Plasmodium formation without change in nuclear DNA content inPhysarum polycephalum

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Abstract
SUMMARY: The Colonia isolate ofPhysarum polycephalumproduces plasmodia within amoebal clones. Wheals demonstrated genetically that amoebae of the C50 strain of this isolate, when crossed with heterothallic amoebae, yielded recombinant progeny. He concluded that nuclear fusion and meiosis occurred in these crosses and suggested that nuclear fusion was also involved in plasmodia formation in clones. He thus designated the strain ‘homothallic’.In the present work genetic evidence is presented which indicates that the Colonia strainCL, when crossed with heterothallic strains, also yields recombinant progeny and thus undergoes nuclear fusion and meiosis. Microdensitometric measurements of nuclear DNA content are reported which indicate thatCLamoebae are haploid like heterothallic amoebae, and crossed plasmodia are diploid. However, clonally formedCLplasmodia were found to have the same G2nuclear DNA content asCLamoebae. This observation excludes the possibility of nuclear fusion when plasmodia form within clones ofCLamoebae and therefore the strain cannot be homothallic. Two alternatives, apogamy and coalescence, are proposed as the most likely mechanisms for clonal plasmodium formation in strainCL.