GENESIS OF MITOCHONDRIA IN NEUROSPORA CRASSA

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Abstract
In logarithmically growing cultures of Neurospora crassa, mitochondrial lipids can be labeled with H3- or C14-choline. Under the conditions of a washout experiment, radioactive choline is preserved in the mitochondrial population and serves as a satisfactory label through 3 mass doubling cycles. Radio-autographic analyses show that the label is randomly distributed in mitochondrial populations, isolated as mitochondrial fractions from fully labeled cells, as well as from cells undergoing 3 subsequent doubling cycles in unlabeled choline. This random distribution does not appear to be the result of the cell fractionation procedures, nor would it seem to be effected in vivo through repeated mitochondrial fusions and fissions. These data, which fit a dispersive distribution of label, exclude the possibility of de-novo formation, and do not favor the hypothesis of structural mitochondrial precursors. The most likely interpretation of the findings is that in growing Neurospora. the mitochondrial mass increases by a continuous process of addition of new choline-containing lipid units to the existing mitochondrial structure. The number of individuals in the population is increased by division of mitochondria and the division process distributes the label at random so that preexisting membrane material is transmitted uniformly or nearly uniformly to all progeny.