Viability of Physarum polycephalum spores and ploidy of plasmodial nuclei

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Abstract
Amoebae of P. polycephalum carrying the mth mating-type allele may differentiate into plasmodia in the absence of mating. Such plasmodia are haploid and, upon sporulation, produce mainly inviable spores. Meiotic and mitotic divisions of viable spores were studied. Using a microfluorometric measurement of the DNA content of individual nuclei, the fraction of viable spores was found to be correlated with the proportion of rare, diploid nuclei contained in the generally haploid plasmodium. When homozygous diploid plasmodia were created by heat shocking, spore viability increased dramatically. Possibly viable spores are produced via meiosis in mth plasmodia, the mth allele has no effect on sporulation per se and the normal source of viable haploid spores is a small fraction of diploid nuclei ubiquitous in haploid plasmodia.