CONTROL OF NUCLEAR RATIOS IN NEUROSPORA HETEROKARYONS

Abstract

IN THE COURSE of studies on varying gene dosage in two-component heterokaryons of Neurospora crassa, the nuclear proportions in the heterokaryons were found to be related to those of the homokaryotic inocula (Pittenger and Atwood, 1954). Some indication that this might be the case has also been noted by Barratt and Garnjobst (1949) in heterokaryons between wild type and colonial. If the experimenter could vary the nuclear proportions at will, he would provide the basis for a variety of experiments on the interactions of nuclei of different genotype, genetic and environmental effects on the relative division rates of the nuclei. alteration of the nuclear proportions by natural selection, and on the dominance of various mutants, as suggested by Beadle and Coonradt (1944). The relation between input and final proportions is examined here in detail using a serie3 of typical biochemical mutants. The nuclear proportions in controlled inocula consisting of mixtures of homokaryotic conidia are compared with those in the resultant heterokaryons. It will be shown that a rather simple relation exists between the input and final proportions over a wide range, and that the differences are caused, in the main, by two separable selective processes which operate during heterokaryon formation, but not afterward. MATERIALS AND METHODS.-Five homokaryotic stocks, of mating type A, were used; their mutant loci are listed in the descriptive index of Barratt et al. (1954): pan,al-1; nic-2,aJ-2; pab-1; lys-3; and arg-6. These stocks phenotypically resemble het+ of Garnjobst (1953 and 1955) and their combinations, when uncontrolled inocula are used, are type I by the criteria of Holloway (1955). The average number of nuclei per conidium was determined for each stock by counting the nuclei in ca. 500 conidia stained according to Heubschman (1952), all samples being from the same cultures used in the experiments. The cultures were grown in 2000-ml. flasks containing 500 ml. of agar medium supplemented with the required growth factors. Conidia were harvested from the mature cultures and held overnight in supplemented liquid medium at 3?C. to allow maximum hydration and absorption of growth factors without germination. The suspensions were then adjusted to approximately equal turbidity, and sample dilutions of each were plated on supplemented sorbose agar to determine the viable concentration. Pairs of suspensions were combined in several volume ratios so that the proportions of a given component ranged from ca. 0.10 to 0.90. This procedure was followed