Jean 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.

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.

Various effects induced by chemical microtubule-inhibitors and neurodrugs on the microtubular system of the Heliozoan Sticholonche zanclea

Summary Various anti-mitotic drugs, microtubule-inhibitors (Colchicine, Vinblastine, Griseofulvine, Podophyllotoxine, I.P.C., Maytansine) and neurodrugs (Chlorpromazine, Halothane, Enflurane) have been applied to the microtubular axoneme of the axopods of Sticholonche zanclea, to see what kind of structures they could induce on the depolymerized tubulins. Three types of structures have been observed: 1 — the classical Vinblastine paracrystals; 2 — helical structures made of the association of macrotubules, similar to those observed under the action of lowering temperature; 3 — and finally, chevron-shaped formations made of linear segments, forming between them angles of about 120°, previously induced by mechanical shocks and ultra-sounds.

Adaptation des dinoflagellés à la vie planctonique

Abstract Adaptive evolution of Dinoflagellates Dinoflagellates are located midway between the plant and the animal realms. They have evolved into very varied forms and have adapted to planktonic life, to ecto‐ or endo‐parasitism, to symbiosis, to pluricellularity. The adaptations to planktonic life are various, particularly among the Gymnodinides, and particularly among Noctiluci‐dae (motility of the whole body, modifications of the cell‐shape during life‐cycle, adaptations to prey capture in sea medium etc.).

The Microtubule-Organizing Centers of Sticholonche zanclea Axopods. Microtubule-Nucleating Template or Linker Nucleation Hypothesis?

Summary The microtubules (MTS) of the axopods in the Heliozoan Sticholonche grow from a hip-joint shaped organelle. Only the upper part of this organelle functions as a MTOC. It is a multilayered dense-substance disc in the center of which is a well with 6 priviledged. MTS which represent the axis of the axoneme. MTS regrowth has been studied by experimental conditions: the 6 axial MTS could represent nucleating-initiators (BRDELE’S linker nucleation hypothesis) while the discoidal plate could have pre-determined sites (TUCKER’S MT-nucleating template) for the other MTS of the axoneme.