SECTORING 265 



include kind and amount of nutrients, temperature, light and other 

 radiations, pH, staling products, and the addition of certain salts 

 and toxic substances, a subject brief! v summarized by Christian- 

 sen (1940). 



Sectoring could be expected to take place among fungi in which 

 hvphal fusions occur or in those with multinucleate spores, as in 

 Botrytis cinerea. Hansen and Smith (1932) have shown that the 

 propagative elements of this fungus are heterocaryotic, resulting 

 from anastomoses that permit the migration of nuclei from one 

 cell to another. In Botryosphaeria ribis, which has multinucleate 

 ascospores and conidia, however, all the nuclei within anv asco- 

 spore or conidium have the same origin and hence are homo- 

 caryotic [Wolf and Wolf (1939)]. The causes of sectoring in 

 this species are unknown, and the phenomenon may be wholly 

 spontaneous. 



Normally when a culture originates from a single conidium it 

 is regarded as clonal and is presumed to be genetically pure. 

 Variations occur in the colonies from these clones, as has been 

 shown by LaRue (1922) in Festalozzia guepiiii, by Christiansen 

 (1932) in Festalozzia funerea, and by Leonian (1929) in many spe- 

 cies and varieties of Fusarium. Sometimes the variants in subcul- 

 tures of Fusarium remained different from the parent type and 

 that of the variant biotype that arose by sectoring. Leonian 

 (1929) concluded, "The presence of distinct strains and variants 

 within the same species and their decidedly different reactions (to 

 various acids and toxic substances) seem to indicate that the con- 

 cept of the species must not be that of a single organism but that 

 of a group of many organisms having in general the same principal 

 characters." 



Brierley (1929) summarized his observations on variation of 

 fungi in cultures, especially of Botrytis cinerea, by stating that his 

 distinct monosporial isolates of B. cinerea remained stable for long 

 periods of time when cultivated under different nutritional and 

 environmental conditions, both in vitro and in vivo. When the 

 different isolates were then brought back to the common stand- 

 ardized environment, all immediately reverted in their conidial 

 dimensions to a common original condition. This phenomenon 

 shows genotypic fixity within the species, which has been re- 

 peatedly demonstrated to occur in other organisms. 



