Action of the organophosphorous insecticide parathion on the free-living marine dinoflagellate Prorocentrum micans Ehrbg.

Growth of cultures of the dinoflagellate Prorocentrum micans Ehrbg. was slowed by parathion greater than 1 ppm. Parathion also decreased chlorophyll content and perturbed cellular ultrastructure, eliciting especially plastoglobuli in their chloroplasts. Toxicity of this organophosphorous insecticide is unlikely to be due to its anticholinesterase activity since P. micans appears not to contain cholinesterase. Fluorescence kinetics show that parathion affects the photosynthetic system, particularly photosystem II.

The native structure of dinoflagellate chromosomes and their stabilization by Ca2+ and Mg2+ cations.

Native chromosomes from the primitive dinoflagellate Prorocentrum micans were isolated without recourse to chemicals from purified nuclei frozen in liquid nitrogen. Thawing of the nuclear suspension resulted in the disruption of nuclear membrane, and the released chromosomes were incubated in various conditions or directly observed by electron microscopy after spreading. (1) Observations of whole mounted chromosomes confirmed that they were organized in a double-helical bundle of chromosomal fibers. (2) Investigations of these high-ordered chromosomal fibers at high magnification revealed helical compaction of nucleofilaments in a hierarchy of six organization levels (from the DNA to the double helical bundle forming the chromosome). (3) This peculiar architecture was shown to be stabilized mainly by divalent cations; the behavior of native chromosomes incubated in a low concentration of either EDTA or EGTA strongly suggested that the chromosome has two different cation sites, one for Ca2+ and the other for Mg2+.

A high level of thymine replacement by 5-hydroxymethyluracil in nuclear DNA of the primitive dinoflagellate Prorocentrum micans E.

The nuclei of dinoflagellate protists display several distinctive features which make it difficult to assign these organisms as either eukaryotes or prokaryotes. We investigated some physical properties of purified nuclear DNA from the primitive species Prorocentrum micans. Nuclear DNA was separated on a CsCl gradient, into two components, which banded with relative densities of 1.7240 g/cm3 for the main peak and 1.7301 g/cm3 for the heavy shoulder. Thermal denaturation of nuclear DNA displayed a broad profile with a Tm of 71 degrees C. A large discrepancy was thus revealed between the apparent (G + C) content as determined from density (65.4%) and that from Tm (41.7%) while the actual (G + C) content determined by 32P nucleotide chromatography was shown to be 57.1%. The abnormal behaviour of this DNA was due to the presence of an unusual nucleotide which was identified as 5-hydroxymethyluridylate (HOMedUMP) from its chromatographic and U.V. spectral characteristics. It amounted to 13.4% of the total nucleotides and replaced an average of 62.8% of the expected thymidylate (dTMP). Composition analysis of different fractions of the CsCl gradient revealed that the unusual pyrimidine, 5-hydroxymethyluracil, was not uniformly interspersed with thymine in the DNA; the substitution rate increased with the relative density of the DNA. A minor component was also found, tentatively identified as 5-methylcytidylate (MedCMP) from its chromatographic properties, which amounted to less than 0.5 mol percent.

Distinctive features of dinoflagellate chromatin. Absence of nucleosomes in a primitive species Prorocentrum micans E.

The absence of nucleosome-like structures from purified nuclei of the primitive dinoflagellate Prorocentrum micans was demonstrated by three means. i) Electron microscopy revealed mostly thin, smooth 6-nm nucleofilaments in chromatin incubated at various ionic strengths and either fixed in 0.1% glutaraldehyde or unfixed. No "beads-on-a-string" structure was found. ii) Analysis of nuclear proteins showed that low amounts of basic proteins were present (basis proteins: DNA less than 0.1), the two major one with molecular weights 12 000 and 13 000 and that histones characteristic of eucaryotes were absent. iii) Digestion of the nuclei with micrococcal endonuclease of DNase I did not result in partially digested DNA fragment repeats. Only about 10% of the bulk of the nuclear DNA was digested by micrococcal endonuclease. The high molecular weight of the remainder suggests particular protection against this type of nuclease. In the light of these distinctive nuclear features, we discuss the evolutionary position of the dinoflagellate protists with respect to the procaryotes and eucaryotes.

Presence of intranuclear micro cables in a primitive dinoflagellate protist: morphological description and discussion of their possible evolutionary significance.

An intranuclear "cable" system has been demonstrated in the nucleus of the so-called primitive dinoflagellate Prorocentrum micans E. by means of a new cryofixation technique. Numerous cables are situated between the chromosomes. They are composed of several helically twisted filaments of 50-60 A diameter. The diameter of the cables varies from 200 A. In most dinoflagellates the microtubular spindle is permanently extranuclear, whereas this filament system is always located in the nucleus and is permanently surrounded by the nuclear envelope. The possible function of these cables is discussed. In evolutionary terms, such a complex fibrillar system must be taken into account as much as the microtubular system, particularly in these cells which are still considered as lower eukaryotes, although the special position of the dinoflagellates becomes more and more separate.

[Action of cadmium on a free dinoflagellate Prorocentrum micans E: growth, cadmium absorption and cell modifications]. / Action du cadmium sur un dinoflagellé libre: Prorocentrum micans E.: croissance, absorption du cadmium et modifications cellulaires.

The study of the growth of Prorocentrum micans E. submitted to sublethal doses of cadmium demonstrated a high sensitivity of this Dinoflagellate to the polluant starting with concentration of 5 microgram/l. The uptake of cadmium was measured by pulse polarographic analysis of the filtered culture medium. Structural modifications, not apparent in living cells, are visible after fixation. The high sensitivity of Prorocentrum micans to cadmium, also its easy culture, makes it a suitable pollution indicator of this heavy metal.

Are chromosomal axial struc-ures and microtubular systems phylogenetic markers in the dinoflagellates?

Two new events concerning nuclear division and chromosomal predivision are described in the free-living marine dinoflagellate Prorocentrum micans E. considered by Loeblich (1976) as a primitive form. These events will be discussed in relation to the evolution of Dinoflagellates. (1) Presence of a paranuclear bundle of microtubules parallel to the extranuclear bundle passing through the dividing nucleus (Soyer, 1975, 1977b) and (2) Presence of a chromosomal axis in the predividing chromosomes (Soyer, 1977c).