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The cell cycle of Physarum polycephalum myxamoebae, as well as haploid and diploid and plasmodia, was analysed using laser microfluorometry. The data obtained indicate that the cell cycle of all strains examined are similar. The DNA profiles also indicate that a haploid ploidy level is still uncertain and show conclusively that spontaneous chromosome loss is a parameter of concern in research involving these organisms.

Thymidine (TK) and deoxycytidine (dCK) kinase levels, chromosome counts and whole culture protein:DNA ratios have been determined in the TK- plasmodial strains of P. polycephalum--TU84 and TU63--and on several wild type (wt) plasmodial and amoebal strains from Wis 1 and Colonia backgrounds. It was found that (a) dCK levels and protein:DNA ratios were similar in all plasmodia, regardless of ploidy or genetic background; (b) TU84 and 63 were haploid; (c) TK specific activity was similar in all wt plasmodia but lower in amoebae; (d) TK activities of mutants and wt were approximately additive in heterokaryons. It was concluded that TK deficiency is due to a defect in a structural gene and that wt strains are TK+/TK+ and mutants hemizygous TK-.

Nuclear DNA content and ploidy have been determined at different stages of the life cycle of the Colonia strain of the myxomycete Physarum polycephalum. Analyses at the plasmodial stage showed that (a) Burton and Fuelgen DNA analyses agreed within 15% with strains which ranged from 0-6 to 3-6 pg of DNA per nucleus; (b) S-phase in Colonia is during the early part of interphase as in the Wisconsin strain; (c) in heterothallic and heterothallic x Colonia crossed strains there are 1-0-1-2 pg of DNA and 70 chromosomes per nucleus and in Colonia 0-6 pg of DNA and 40 chromosomes. Germinating spores of all strains contained one population of cells with about 0-5 pg of DNA and 40 chromosomes and another of larger cells with up to 2-5 pg of DNA and 200 chromosomes. The polyploid nuclei comprised 2-20% of the total in heterothallic strains, 2-65% in heterothallic x Colonia crosses and 25-75% in Colonia. A method was devised for making chromosome spreads of amoebae grown on bacterial lawns. Cells were first exposed to dilute formaldehyde at 26 degrees C for 30 min, then spread on slides with hot lactic acid and strained. Such spreads of CLd (Colonia) and RSD4 (heterothallic) amoebae both contained about 40 chromosomes. The data are consistent with the view that Colonia is haploid throughout its life cycle and that chromosome number is neither halved during sporulation nor doubled during plasmoidal formation. However, the possibility exists that an alternance of ploidy occurs by way of the few diploid nuclei present in the plasmodium.

A method has been developed for growing Physarum polycephalum plasmodia that are 8 to 10 times larger than those obtained in the petri dish cultures used by Nygaard, Guttes, and Rusch. In the large-scale procedure, plasmodia were grown in metal trays on a membrane supported by filter paper on stainless-steel screen. Plasmodia were started from a ring of inoculum to allow inward and outward migration and were incubated on a rocker so that nutrient medium would flow back and forth, wetting the undersurface of the plasmodium. Rocker and petri dish cultures had similar growth characteristics: (i) the interphase time between mitoses I and II and between II and III was about 8 hr; (ii) ribonucleic acid and protein increased essentially logarithmically throughout the cell cycle; and (iii) deoxyribonucleic acid increased only during early interphase and it doubled in approximately 3 hr after each mitosis. Rocker cultures were not as nearly synchronous as petri dish cultures and had a range in metaphase time (at mitosis III) within individual plasmodia of 15 to 45 min, as compared with 5 to 10 min in petri dish cultures.