The Inheritance of Growth Rate in Neurospora crassa with Special Reference to Hybrid Vigour and Cytoplasmic Inheritance

Eight crosses have been made between different strains of Neurospora crassa and the growth rates of the segregants and parents determined. Growth rates smaller than, and intermediate between, those of either parent have been found among the segregants, but in only one case has an apparent example of hybrid vigour, i.e. of a greater growth rate, been established. This example of an apparently greater growth rate has been shown on analysis to be due to the segregant type having a respone to temperature different from that of its parents. In consequence it is to be emphasized that in questions of the inheritance of physiological characters deductions based on measurements made under only one set of environmental conditions are unsatisfactory and may readily lead to wrong conclusions. The inheritance of certain morphological characteristics and of two factors affecting the rate of spread of the fungus are recorded. Evidence is adduced to show that cytoplasmic inheritance is demonstrable in this fungus and that it affects both morphological and physiological characteristics. Experiments carried out by the author on Coprinus sphaerosporus are reviewed and are shown to indicate the presence of hybrid vigour in that fungus. A suggestion arising from this discussion indicates a possible interpretation of certain unexpected results in experiments carried out by Ashby and Luckwill. The author has much pleasure in thanking Professor V. H. Blackman for his constant advice during these experiments and also Professor F. G. Gregory for numerous suggestions.