Analysis of Vegetative Segregation and Recombination in Penicillium Chrysogenum.

N the basis of an analysis of vegetative segregation in heterozygous diploids of 0 Aspergillus nidulans, PONTECORVO et al. (PONTECORVO 1953 ; PONTECORVO, TARR GLOOR and FORBES 1954) concluded that the segregant nuclei arise as a consequence of two independent processes. The first is mitotic crossing over (STERN 1936), giving rise to diploid nuclei which are homozygous for some of the markers. The other is irregular distribution of whole chromosomes a t mitosis; this produces haploid nuclei in a proportion of cases. ROPER and PRITCHARD’S (1955) recovery of complementary products of mitotic crossing over provided conclusive evidence for the first of these processes. PONTECORVO and KAFER (1956) have shown with Aspergillus nidulans the practicability of using mitotic crossing over and haploidization for mapping chromosomes. The two types of vegetative segregant, haploid and diploid, have also recently been identified in Penicillium chrysogenum (SERMONTI 1956), so that it is a reasonable assumption that vegetative segregation takes place by means of the same processes in this species. In the same paper the author shows that the spore size of a segregant is diagnostic for its ploidy: diploid spores are about twice the volume of haploid spores. Observations of spore diameter have not been carried out in the present experiments, and the distinction of strains into haploid and diploid has been based on the phenotype of the segregants and on second-order segregation data.