Spontaneous somatic segregation in a strain of Humicola

Abstract

Colonies of Humicola sp. strain 20–31 segregated into black (20–31 b) and white (20–31 w) colony colour types when propagated by single phialospores, single aleuriospores, single hyphal tips and macerated mycelium. Although the segregation ratio varied, depending upon the particular subculture of 20–31 used and the method of propagation, the spontaneous proportion of white segregants never exceeded 40%. Exposure of aleuriospore populations from 20–31 to high doses of far ultra-violet light increased the proportion of white segregants. Strains 20–31, 20–31 b and 20–31 w differed in their level of production of the pigmented aleuriospores and this was primarily responsible for the observed differences in colony colour. The three strains also differed in growth rate and growth yield in submerged culture and in the sensitivity of their aleuriospores to far ultra-violet light, those from 20–31 w being more resistant than those from 20–31 b . All first generation cultures of 20–31 b and 20-31 w were true breeding in further generations, whatever the method of propagation used. Only when the parental strain, 20–31, was subcultured by blocks of hyphae were the resultant first generation cultures themselves capable of segregating into the black and white types. Cultures of 20–31 b and 20-31 w were also stable throughout periods of storage, repeated subculture and following ultra-violet irradiation. These results suggest that strain 20–31 is a mixture of homokaryotic mycelia of strains 20–31 b and 20–31 w . It is considered that 20–31 b represents the natural state of this Humicola isolate and that strain 20–31 arose in culture through mutation of a 20–31 b nucleus to 20–31 w , the resultant mixture being subsequently maintained over many generations of mass hyphal transfer. The change in segregation ratio following ultra-violet irradiation of aleuriospores of strain 20–31 can be explained by the differential survival of the two component genotypes. Apparently diverse segregants can arise in culture from simple underlying genetic changes. A full understanding of the significance of these variants requires a combination of classical morphological data with information from genetic and biochemical studies.