Interactions between an Extrachromosomal Factor, Poky, and Nuclear Genes in NEUROSPORA CRASSA.

’ ~ ~ T c ~ ~ ~ ~ Lnd MITCHELL 1952). Its phenotype is characterized by a long lag period, slow growth and defects in the cytochrome system during the early growth period. Older poky cultures recover to a considerable extent, approaching, but not attaining, either the wild-type growth rate or normal concentrations of the cytochromes (HASKINS, TISSIERES, MITCHELL and MITCHELL 1953; MITCHELL, MITCHELL and TISSIERES 1953). This asexual cycle of growth and cytochrome abnormalities is repeated on conidial transfer. This study was undertaken to attempt to modify the life history of poky. Since poky is a transient defect, and is reestablished from a “cured” culture on conidial transfer, experiments were undertaken to perpetuate the “cured” state by preventing conidiation in two ways: by continuous hyphal growth, and by introducing the gene, fluffy, which blocks conidiation. Since poky mycelia, unlike wild-type, accumulate large quantities of free fatty acids ( HARDESTY and MITCHELL 1963), attempts were made to determine whether the combination of a fatty acid-requiring mutant with poky cytoplasm would result in complementation, so that normal growth would prevail with suitable concentrations of oleic acid in the medium. Other chromosomal genes were introduced into the poky strain to attempt to modify its phenotype with the hope of gaining further insight into nuclear-cytoplasmic interactions. The gene f (“fast”) , a modifier of the slow growth but not of the defective cytochromes of the poky phenotype (MITCHELL and MITCHELL 1956), was mapped in order to distinguish it from other modifiers which might be found. These experiments show that the slow-growth aspects of poky can be modified to approach wild type by continuous hyphal growth over a long period, but that certain mutant genes introduced into the poky strain greatly decrease the rate of growth.