Boris Ephrussi and the Early Days of Cytoplasmic Inheritance in Saccharomyces

Abstract

This volume on mitochondrial genes is dedicated to the memory of Boris Ephrussi,1 whose pioneering investigations of cytoplasmic inheritance in yeast set the stage for what is now a rapidly expanding area of molecular biology. I was fortunate to spend 1952–1953 in his laboratory, when he and his students were preoccupied with understanding the petite mutation, so called because its small-colony phenotype was expressed when mutant cells were grown on solid medium. The small size was due to a respiratory deficiency of the mutant cells. One of the students, Piotr Slonimski, concerned himself with the biochemistry of the petite mutation while Ephrussi was working out the genetics and physiological aspects of petiteness—the same Piotr Slonimski who is an editor of this volume. He and his collaborators have been the principal contributors to the theory of the structure and function of yeast mitochondrial DNA (mtDNA) and of its relation to petiteness.

INHERITANCE OF THE PETITE PHENOTYPE
The first evidence that the original petite mutation was cytoplasmic in its inheritance was reported in 1949 (Ephrussi et al. 1949b). Mutant cells, when mated with normal cells, gave a normal diploid; upon sporulation, this diploid produced asci in which all four spores were generally normal. Repeated backcrosses to the mutant gave the same result. Thus, since a 2:2 segregation would be expected if the mutation were nuclear in origin, it was concluded that the mutation was in fact cytoplasmic.

The petite story is an example of the unexpected discovery and the logical...