, frequency of aneuploid looking colonies in a few crosses between related strains and to identify some of these as hyperhaploids of the n 41 type. I t seemed likely that meiotic nondisjunction was increased in these crosses as a result of heterozygosis for one or more chromosomal aberrations since aberrations are known to increase meiotic nondisjunction in higher organisms ( STURTEVANT 1926; ANDERSON 1929; DOBZHANSKY 1930, 1933; DOBZHANSKY and STURTEVANT 1931; BEADLE 1932; GLASS 1933, 1935) and translocations have been found in the pedigrees of several Aspergillus strains (KXFER 1965). The results presented here are consistent with this hypothesis. On the other hand, no similar effects on the frequency of the various types of n + 1 aneuploids resulting from mitotic segregation were detectable in diploids heterozygous for translocations.