Monique Cachon

Observations on the mitosis and on the chromosome evolution during the lifecycle of Oodinium, a parasitic dinoflagellate

The life cycle of the dinoflagellate Oodinium alternates between an ectoparasitic trophic phase and a phase of multiplication as free-living flagellates. The nucleus of the young ectoparasite has rod-like chromosomes similar to those of free-living dinoflagellates. As growth of the trophont proceeds the nucleus becomes increasingly homogeneous. When Oodinium leaves its host, nuclear reorganization processes occur rapidly; they correspond to a peculiar prophase of the first sporogenetic division. The following division stages are similar. A conspicuous fusorial system appears between two archoplasmic areas which are responsible for daughter-chromosome segregation. The nuclear envelope remains intact while the fusorial microtubules are attached at distinct, kinetochore-like structures onto the nucleus. As the chromosomes become more condensed the kinetochore-like formations disappear.

Movement by non-actin filament mechanisms

Non-actin contractile MFS exist in Protozoa. Their chemical constitution is partially known. They are ATP-independent, but their various physiological characters can be ultrastructurally grouped into two types: either, as it is the case for actin, they are closely associated to the endoplasmic reticulum apparatus, or they form myonemal bundles which are not related to any other organelle.

Swimming behaviour of the unicellular biflagellate Oxyrrhis marina: in vivo and in vitro movement of the two flagella

Summary— The movement of the 2 flagella of Oxyrrhis marina was examined with respect to their individual waveforms and the swimming behaviour of the organism. The longitudinal flagella propagated helicoidal waves whose amplitude decreased toward the tip of the flagellum. Their beat frequencies were 50–60 Hz. The transverse flagella beat helicoidally within a furrow. Sudden changes in the direction of the cell trajectories were generated by transient arrests of the longitudinal flagellum beat, which were accompanied by a switch from the backward orientation to a forward one. This sweeping motion generated the rotation of the cell body. Ca2+ ions highly stimulated the frequencies of this arrest response, which compared to the “walking‐stick” behaviour of sea urchin spermatozoa.

Isolation of the major basic nuclear protein and its localization on chromosomes of the dinoflagellate, Oxyrrhis marina.

Basic nuclear proteins were extracted from isolated nuclei of Oxyrrhis marina. The HPLC pattern of the extract showed a single major peak, which consisted of a major band with an apparent molecular mass of 23 kDa on an SDS‐PAGE gel. We designated this protein as Np23 because of its apparent molecular mass. The amino acid composition of this protein revealed its extremely basic nature with a high lysine content. Polyclonal antibodies were raised against Np23. Immunofluorescence microscopy showed that Np23 was localized within the nucleus of dividing and non‐dividing cells as well, and immuno‐electron microscopy showed that the protein was localized only on chromosomes. These data established that Np23 is the major basic chromosome protein of Oxyrrhis marina.

Ultrastructure of the flagellar apparatus of Oxyrrhis marina

Summary— Oxyrrhis marina, like all dinoflagellates, possesses one transverse and one longitudinal flagellum, which show structural differences.

Analysis by polarizing microscopy of chromosomal structure among dinoflagellates and its phylogenetic involvement

Dinoflagellate chromosomes are highly interesting because of their condensed state, their lack of histone, and their ultrastructure reminiscent of that of bacterial nucleoid.

The nuclear division of Oxyrrhis marina: An example of the role played by the nuclear envelope in chromosome segregation

Summary Oxyrrhis marina is a marine Protozoa which has up-to-now no clear position among the Dinoflagellates though its mitosis was considered by Grasse (1952) as typical of the group. The electron microscopical observations show that this mitosis is singular. The nucleus is deeply hollowed by a furrow as it is usual in Dinoflagellates but no microtubules are observed in it in spite of all our attempts. The chromosomes are rod-shaped and made of longitudinal fibrils. They are attached by one end on the nuclear envelope and at these levels no kinetochore is seen. The separation of the daughter-chromosomes and their distribution in two sets seem to be realized by an elongation of the nuclear envelope. Cytophotometrical studies have been made and they are in agreement with our interpretation: there is a progressive and continuous DNA synthesis during the resting-stage and an equal distribution of DNA among the two telophasic daughter-nuclei.

Singular kinetochore structure in a parasitic dinoflagellate

Summary Normally kinetochores have three-layered structures: the inner part containing DNA, the outer part RNA, and between them a transfer area. This structure can be observed as well in higher organisms as in Protozoa. But in some Dinoflagellates such as Apodinium and most of the free-living ones (Amphidinium …) this structure is quite different. Our observations allow us to conclude that only the chromosomes which bear histones may have typically trilaminar kinetochores.

Analysis of the mechanism of dinoflagellate flagella contraction-relaxation cycle

Summary— Dinoflagellates possess two flagella. One of them, the longitudinal flagellum, retracts from time to time in some species, such as Ceratium and Peridinium. Additional structures which run along the axoneme seem to be responsible for this particular behaviour. The retraction which is rapid (less than 60 ms) may be subdivided into several steps: i) the undulating movement stops; ii) the flagellum appears then as a jagged line during 20 ms; iii) finally a rapid retraction (20 ms) takes place, the flagellum being folded 20 times inside the cylindrical flagellar pocket. The measurements on video‐records suggest that the R‐fibre shortens to 30% of its original length. The contraction and relaxation mechanism of nanofilaments is proposed to be through coiling and uncoiling dependent on Ca2+ concentration.

Nanofilament dependent motility in dinoflagellates

Summary— In dinoflagellates, as in many eukaryotes, several kinds of nanofilaments have been described: some are organized in bundles showing a strict periodicity, some are not; most are contractile upon a rise in Ca2+ concentration, some are not. In any case, their contractile properties appear to be ATP independent. They are present in all cell types studied so far, and are typically found in association with the centrioles‐basal bodies where they are suggested to play a role in MTOC structure, position and function. Nanofilaments are all about 2–4 nm in diameter, interact with microtubules, and are insoluble filaments, though to a variable extent. We propose that the nanofilaments may play a structural and/or active role complementary to intermediate filaments.