Specific nuclear elimination in polyploid plasmodia of the slime mold physarum polycephalum

Abstract

In growing plasmodia of the myxomycete Physamm polycephalum (G2-phase), three distinct classes of nuclei with a relative DNA content of lx, 2x, and 4x are observed in the presumed haploid strain CL. The 2x and 4x species comprise up to 35% and 5% of the nuclei. Quantitative cytofluorometric studies of nuclei isolated in either G2-or S-phase or after FUDR treatment (G1 arrest) show that the three nuclear populations undergo a synchronous mitotic cycle and that the relative DNA content of the nuclear fractions in G-2 phase reflects the 2c, 4c, and 8c state.The heterogeneity of the nuclear population does, however, seem to be restricted to the growth phase. During a starvation period of 4 days that always preceeds sporulation (and also meiosis), the 4c nuclear population is reduced to 7%, 8c nuclei are no longer detected. These results suggest that a mechanism exists in Physarum for the selective detection and elimination of polyploid nuclei.Key terms: Macroplasmodia, myxomycete, heploid stain, starvation, cell growth Plasmodia of the myxomycete Physarum. polyce phalum are unicellular, multinucleate organisms. The plasmodia1 cell cycle is characterized by synchronous nuclear divisions (6,9,15) and the lack of a G1-phase (21). After nutrient deprivation for 3 to 5 days, plasmodia are induced to sporulate by light (4). The formation of spores is accompanied by meiotic divisions (11). Germinating spores release single uninucleate amoebae, which, upon fusion with amoebae of different mating type, reform plasmodia,The colonia (CL) isolate differs from heterothallic P h y sarurn strains in that plasmodium formation occurs without amoeba1 fusion. CL plasmodia are therefore haploid in contrast to the generally diploid heterothallic strains (1,131. A low degree of diploidization, however, appears to be essential for the formation of viable spores in the CL strain (12). It has recently been reported that haploid strains, which were kept for extended periods in submerse culture, revealed a high degree of mixoploidy (10).A specific elimination of nuclei has been observed in Physarum heterokaryons. In a genetic analysis, Dee and Anderson (5) reported the elimination of diploid nuclei from haploid/diploid heterokaryons during growth phase; a similar effect was observed under starvation conditions (14).In this report we show that polyploidization in the strain CL may occur at a high level in plasmodia during growth and that the polyploid nuclei are gradually reduced to a minute amount during a prolonged starvation period. Thus we extend earlier findings of nuclear d e g radation in Physarurn heterokaryons to a naturally polyploidized strain, suggesting that a selective mechanism exists to include polyploid nuclei from meiosis. MATERIALS AND METHODSPlasmodia1 Culture Plasmodia of the colonia strain (CL) were generated by plating spores on agar made from dilute (33%) semidefined growth medium (3). Surface plasmodia were scraped from the agar, transferred to shake flasks containing growth medium, and cultured as microplasmodia at 24...

Recent advances of flow cytometry in fundamental and applied microbiology

“…Cell cycle studies have also been performed on the ciliate Tetrahymena [34], amoebae [96,188], and slime moulds [102,203], where FCM analysis has revealed some unusual features, such as the absence of a G I phase (see also [76] for older references), and dinoflagellates [87,197].…”

Physarum polycephalum mutants in the photocontrol of sporulation display altered patterns in the correlated expression of developmentally regulated genes

“…During growth, the protoplasmic mass continually increases and the nuclei suspended in the vigorously streaming protoplasm synchronously divide by undergoing a closed mitosis, while the plasmodial cell does not divide. A plasmodium covering a 9 cm diameter Petri dish, for example, then contains between 10 7 and 10 8 nuclei, which are still naturally synchronous in cell cycle progression and during differentiation (Rusch et al 1966;Sachsenmaier et al 1972;Werenskiold et al 1988).…”