Igor B. Raikov

Meiosis in protists: Recent advances and persisting problems

Summary This review summarizes the results obtained on meiosis in protists in the last 14 years. Special attention is drawn to the discovery of sexual phenomena in protozoan groups which were earlier considered asexual and to the rather widespread occurrence among the protists of heterophasic life cycles (with “intermediate” meiosis). Synaptonemal complexes are considered markers of early parallel chromosome pairing which is typical of two-divisional meiosis. Two-divisional meiosis (though sometimes achiasmatic) seems to be the general rule among protists, including the Apicomplexa and the dinoflagellates. One-divisional meiosis seems to be rather a rarity having independent and secondary origin (in some flagellates from the gut of Cryptocercus). These cases need reinvestigation with modern methods.

The nuclear apparatus of some primitive ciliates, the karyorelictids: Structure and divisional reorganization

Abstract This review considers the peculiarities of the generative nuclei (micronuclei) and the somatic nuclei (macronuclei) of the presumably primitive ciliates of the order Karyorelictida. Their nuclei display the tendency to form nuclear clusters. Whereas the karyorelictid micronuclei are of the type usual for other ciliates and divide by mitosis, the macronuclei never divide and belong to the vesicular type of nuclei, being rather poor in condensed chromatin, and near‐diploid as regards DNA content. The macronuclei also contain nucleoli and often nuclear bodies in the form of either spheres or crystalloids containing protein and sometimes RNA. The division of the ciliate is always accompanied by nuclear reorganization involving repeated mitoses of the micronuclei and differentiation of some excess ones into new macronuclei,the old ones being segregated without division. The development of the new macronuclei involves one round of DNA replication but no signs of further polytenization and/or polyploidi...

Somatic Cortical and Endoplasmic Fine Structure of Remanella granulosa Kahl (Ciliophora, Karyorelictida)

Summary Remanella granulosa Kahl (Family Loxodidae) is a flattened lancet-shaped ciliate with 10 kineties on its right (physiologically ventral) body side, and two kineties along the margins of the otherwise barren left body side. In the right side kineties, the kinetosomes are paired, both being ciliated. In each pair, the rear kinetosome gives rise to a postciliary ribbon of microtubules which are stacked to form prominent lamellated postciliodesmata. The rear kinetosome has also a medium-long striated kinetodesmal filament which contacts the postciliary ribbon of the preceding pair. The antherior kinetosome has a short ribbon of transverse microtubules. There are numerous mitochondria in the interkinetal ridges, but no obvious myonemes. In the two left-side kineties, the kinetosomes are also paired, the anterior being ciliated, the posterior barren. Some of the latter are connected with the inner bodies of the Mullers vesicles, likely to be gravity receptors. The endoplasm contains many skeletal spicules, especially near the right body side, and many food vacuoles, frequently with diatoms inside. There are also envacuolated bacteria likely to be symbiotic. Some endoplasmic mitochondria are conspicuously surrounded by membrane vesicles and lack definite cristae. Curious tiny bottle-shaped organelles, likely to be extrusomes, occur in groups between some kineties on the ciliated body side. On both sides of the body, the plasma membrane is underlain by microtubules and provided with envacuolated subpellicular pigment granules.

Fine Structure of Conjugation of the Ciliate Blepharisma japonicum II. Changes of Meiotic and Ameiotic Micronuclei and Development of Meiotic and Ameiotic Macronuclear Anlagen

Summary During conjugation of Blepharisma, only some micronuclei enter meiosis. Other micronuclei, called somatomicronuclei, do not degenerate but differentiate directly into macronuclear anlagen (secondary anlagen) without meiosis and karyogamy. Normal (primary) anlagen develop from synkaryon derivatives. We observed these processes at the ultrastructural level. In early conjugation (0–2 hours after pair formation), all micronuclei swell. This correlates with decondensation of the micronuclear chromatin. At 3 hours, the micronuclei differentiate into somatic and meiotic (leptotene) ones: the latter develop bundles of microtubules. The somatomicronuclei remain homogeneous and lack microtubules. At 8–9 hours meiotic micronuclei display synaptonemal complexes and thus are in pachytene; at the same time, structures in form of loose chromatin patches first appear in somatomicronuclei. The patches gradually condense and become conspicuous at 10–12 hours (stages from diplotene to metaphase I of meiosis). At about 12 hours, the meiotic micronuclei are in metaphase I and display acentric intranuclear spindles with blunt poles and homogenous polar caps; the bivalents have prominent kinetochores. At 16 hours, the somatomicronuclei contain numerous chromatin patches which are possibly subchromosomes, and first nucleoli appear in them. At 16–18 hours, the stage of pronuclei is reached; and other meiotic products start degenerating. The migratory pronuclei show concentration of the chromatin at the centre of the nucleus. At 20, 26, 28 and 34 hours, the fine structure of somatomicronuclei (secondary anlagen) changes little. Their size remains constant (about 5–6 μm). However the nucleoli enlarge at 34 hours. The first division of the synkaryon has protruding poles and no polar caps, unlike meiosis I; though, the nuclear envelope remains intact even at the poles. The synkaryon divisions give rise to new micronuclei and primary macronuclear anlagen. The primary (meiotic) macronuclear anlagen differentiate in number of 2–4 at 22–24 hours. They are much larger than secondary anlagen (up to 20 μm) and, at early stages of development, their chromatin is so strongly decondensed that the anlagen look “empty”. However later (by 34 hours) loose chromatin patches, small bodies of condensed chromatin and nucleolar primordia appear in them, like in somatomicronuclei, and the primary anlagen at 34 hours show additionally a karyosome-like central condensation of the chromatin.

The orthonematocyst, a new type of extrusome found in Remanella rugosa and Remanella brunnea (Ciliophora: Karyorelictida)

A new type of extrusomes has been found in Remanella rugosa Kahl and R. brunnea Kahl (Karyorelictida) at the non-ciliated (left) body side and the non-ciliated rims of the flattened body. The extrusomes are spindle-shaped membrane-bound organelles measuring about 2.5 urn in length and 0.75 μm in diameter. The extrusome consists of a lamellated (myelin-like) outer wall, an apical cone, a thin-walled capsule, and a straight tubular filament inside the intracapsular matrix. Extrusion of the filament has not been observed. The structure of the extrusomes seems to be intermediate between that of the nematocysts of Remanella multinucleata which have coiled filaments [24] and that of toxicysts. They are termed orthonematocysts.

Ultrastructure of the gamont shell and nucleus in the polythalamous foraminifer Elphidium ponticum

Summary The nucleus and shell structure of the growing and mature uninucleate gamonts of the polythalamous foraminifer Elphidium ponticum were studied using the material collected at the Karadag Biological Station (Black sea). The mature gamont has 12–14 chambers and a single voluminous nucleus measuring up to 70 μm in diameter. It usually lies in the eighth to tenth chamber. The outline of the nucleus of the growing gamont is smooth, it stains light in semithin sections; the nucleus of the adult gamont is slightly smaller and stains dark, its outlines being lobulated. The chromatin is fully decondensed inside the nucleus and invisible in ultrathin sections. Multiple nucleoli are mainly peripheral. They are more numerous, larger and denser in the grown gamont. The nucleoli are mainly dense fibrillar and contain fibrillar centres. They are linked to the nuclear envelope by fibrogranular aggregates, possibly chromatin. This connection is more clear in the growing gamont than in the adult one. A study of the texture of the shell by scanning electron microscopy allowed to observe some new features of its structure and to identify the variability of the shell structure of specimens from the same locality.

Fine structure of conjugation of the ciliate Blepharisma japonicum. I: Changes of the old macronucleus

Summary The macronucleus of vegetative cells of Blepharisma japonicum contains numerous discrete chromatin bodies and nucleoli of the compact type. The nucleoli contain very dense inclusions likely to be the dense fibrillar component of the nucleoli themselves. The macronuclear envelope displays tightly packed pore complexes. Until the 10th hour of conjugation, the old macronucleus shows the same fine structure as in vegetative cells. At 12 hours (approximately during metaphase I), the fibrillar and granular parts of the nucleoli segregate and the latter gradually disappear. At 14 hours, all nucleoli are reduced to their fibrillar parts that have the aspect of small dense bodies scattered in the nucleus. Simultaneously, the chromatin bodies begin to unite into a common network. This process continues at 16 hours (stage of pronuclei). The chromatin network is connected to the nuclear envelope via submembrane bodies of condensed chromatin tightly applied to the inner nuclear membrane. The macronuclear envelope has no coating of dense material in these areas. Thereafter (24 and 26 hours), areas of nuclear matrix free of the chromatin network appear in the macronucleus. At 28 and 34 hours, the chromatin network condenses and the residual nucleoli are caught in its meshes, being incorporated into the chromatin. The points of contact of the network with the nuclear envelope become less numerous.

Comparative Ultrastructure of the Cytoplasm in Species of the Genus Tracheloraphis (Ciliophora: Karyorelictida) I. Somatic Cortex

Summary: The somatic kineties, the myonemes, the dorsal non-ciliated stripe, the cortical granules and the rhabdocysts are described in four species of the genus Tracheloraphis: Tr. phoenicopterus, Tr. totevi, Tr. crassus, and Tr. caudatus. The somatic kineties carry kinetosome pairs of which both are usually ciliated. Less frequently, only the anterior kinetosome is ciliated. Each kinety is accompanied on its right by a strong lamellated postciliodesma and, on its left, by a prominent longitudinal microfilamentous myoneme. Tr. totevi has also thinner transverse myonemes which are more superficial than the longitudinal ones. The kinetosome pairs show a set of typical fibrillar derivatives: the posterior kinetosome has a strong postciliary fibre joining the postciliodesma and a short hooked kinetodesmal filament; the anterior kinetosome shows a transverse fibre of seven microtubules. Each ciliary row is underlain by a subkinetal fibre directed backwards and consisting of up to 10 sheets of microtubules. The basal end of a kinetosome is closed by a dense basal plate; the intermediate zone is short and consists of a cup-formed septum and a large axial granule. The cortex displays refractile subpellicular bodies which are pigmented in some species and contain dense granular material, and rhabdocysts consisting of a concentrically-structured “head”, a shaft and a basal vesicle. Incomplete extrusion of rhabdocysts has been observed. The basal vesicle swells and the shaft moderately elongates upon extrusion.

Comparative Ultrastructure of the Cytoplasm in Species of the Genus Tracheloraphis (Ciliophora, Karyorelictida). III. Buccal Apparatus

Summary The structure of the apical mouth has been studied in Tracheloraphis dogieli, Tr. phoenicopterus, Tr. totevi and Tr. caudatus . There is no permanent cytostome in any of them. The apical surface is covered by only the plasma membrane with many extrusomes of the type of subpellicular bodies (pigmentocysts) beneath it. In Tr. dogieli , the apical area is flat and surrounded with a circular “lip”. In Tr. phoenicopterus , the apical surface is dome-shaped, and the cytoplasm of the body “head” is filled with refringent oval electron-lucent granules up to 2 μm in size, clearly different from the extrusomes. The peribuccal ciliature of Tracheloraphis includes a specialized circumoral kinety independent of the meridional somatic kineties. In most species, this kinety consists of paired kinetosomes, one ciliated and one barren. However, in Tr. dogieli the circumoral ciliature consists of a series of short radial rows of kinetosomes, at least four kinetosomes in each row (2 ciliated and 2 barren). The innermost kinetosome, which is barren, sends a ribbon of transverse microtubules towards the apical area. This kinetosomal array is immediately outside the circular “lip”. The cilia of these kinetosomes are directed forward and form the apical ciliary “crown”. Nemadesmata start from the kinetosomes of the “crown”, both ciliated and barren, and are directed obliquely backwards. The somatic kineties, formed by dikinetids, begin well behind this “crown” and are not connected with it. They bear no nemadesmata.

Comparative Ultrastructure of the Cytoplasm in Species of the Genus Tracheloraphis (Ciliophora: Karyorelictida) II. Endoplasmic Organelles and Genesis of Rhabdocysts

Summary The endoplasm of five Tracheloraphis species, Tr. dogieli, Tr. phoenicopterus, Tr. totevi, Tr. crassus and Tr. caudatus , has been investigated. The endoplasm of all species is permeated with an extensive general lacunary system, the ground cytoplasm being reduced to a network of interconnected cytoplasmic islets which contain the cell organelles. The mitochondria have tubular cristae and variably dense matrix. They accumulate beneath the myonemes and under the non-ciliated stripe. Some mitochondria of Tr. dogieli show reduced cristae. The Golgi bodies are numerous near the ciliated body surface and consist of up to ten flattened cisternae. Some species (Tr. phoenicopterus, Tr. crassus) contain refractile electron-lucent inclusions lying inside the endoplasmic lacunae, others (Tr. dogieli) contain large protein globules, devoid of own membrane, lying in the ground cytoplasm. The lipid droplets also lie in the ground cytoplasm and are not surrounded with an own membrane. The rhabdocysts are formed within membrane vesicles inside endoplasmic islets which are rich in ribosomes. In the vesicles, the head and the shaft of the extrusomes are progressively formed. At last, rhabdocysts migrate to the periphery of the cell and become docked in the interkinetal papillae accompanying ventral kineties.