Investigations on the genetics of the homothallic ascomycete Aspergillus nidulans (Eidam) Winter

Aspergillus nidulans is a homothallic fungus with both a vegetative and a sexual cycle. Genetical analysis of Aspergillus nidulans had previously been carried out using a technique of recombinant selection. This method gives no information about the position of the centromeres, the incidence and types of unusual asci, if any, or certain modalities of crossing-over. The forst object of the work reported in this thesis was to investigate some of these problems. To this end a technique of ascus dissection suitable for A. nidulans was developed. Among 136 asci of crossed origin isolated from crosses of various strains of A. nidulans, eleven abnormal asci were found. The three main probable causes of abnormalities were: a) mutation during the first meiotic division (3 asci); b) supernumerary divisions in the ascus (2 asci); c) inclusion of two nuclei, instead of one, in the ascopore (6 asci). The incidence of inclusion of two nuclei in one ascopore was found to be about 2%. That A. nidulens is homothallic, was rigorously established in four original strains by means of dissection of all the spores of one ascus through two successive complete sexual cycles. To locate some of the centromeres in A. nidulans, a cross in which four loci were segregating (w/W, lys/LYS, y/Y and bi/BI) was studied. The choice of strains proved unfortunate as low viability of the ascopore from"crossed" asci occurred. Backcross to the wild type were carried out, but little improvement was shown after two backcrosses. From these and other results of ascus analysis, it appeared that genetic and/or cytoplasmic differentiation had taken plase between the original white strain (and its derivatives) and the wild type strain (and its derivatives). The former had been isolated in 1936 from wild type and kept genetically separated since. Some of the details of kyryogamy in heterokaryons were investigated, and the interesting fact was discovered that the asci of one perithecium tend to be all of one type (either all selfed or all crossed). This made it possible to develop a new method of genetical analysis, applicable to A. nidulans and presumably also to other homothallic species, which did not necessitate the selection of particular classes of recombinants. Perithecia containing only (or almost only) "crossed" asci were picked, and classification as to all genes segregating was carried out, without selection of ascospores, from those perithecia which showed correct genetic ratios for certain "markers". This technique ("Perithecium analysis") is used to most advantage in crosses where the proportion of perithecia containing "crossed" asci is high and the loci, between which recombination has to be estimated, are not too closely linked