Quantitative intrachromosomal changes arising at mitosis in Aspergillus nidulans.

novel and spontaneous process, giving quantitative change of genotype at A mitosis in Aspergillus nidulans, was described by BAINBRIDGE and ROPER (1966). Segregants carrying a segment of linkage group 111 in duplicate were derived from crosses of strains free of translocations to strains with an apparently non-reciprocal III-VI11 translocation. These duplication strains had reduced linear growth rate and a distinctive morphology; they were designated “crinkled.” Crinkled colonies showed vegetative instability by producing sectors which, in varying degree, approached wild type in growth rate and morphology. Genetic analysis showed that the sectors had lost a variable amount of the chromosome segment carried in duplicate; loss occurred from either the untranslocated segment of linkage group I11 or from that segment of linkage group 111 translocated to VIII. The investigations reported here extend these observations by seeking answers to the following questions. First, is the phenomenon of chromosome loss confined to the particular linkage group I11 duplication or, as might reasonably be supposed, is it of general occurrence? Second, are the losses terminal and are there indications of preferential regions of loss? This required, ideally, duplications with several markers in heterozygous condition. Third, does loss involve exchange of markers between the homologous segments? The answer would cast light on the mechanism involved. Finally, and this is to some extent related to the third question, is the change always one of loss and never of gain? Losses might arise by, for example. unequal mitotic crossing over or by a mitotic version of the intrachromosomal meiotic process proposed by LAUGHNAN ( 1955, 1961 ) , PETERSON and LAUGHNAN (1961, 1963) and BOWMAN (1965). Either mechanism would generate, in addition to losses, tandem duplications. Such duplications might he detected by a further deterioration in phenotype.