Plasmodium formation without change in nuclear DNA content in Physarum polycephalum

Genetic regulation of differentiation inPhysarum polycephalum

Plasmodial formation in the MyxomycetePhysarum polycephalumis controlled by a mating type (mt) locus. There are a number of different heterothallicmtalleles; and also a variant,mth, that allows plasmodial formation in pure clones. This paper reports an analysis of this differentiation system. The strainCL(mth) forms plasmodia in pure clones (i.e. it selfs).CLwas mutagenized with NMG and 21 mutants unable to differentiate into amoebae were isolated. The mutants, together withCLd, fell into two complementation groups, twelve indifAand ten indifB. Both complementation groups are closely linked or allelic to themtlocus.difArepresents a gene essential for plasmoclial formation, but it is suggested thatdifBrepresents a class of revertants to themt2heterothallic state. A model of the control of plasmodial formation is proposed in which themtlocus is suggested to code for the repressor of thedifAgene. Genetic control is explained in terms of the dilution of allele-specific repressors.

Genetics and biochemistry of 5-Bromodeoxyuridine resistance inPhysarum polycephalum

5-Bromodeoxyuridine (BrdU)-resistant mutants ofPhysarum polycephalumwere isolated as colonies of myxamoebae growing on BrdU-substituted bacteria after exposure to long-wave ultraviolet light (UV). Twenty-four such mutants were studied. They all show Mendelian segregation in crosses with wild type. Plasmodia constructed from mutant amoebae were all deficient in deoxythymidine incorporation. Extracts made from selected plasmodia showed that all except one had low thymidine kinase activity.

Genetical and biochemical complementation studies revealed two complementation groups: 23 mutants,burA, had low thymidine kinase while 1 mutant,burB, had normal thymidine kinase levels.

Genetic analysis of a cross between two homothallic strains ofPhysarum polycephalum

The homothallic amoebal clones ofPhysarum polycephalumare of potential use in understanding the developmental genetics of this organism. Such an application requires that complementation and recombination analysis be possible between pairs of homothallic clones. This paper is a report of the formation of mixed plasmodia by pairs of homothallic amoebal clones. In order to detect such mixed plasmodia use was made of two marker genes involved in plasmodial fusion,fusA andfusB. Sporulation of a mixed plasmodium formed from two homothallic (delayed) amoebal clones yielded progeny amoebae which were genetically recombinant. It is deduced from the ratios of various genotypes in these progeny clones that the mixed plasmodium was diploid and that meiosis was associated with sporulation. There is therefore no impediment to the use of the homothallic strains for genetical analysis. The progeny amoebal clones were observed to be showing segregation for the characters homothallic (rapid) and homothallic (delayed). This observation, taken together with other related observations, suggests that the homothallic (delayed) character is produced by mutation of the homothallic (rapid) character. The rare plasmodia formed by a homothallic (delayed) amoebal clone are the result of reversion of this mutation. Amoebal clones of the homothallic (delayed) type are therefore developmental mutants unable to perform the differentiation from amoeba to plasmodium.

A new type of plasmodium formation inPhysarum polycephalum

Haploid amoebae ofPhysarum polycephalummay form plasmodia sexually by ‘crossing’, which involves cellular and nuclear fusion, or asexually by ‘selfing’, which occurs without nuclear fusion. In most amoebal strains, selfing is seen in clonal cultures only at very low frequency. In the present study, we have shown that selfing occurs at a similarly low frequency in mixtures of crossing-incompatible amoebae, but is stimulated in crossing-compatible mixtures. In certain compatible mixtures involving mutant strains, where crossing is temperature-sensitive, selfing may be stimulated even at a temperature that largely or wholly abolishes crossing. The extent to which selfing is stimulated appears to be influenced bymatB, a locus which is known to affect the frequency of amoebal fusion. We have failed to detect any filter-transmissible factor that might be responsible for the effects we have observed. We suggest a sequence of events that might bring about ‘stimulated selfing’ as a consequence of abortive crossing.

Map and function ofgadmutations inPhysarum polycephalum

Amoebae and plasmodia are alternate vegetative forms in the life cycle of the acellular slime mouldPhysarum polycephalum. Haploid amoebae carrying heterothallic alleles of thematA(ormt) locus ordinarily form plasmodia only by crossing, but occasionally give rise to mutants that form plasmodia by selfing as well as by crossing. Twelve independently isolated mutants of this type have been studied. Eight carry mutations (termedgador greater asexual differentiation mutations) within approximately 0·2 map units ofmatA. Another mutation (gad-12) is linked neither tomatAnor to any of 9 other markers tested. The remaining three mutations are linked tomatAand map as follows:matA–0·5 units –gad-4–4 units –gad-6– 8 units –gad-11. One mutation,gad-11, has been tested in strains carrying each of the fivematAalleles (matAl, 2, 3, 4, andh) available in a common genetic background; the mutation is expressed with all five alleles. The mutationnpfF1(formerlyaptA1), which was isolated as a suppressor of selfing in Colonia (matAh) amoebae, suppresses the action of each of the 12gadmutations. The similarly isolated mutationnpfA1is also epistatic to eight of the mutations, but permits selfing withgad-5, 6, 12and13. For double mutant strains containinggad-12andgad-1, 2, 4, 6or11, the selfing behaviour of each double mutant differs from that of either single mutant. Mixtures ofgad−npfF1withgad+npf+amoebae readily form plasmodia, a result suggesting thatgadmutations are dominant or semi-dominant. We conclude that the commitment of a cell to differentiate into a plasmodium is under the control of a complex group of genes linked tomatA.

A gene unlinked to mating-type affecting crossing between strains ofPhysarum polycephalum

Two alleles of a gene (rac) unlinked to the mating-type locus (mt) have been identified in strains ofPhysarum polycephalumfrom different laboratories. Heterothallic strains differing inmtalleles cross more rapidly if they differ also in theirracalleles. The recovery of hybrid plasmodia from crosses between apogamic (mth) and heterothallic strains is more likely to be achieved if strains of differentracgenotype are used.

Enrichment and screening of heat-sensitive mutants of Physarum polycephalum

A new method for the isolation of temperature-sensitive mutants of Physarum polycephalum is described. It involves enrichment and prescreening of mutagenized amoebae followed by screening at both the plasmodial and amoebal stage. A total of 74 temperature-sensitive strains were recovered of which 26 were temperature-sensitive only as plasmodia, 35 only as amoebae and 13 in both stages. After a shift to the nonpermissive temperature, DNA and protein synthesis were followed in temperature-sensitive plasmodia to discover if the lesion affected functions of the nuclear cycle.

Isolation and Analysis of Amoebal–Plasmodial Transition Mutants in the Myxomycete Physarum polycephalum

Plasmodium formation in the Myxomycete Physarum polycephalum normally involves fusion of haploid amoebae, carrying different alleles at the mating type (mt) locus, to give diploid plasmodia. Strains carrying the mth allele are capable of undergoing the amoebal–plasmodial transition with high efficiency within amoebal clones, resulting in the formation of haploid plasmodia. NMG mutagenesis of mth amoebae, followed by an enrichment procedure, was used to isolate mutants in which such clonal plasmodium formation was either delayed or absent. Thirteen mutants of the second type were analysed. Three of these were temperature-sensitive for plasmodium formation. All thirteen mutants were able to form diploid crossed plasmodia when mixed with a mt1 strain. Three new genes were identified and designated npfA, npfB and npfC. A mutant allele of npfA rendered clonal plasmodium formation temperature-sensitive, but did not prevent crossing at the non-permissive temperature with derived strains carrying the same mutant allele. No recombination was detected between npfB or npfC and mt, but npfA was unlinked to mt and a locus (apt-1) shown in a previous study to be involved in plasmodium formation. The genes npfB and npfC were distinguished by complementation analysis. Strains of the genotype npfB−; npfC+ behaved in the same way as strains carrying the mt2 allele. The nature of the mutants and the role of the mating-type locus in the initiation of plasmodium formation are discussed.

Non-selfing mutants from selfing (Het−) strains ofPhysarum polycephalum

Plasmodial formation in the MyxomycetePhysarum polycephalumis under heterothallic control by a mating type (mt) locus. In natural isolates only amoebae with differentmtalleles are able to cross to form diploid plasmodia. A class of mutants isolated from heterothallic amoebae, together with the variant strainCL, is able to form plasmodia in pure clones, designated as selfing. Non-selfing mutants have been isolated fromCLand from other selfing amoebae.

This paper reports the isolation and analysis of 64 non-selfing derivatives (designated Npf−) from seven selfing (Het−) strains. The Npf−mutants could be grouped into eight classes on the basis of their crossing and complementation patterns. The possible significance of these mutants is discussed.

Selfing mutants from heterothallic strains ofPhysarum polycephalum

Plasmodial formation in the myxomycetePhysarum polycephalumis controlled by a mating type (mt) locus, with heterothallic amoebae normally being unable to form plasmodia in pure clones. We report the isolation by mutagenesis of selfing mutants from heterothallic strains, and their analysis. Various amoebal strains of different mating types were mutagenized with a range of mutagens, and a number of selfing mutants (designated Het−) were isolated. A specific sensitivity ofmt2amoebae to mutagenesis by NMG was observed. This sensitivity segregated as a single locus closely linked or allelic to themt2locus. When the Het−clones were incubated at 30 °C, selfing was greatly inhibited. This property was used to determine themtspecificities of four Het−clones. The process of plasmodial induction in pure clones ofCLwas also studied using the 30 °C temperature effect.

cDNA cloning of Physarum polycephalum DNA topoisomerase I and expression analysis in plasmodia treated with cAMP

cDNA encoding DNA topoisomerase I from Physarum polycephalum was isolated from a poly(A)+ -primed library (3'-region) and by PCR (5'-region). The coding region of cDNA was 3045 bp, encoding a polypeptide of molecular mass of 112 kDa. Identity between predicted amino acids sequences of conserved domains and corresponding domains from another eukaryotic type I DNA topoisomerases varied from 33.2 to 53.5% for the core domain and from 33.8 to 57.4% for the C-terminal domain. A peculiar feature of Physarum DNA topoisomerase I was a stretch of repeated KPAX...X motifs in the N-terminal domain of the polypeptide. Although treatment of the plasmodia with db-cAMP increased relaxing activity of the DNA topoisomerase I several-fold, there was only a slight increase in the mRNA level.

Molecular cloning, over-expression, developmental regulation and immunolocalization of fragminP, a gelsolin-related actin-binding protein from Physarum polycephalum plasmodia

FragminP is a Ca2+-dependent actin-binding and microfilament regulatory protein of the gelsolin family. We screened a Physarum polycephalum cDNA library with polyclonal fragminP antibodies and isolated a cDNA clone of 1,104 bp encoding 368 amino acids of fragminP, revealing two consensus phosphatidylinositol 4,5 bisphosphate-binding motifs in the central part of the protein. The first methionine is modified by an acetyl group, and three amino acids were missing from the protein coded for by the cDNA clone. Full-length recombinant fragminP was generated by PCR, purified after over-expression from Escherichia coli and displayed identical properties to native Physarum fragminP. Northern blot analysis against RNA, isolated from cultures at various stages of development, indicated that fragminP is absent from amoebae and that expression is initiated at an early stage during apogamic development, in a similar way to that observed for the profilin genes. In situ immunolocalization of fragminP in Physarum microplasmodia revealed that the protein is localized predominantly at the plasma membrane, suggesting a role in the regulation of the subcortical actin meshwork. Our data indicate that we have isolated the plasmodium-specific fragminP cDNA (frgP) and suggest that, in each of its two vegetative cell types, P. polycephalum uses a different fragmin isoform that performs different functions.

A single gamma-tubulin gene and mRNA, but two gamma-tubulin polypeptides differing by their binding to the spindle pole organizing centres

Cells of eukaryotic organisms exhibit microtubules with various functions during the different developmental stages. The identification of multiple forms of alpha- and beta-tubulins had raised the question of their possible physiological roles. In the myxomycete Physarum polycephalum a complex polymorphism for alpha- and beta-tubulins has been correlated with a specific developmental expression pattern. Here, we have investigated the potential heterogeneity of gamma-tubulin in this organism. A single gene, with 3 introns and 4 exons, and a single mRNA coding for gamma-tubulin were detected. They coded for a polypeptide of 454 amino acids, with a predicted molecular mass of 50,674, which presented 64–76% identity with other gamma-tubulins. However, immunological studies identified two gamma-tubulin polypeptides, both present in the two developmental stages of the organism, uninucleate amoebae and multinucleate plasmodia. The two gamma-tubulins, called gamma s- and gamma f-tubulin for slow and fast electrophoretic mobility, exhibited apparent molecular masses of 52,000 and 50,000, respectively. They were recognized by two antibodies (R70 and JH46) raised against two distinct conserved sequences of gamma-tubulins. They were present both in the preparations of amoebal centrosomes possessing two centrioles and in the preparations of plasmodial nuclear metaphases devoid of structurally distinct polar structures. These two gamma-tubulins exhibited different sedimentation properties as shown by ultracentrifugation and sedimentation in sucrose gradients. Moreover, gamma s-tubulin was tightly bound to microtubule organizing centers (MTOCs) while gamma f-tubulin was loosely associated with these structures. This first demonstration of the presence of two gamma-tubulins with distinct properties in the same MTOC suggests a more complex physiological role than previously assumed.

RNA editing of the coI mRNA throughout the life cycle of Physarum polycephalum

Editing of RNA via the insertion, deletion or substitution of genetic information affects gene expression in a variety of systems. Previous characterization of the Physarum polycephalum cytochrome c oxidase subunit I (coI) mRNA revealed that both nucleotide insertions and base substitutions occur during the maturation of this mitochondrial message. Both types of editing are known to be developmentally regulated in other systems, including mammals and trypanosomatids. Here we show that the coI mRNA present in Physarum mitochondria is edited via specific nucleotide insertions and C to U conversions at every stage of the life cycle. Primer extension sequencing of the RNA indicates that this editing is both accurate and efficient. Using a sensitive RT-PCR assay to monitor the extent of editing at individual sites of C insertion, we estimate that greater than 98% of the steady-state amount of coI mRNA is edited throughout the Physarum developmental cycle.

Variable pathways for developmental changes of mitosis and cytokinesis in Physarum polycephalum

The development of a uninucleate ameba into a multinucleate, syncytial plasmodium in myxomycetes involves a change from the open, astral mitosis of the ameba to the intranuclear, anastral mitosis of the plasmodium, and the omission of cytokinesis from the cell cycle. We describe immunofluorescence microscopic studies of the amebal-plasmodial transition (APT) in Physarum polycephalum. We demonstrate that the reorganization of mitotic spindles commences in uninucleate cells after commitment to plasmodium formation, is completed by the binucleate stage, and occurs via different routes in individual developing cells. Most uninucleate developing cells formed mitotic spindles characteristic either of amebae or of plasmodia. However, chimeric mitotic figures exhibiting features of both amebal and plasmodial mitoses, and a novel star microtubular array were also observed. The loss of the ameba-specific alpha 3-tubulin and the accumulation of the plasmodium-specific beta 2-tubulin isotypes during development were not sufficient to explain the changes in the organization of mitotic spindles. The majority of uninucleate developing cells undergoing astral mitoses (amebal and chimeric) exhibited cytokinetic furrows, whereas cells with the anastral plasmodial mitosis exhibited no furrows. Thus, the transition from astral to anastral mitosis during the APT could be sufficient for the omission of cytokinesis from the cell cycle. However, astral mitosis may not ensure cytokinesis: some cells undergoing amebal or chimeric mitosis contained unilateral cytokinetic furrows or no furrow at all. These cells would, most probably, fail to divide. We suggest that a uninucleate committed cell undergoing amebal or chimeric mitosis can either divide or else form a binucleate cell. In contrast, a uninucleate cell with a mitotic spindle of the plasmodial type gives rise only to a binucleate cells. Further, the decision to enter mitosis after commitment to the APT is independent of the developmental changes in the organization of the mitotic spindle and cytokinesis.

Changes in phospholipid composition and phospholipase D activity during the differentiation of Physarum polycephalum

Changes in phospholipid composition and phospholipase D activity were observed during a differentiation from haploid myxoamoebae to diploid plasmodia of a true slime mold, Physarum polycephalum. In the amoeboid stage, the main components of phospholipid fraction were phosphatidylethanolamine (PE, 43.3%), phosphatidylcholine (PC, 28.8%) and phosphatidylinositol (PI, 8.0%), but in the plasmodial stage, PC was dominant (40.7%) and other main components were PE (31.5%) and phosphatidic acid (PA, 11.0%). The specific activity of phospholipase D in the plasmodia was 5.7-times higher than that in the myxoamoebae when measured in the presence of Ca2+ at the alkaline pH. In the amoeboid stage, phospholipase A activity (A1 or A2) was detected at the alkaline pH with Ca2+. Phospholipase D activity in the plasmodia was characterized: pH optimum was 6.0; Ca2+ was required for the reaction and Ba2+ could substitute partly for Ca2+; PE was the best substrate for the hydrolytic activity and PC and PI were not appreciably hydrolyzed; and all detergents tested inhibited the enzyme activity.

Recognition of specific Physarum alpha-tubulin isotypes by a monoclonal antibody. Sequence heterogeneity around the acetylation site at lysine 40

The monoclonal antibody 6-11B-1 recognises specifically the acetylated form of alpha-tubulin. The acetylation event occurs on a unique lysine residue, lysine 40. Using 6-11B-1, acetylated alpha-tubulin was detected in myxamoebae but not plasmodia of Physarum polycephalum. Following chemical acetylation plasmodial alpha-tubulin was detected by 6-11B-1. The monoclonal antibody KMP-1 recognises certain Physarum alpha-tubulin isotypes but only in non-acetylated form. Whilst recognising all the non-acetylated fraction of myxamoebal alpha-tubulin only a proportion of plasmodial alpha-tubulin was recognised by KMP-1. Peptides were synthesised corresponding to the acetylation domains (containing lysine 40) of myxamoebal alpha-tubulin and the inferred acetylation domains of two plasmodial-specific alpha-tubulin isotypes. The only difference between the two peptides was at a single residue corresponding to amino acid 44 in the polypeptide. Tyrosine was present in myxamoebal alpha-tubulin and glycine was present in the plasmodial specific peptides; the peptides are referred to as the Tyr44 and Gly44 peptides respectively. Both peptides in acetylated form blocked 6-11B-1 reactivity towards acetylated myxamoebal alpha-tubulin. The Tyr44 but not the Gly44 peptide blocked KMP-1 reactivity towards non-acetylated myxamoebal alpha-tubulin. Tyrosine at position 44 is not found in any other known alpha-tubulin. Thus a unique antigenic determinant exists in certain Physarum alpha-tubulin isotypes, close to the acetylation site at lysine 40. This antigenic determinant forms part of the KMP-1 recognition epitope and explains the unique isotype selectivity of this monoclonal antibody.

Purification of myxamoebal fragmin, and switching of myxamoebal fragmin to plasmodial fragmin during differentiation of Physarum polycephalum

We have isolated and purified an activity from amoebae of Physarum polycephalum that reduces the flow birefringence of a solution of F-actin in a Ca2+-dependent manner. The purified activity from 100 g of amoebae consisted of 1 mg of a 40,000 mol. wt protein. DNase I-affinity chromatography demonstrated that the protein binds to Physarum actin in a Ca2+-dependent manner, and the binding is not reversed by excess EGTA. Viscometric measurement indicated that the protein (i) accelerates polymerization of G-actin, and (ii) severs F-actin, in a Ca2+-dependent manner. Thus, the protein appeared functionally similar to the fragmin previously isolated from Physarum plasmodia (plasmodial fragmin). However, the two proteins had slightly different mobilities on urea-SDS-PAGE, and antibodies raised against the two proteins scarcely cross-reacted with each other. Hence, we conclude that the two proteins are closely related to but are different from each other, and we have named the novel protein 'myxamoebal fragmin'. Immunoblot analysis indicated that myxamoebal and plasmodial fragmins are specifically present in amoebae and plasmodia, respectively. Results of immunofluorescence staining suggest that the synthesis of plasmodial fragmin is switched on coordinately with the synthesis of the heavy chain of plasmodial myosin and other plasmodium-specific contractile proteins during the apogamic differentiation of amoebae to plasmodia.

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

In growing plasmodia of the myxomycete Physarum polycephalum (G2-phase), three distinct classes of nuclei with a relative DNA content of 1x, 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.

Flow cytometry of the differentiation of Physarum polycephalum myxamoebae to cysts

Myxamoebae of Physarum polycephalum, strain Cld, were grown on agar lawns on live bacteria. Myxamoebae were harvested, fixed and stained with propidium iodide. Flow cytometry showed that, as in the case of Physarum plasmodia, there is no G1 phase during rapid exponential growth. However, an apparent G1 phase was observed at the end of exponential growth when the culture arrested with the G1 DNA content for about a day between growth and differentiation. Most myxamoebae differentiated into cysts, but some formed microplasmodia and others appeared to lose DNA. The cysts possessed the G2 phase DNA content and there was an S phase connecting the G1-arrested state with the encysted state. Encystment was blocked by hydroxyurea (HU) suggesting that DNA synthesis is essential for encystment. The natural temporary synchronization in G1 phase may provide the basis of a method for selecting mutants with a conditional block in G2 or M phases.

Effect of the thermal variation on the molecular weight estimation of the acid phosphatase from Physarum polycephalum

A Binding Protein (BP) of nearly 95,000 daltons interacts with the acid phosphatase at the permissive temperature, elevating its molecular weight from 48,000 +/- 2,000 to 136,000 +/- 7,000 in CL and CHR strains of Physarum polycephalum plasmodia. The same molecule has not been observed in the extracts of M3C VIII and K strains. 28-31 degrees C is the critical temperature range for the breaking of enzyme-BP association in CL strains. This critical temperature is higher than 39 degrees C for the extract of CHR strains. CHR BP recognizes and binds CL acid phosphatase as strongly as CHR acid phosphatase. Km determination at 32 degrees C and thermoinactivation patterns at 66 degrees C indicate that the BP has no apparent regulatory role for the acid phosphatase activities. The presence or the absence of the BP in some of the strains may be used as a marker in the genetical studies of this organism.

Induction of heat-shock proteins at permissive growth temperatures in the plasmodium of the myxomycete Physarum polycephalum

When synchronous plasmodia of the myxomycete Physarum polycephalum were submitted to temperature shifts from 22 degrees C to 32 degrees C, the highest physiological temperature, protein synthesis was increased during at least 10 h. Moreover during 2 h, four proteins (69, 74, 82 and 105 kDa) showed a transient increase of their synthesis, independently of the period of the temperature shift during the cell cycle. The stability of these proteins and the susceptibility of their synthesis to actinomycin suggested that they corresponded to four different proteins. Temperature shifts from 22 degrees C or 29 degrees C to 37 degrees C, a non-physiological temperature, demonstrated that the 69-kDa, 74-kDa, 82-kDa and 105-kDa proteins were identical to the four heat-shock proteins which could be detected in Physarum. Although the physiological significance of these heat-shock proteins remained unclear, comparison between the extent of their synthesis and the length of the mitotic delays induced by various temperature shifts ruled out a direct relationship between mitotic delays and synthesis of the 74-kDa, 82-kDa and 105-kDa proteins.

Periodic synthesis of microtubular proteins in the cell cycle of Physarum

Periodic polypeptide labeling over the naturally synchronous nuclear replication cycle of Physarum polycephalum was analyzed by fluorography of two-dimensional electropherograms. Two sets of polypeptides, denoted as P and Q, showed strong periodicity; they were maximally labeled just prior to mitosis. This periodicity was shown to reflect synthesis rather than turnover or recovery. Both P and Q copolymerized with porcine microtubular proteins and displayed electrophoretic properties similar to those of porcine tubulins. The significance of the periodic synthesis of these microtubular proteins is discussed as a possible component within the chain of events that establishes the high mitotic synchrony of Physarum syncytia.

Isolation of cell cycle mutants of Physarum polycephalum

Asynchronous amoebal cultures of temperature-sensitive mutants of Physarum polycephalum were examined cytologically, and two cell cycle mutants were identified. Genetic analysis indicated that each mutant carried a single mutation that was expressed in both amoebal and plasmodial phases. Thus it is possible to isolate cell cycle mutations expressed in plasmodia by initial isolation and analysis of amoebal mutants, a quicker procedure than the alternative of isolating plasmodial mutants directly. The two mutants were studied further by measuring nuclear DNA contents and synthesis of macromolecules. Both mutants gave results consistent with a block in nuclear division.

Polyploidy in fungi

There is evidence supporting a concept of polyploid evolution in a number of groups of fungi. These typically have dominant diploid phases in their life-histories. There are a number of reports of suspected polyploidy in other fungi, but these should be considered speculative at this time.

Temperature-sensitive mutants of Physarum polycephalum--expression of mutations in amoebae and plasmodia

Over 100 temperature-sensitive mutants ofmt-h (apogamic) strains ofPhysarum polycephalumwere isolated either by testing clones of mutagenized amoebae (ATS mutants) or by the more laborious method of testing plasmodia derived from such clones (PTS mutants). When amoebae and plasmodia of each mutant were tested for growth temperature sensitivity on different media (to give optimum growth of each phase), only 21% of 73 ATS mutants and 32% of 31 PTS mutants appeared to be temperature-sensitive in both phases, suggesting that the majority of mutants are phase-specific, as concluded from several similar studies by previous authors. When the mutants were tested on a third medium which allows growth of both amoebae and plasmodia, many of the mutants no longer had a temperature-sensitive phenotype in either phase. Among the remainder, 51% of ATS mutants and 67% of PTS mutants were temperature-sensitive in both phases. It was suggested that certain media have a remedial effect on some temperature-sensitive mutants so that the phenotype is apparently normal. Thus, the proportion of phase-specific mutants may be over-estimated if tests of temperature-sensitivity are done on the different media commonly used for culture of amoebae and plasmodia respectively. It was concluded that the most efficient procedure for isolation of temperature-sensitive mutants expressed in plasmodia is to screen clones of amoebae on a medium resembling as closely as possible that which is to be used for testing plasmodia.

Temperature-sensitive mutants of Physarum polycephalum: viability, growth, and nuclear replication

Using a selfing strain of Physarum polycephalum that forms haploid plasmodia, we have isolated temperature-sensitive growth mutants in two ways. The negative selectant, netropsin, was used to enrich for temperature-sensitive mutants among a population of mutagenized amoebae, and, separately, a nonselective screening method was used to isolate plasmodial temperature-sensitive mutants among clonal plasmodia derived from mutagenized amoebae. Complementation in heterokaryons was used to sort the mutants into nine functional groups. When transferred to the restrictive temperature, two mutants immediately lysed, whereas the remainder slowed or stopped growing. Of the two lytic mutants, one affected both amoebae and plasmodia, and the other affected plasmodia alone. The growth-defective mutants were examined for protein and deoxyribonucleic acid synthesis and for aberrations in mitotic behavior. One mutant may be defective in both protein and deoxyribonucleic acid synthesis, and another only in deoxyribonucleic acid synthesis. The latter shows a striking reduction in the frequency of postmitotic reconstruction nuclei at the restrictive temperature. We believe that this mutant, MA67, is affected in a step in the nuclear replication cycle occurring late in G2. Execution of this step is necessary for both mitosis and chromosome replication.

Pyrimidine metabolism in microplasmodia of Physarum polycephalum

If microplasmodia of Physarum polycephalum are exposed to 14C-labelled pyrimidine nucleosides or bases, an unusual pattern of metabolism is found. Only the nucleosides are taken up. Analysis of the distribution of the radioactivity in the cells revealed that ribonucleosides and deoxyribonucleosides are incorporated into nucleotides; however, a substantial catabolism takes place. Thus incubation with [2-14C]pyrimidine nucleosides readily gives rise to [14C]O2, particularly in the case of [2-14C]thymidine. Due to this a significant part of the trichloroacetic-acid-insoluble radioactivity from exogenously supplied [2-14C]thymidine is not associated with DNA. The pattern of labelling of nucleoside triphosphates from exogenously supplied nucleosides indicated that the de novo synthesis of nucleotides was only partly repressed. An unusual conversion of deoxycytidine into cytidine was noted. Enzyme analysis on cell-free extracts revealed that pyrimidine nucleosides can be salvaged by kinases and that their initial catabolism is initiated by hydrolases. Incubation of microplasmodia with pyrimidine analogues showed that only nucleoside analogues are toxic. The experimental results have led us to propose a scheme for the metabolism of pyrimidine nucleosides and bases in Physarum polycephalum.

Viability of Physarum polycephalum spores and ploidy of plasmodial nuclei

Amoebae of Physarum 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. We have asked whether the viable spores arise from meiotic or mitotic divisions. Using a microfluorometric measurement of the deoxyribonucleic acid content of individual nuclei, we found the fraction of viable spores to be correlated with the proportion of rare, diploid nuclei containing in the generally haploid plasmodium. When homozygous diploid plasmodia were created by heat shocking, spore viability increased dramatically. We suggest that viable spores are produced via meiosis in mth plasmodia, that the mth allele has no effect on sporulation per se, and that the normal source of viable haploid spores is a small fraction of diploid nuclei ubiquitous in haploid plasmodia.

A mutation (gad) linked to mt and affecting asexual plasmodium formation in Physarum polycephalum

Amoebae of the acellular slime mold Physarum polycephalum convert to plasmodia both asexually and sexually. Genetic analysis of a mutant that exhibits enhanced asexual plasmodium formation is reported. The mutant carries a single lesion (gad-11) located 12.3 map units from mt, a gene that controls mating specificity in sexual plasmodium formation. The mutation, which was isolated in an mt3 strain, is also expressed in mth and mt4 strains.

Temperature-sensitive mutants of the slime mould Physarum polycephalum. I. Mutants of the amoebal phase

A replica plating method for isolating it amoebal mutants of Physarum polycephalum has been devised. Temperature-sensitive mutations occur at a frequency after nitrosoguanidine mutagenesis of 10(-3) per survivor, are stable but are not usually expressed in the plasmodia formed from these amoebae in clones. Some of these mutants appear to be cell-cycle stage specific.

Temperature-sensitive mutants of the slime mould Physarum polycephalum. II. Mutants of the plasmodial phase

Methods are described for the isolation and testing of temperature-sensitive plasmodial strains of Physarum polycephalum. Nineteen temperature-sensitive strains were found by screening plasmodia derived from mutagenised amoebae and the properties of these are described. A scheme is outlined for the detection of specific mitotic cycle lesions amongst temperature-sensitive strains, and the properties of a presumptive mitotic cycle mutant are described.