Section 3 — Molecular and Microbial Genetics 



produce biotypes with increased virulence and 

 wider host range. Heterokaryosis alone, without 

 mitotic recombination, immediately increases 

 virulence when two mildly virulent strains ana- 

 stomose. The nuclear ratio in inoculum of 

 heterokaryons containing nuclei of labelled 

 pathogenic and non-pathogenic strains deter- 

 mines the extent of wilt in the host, the rate the 

 rhizosphere is colonized and the ability to survive 

 in soil. Haploid nuclei bearing genes conditioning 

 virulence are selectively increased in hetero- 

 karyons during host infection and their rate of 

 multiplication in the heterokaryon is directly 

 related to the development of wilt symptoms. 

 Similarly, material released from host roots into 

 the rhizosphere selectively increase nuclei con- 

 ditioning virulence, but a few nuclei from non- 

 pathogenic strains also enter the host as com- 

 ponents of a heterokaryon. In soil, a few nuclei 

 conditioning virulence survive in heterokaryons 

 largely composed of nuclei from purely sapro- 

 phytic strains. Heterokaryotic chlamydospores, 

 the resting bodies of F. oxysporum, have been 

 isolated from soil. 



The control of both the parasitic phase and 

 survival in soil through adjustments of the nu- 

 clear ratio in heterokaryons provides the wilt 

 fungi with a rapidly adapting mechanism that 

 helps explain some of their variability. 



ing, typical wild type colonies characterized by 

 abundant cleistothecial formation. Ascospores of 

 this isolate were exposed to ultraviolet light for a 

 period of time such that 2.5 per cent of the asco- 

 sporessurvived the treatment. Among the sur- 

 viving colonies, three strains were isolated which 

 were capable of producing conidia abundantly 

 over the entire colony surface at both 25° and 

 37°C. One of the conidiating mutants was selected 

 for the isolation of conidial color, auxotrophic 

 and antimicrobial agent resistant mutant strains. 

 Since the conidial mutant retained the ability to 

 form cleistothecia, the markers carried by the 

 conidial strain have been mapped based on ran- 

 dom ascospore analyses. Additional conidial 

 mutants have been isolated from the original 

 non-conidial parental strain. Each conidial strain 

 has been marked genetically and intercrossed 

 with other conidial mutant strains in an attempt 

 to locate the gene or genes which control conidi- 

 ation in this strain. Since the conidial strains 

 differ from one another morphologically, it is 

 probable that more than one locus is involved or 

 that there exist several alleles of the same gene. 



3.48. Sexual Polarity in Streptomyces. G. Sermonti, 



I. Spada-Sermonti and S. Casciano (Rome, 

 Italy). 



3.46. Utilization of a Dikaryotic Organism (Coprinus 

 radiatus) for the Study of Lethals. Madeleine 

 Gans (Gif-sur-Yvette, France). 



A method is described which enables us to 

 discover, preserve and study from a genetical 

 point of view the compensable and non-com- 

 pensable lethal mutations (according to Atwood) 

 induced in a dikaryotic mycelium. 



Preliminary results show that from about 

 twenty lethals induced by nitrous acid, two only 

 are compensable by the growth medium. 



3.47. Genetic Control of Conidiation in Aspergillus 

 rugulosus. R. W. Tuveson and D. O. Coy 

 (Chicago, U.S.A.). 



A group of intersterile strains (R~) has been 

 found in Streptomyces coelicolor A3(2). They are 

 fully fertile with all the other strains, which in 

 turn are fertile among themselves. In summary: 



R+ x R+ = fertile 



R+xR' = fertile 



R" x R~ = sterile 

 In R + x R^ crosses, the contribution to the 

 progeny of markers of the R + strain is, as a rule, 

 reduced as compared with that of the R" markers, 

 although in rare cases the reverse is observed. 



The transfer (or integration) of the R+ markers 

 of the two linkage groups appears to be corre- 

 lated in the different zygotes. When defects are 

 observed on the R"" side, the deficiencies in the 

 two linkage groups also appear to be correlated. 



A strain of Aspergillus rugulosus isolated from 

 sand in the Indiana Dunes State Park produces 

 abundant cleistothecia when grown on complex 

 and minimal medium at 37°C. Under these con- 

 ditions, conidial production is limited. When this 

 isolate is incubated at 25°C for extended periods 

 (14 days or more), conidia are produced abun- 

 dantly at the colony periphery. These conidia 

 when transferred to complex or minimal medium 

 and incubated at 37°C give rise to non-conidiat- 



3.49. Directed Mutation of the Mating Type Alleles 

 as an Explanation of Homothallism in Yeast. 

 Donald C. Hawthorne (Seattle, U.S.A.). 



Hybrids of homothallic Saccharomyces cheva- 

 lieri and heterothallic Saccharomyces cerevisiae 

 yield asci in which two of the spores give rise to 

 heterothallic cultures, i.e. they remain haploid 

 and show a mating reaction when mixed with 



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