NUCLEAR BEHAVIOR IN THE DIPLOIDIZATION OF HAPLOID INFECTIONS OF PUCCINIA HELIANTHI

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Abstract
Nuclei of Puccinia helianthi occur in two principal but dissimilar states: an expanded one, characteristic of resting nuclei, and an unexpanded one associated with the process of division. Transitional stages intervene. Preliminary to division, the resting nucleus extrudes its nucleolus and contracts to an unexpanded state. Division gives rise to unexpanded daughter nuclei that in the living thallus soon undergo expansion. The nuclei of germinating urediniospores conform to this general pattern. Pycniospore nuclei, though daughter nuclei, fail to expand because of their isolation. When, however, through fusions of pyeniospores and flexuous hyphae they gain access to compatible haploid mycelia, they assume an expanded form. As expanded nuclei they then continue their migration to the basal region of the protoaecia, dividing perhaps but rarely on their way. There, by rapid successive divisions, they provide nuclei of pycniospore origin for the diploidization of the protoaecia. Cells of the intercellular mycelia remain uninucleate.