Localized Kinetochore Research Articles

Early Diverging Fungus Mucor circinelloides Lacks Centromeric Histone CENP-A and Displays a Mosaic of Point and Regional Centromeres.

Centromeres are rapidly evolving across eukaryotes, despite performing a conserved function to ensure high-fidelity chromosome segregation. CENP-A chromatin is a hallmark of a functional centromere in most organisms. Due to its critical role in kinetochore architecture, the loss of CENP-A is tolerated in only a few organisms, many of which possess holocentric chromosomes. Here, we characterize the consequence of the loss of CENP-A in the fungal kingdom. Mucor circinelloides, an opportunistic human pathogen, lacks CENP-A along with the evolutionarily conserved CENP-C but assembles a monocentric chromosome with a localized kinetochore complex throughout the cell cycle. Mis12 and Dsn1, two conserved kinetochore proteins, were found to co-localize to a short region, one in each of nine large scaffolds, composed of an ∼200-bp AT-rich sequence followed by a centromere-specific conserved motif that echoes the structure of budding yeast point centromeres. Resembling fungal regional centromeres, these core centromere regions are embedded in large genomic expanses devoid of genes yet marked by Grem-LINE1s, a novel retrotransposable element silenced by the Dicer-dependent RNAi pathway. Our results suggest that these hybrid features of point and regional centromeres arose from the absence of CENP-A, thus defining novel mosaic centromeres in this early-diverging fungus.

Early Diverging Fungus <i>Mucor circinelloides</i> Lacks Centromeric Histone CENP-A and Displays a Mosaic of Point and Regional Centromeres

Centromeres are rapidly evolving across eukaryotes, despite performing a conserved function to ensure high fidelity chromosome segregation. CENP-A chromatin is a hallmark of a functional centromere in most organisms. Due to its critical role in kinetochore architecture, the loss of CENP-A is tolerated in only a few organisms, many of which possess holocentric chromosomes. Here, we characterize the consequence of the loss of CENP-A in the fungal kingdom. Mucor circinelloides, an opportunistic human pathogen, lacks CENP-A along with the evolutionarily conserved CENP-C, but assembles a monocentric chromosome with a localized kinetochore complex throughout the cell cycle. Mis12 and Dsn1, two conserved kinetochore proteins were found to bind nine short overlapping regions, each comprising an ~200-bp AT-rich sequence followed by a centromere-specific conserved motif that echoes the structure of budding yeast point centromeres. Resembling fungal regional centromeres, these core centromere regions are embedded in large genomic expanses devoid of genes yet marked by Grem-LINE1s, a novel retrotransposable element silenced by the Dicer-dependent RNAi pathway. Our results suggest that these hybrid features of point and regional centromeres arose from the absence of CENP-A, thus defining novel mosaic centromeres in this early-diverging fungus.

Holocentric chromosome behaviour inCuscuta (Cuscutaceae)

The present study demonstrates thatCuscuta babylonica Choisy has holocentric chromosomes. Evidence for this phenomenon comes from three different observations. (1) Mitosis: During metaphase and anaphase the sister-chromatids are situated parallel to the equatorial plane with no sign of localized kinetochore activity. (2) Inverted meiosis in microsporocytes. (3) X-rayed microsporocytes, in which the numerous chromosome fragments do not show any lagging or formation of micronuclei. We assume that only one out of the three subgenera inCuscuta, namely subg.Cuscuta, has holocentric chromosomes, while the two other subgenera have monocentric chromosomes.

Mitosis in eukaryotic cells: an overview of chromosome distribution.

The usual outcome of mitosis is 2 daughter cells with equal numbers and kinds of chromosomes (euploidy) rather than unequal numbers (aneuploidy). The more we learn about the underlying mechanisms the more remarkable this seems: the precise distribution of chromosomes depends critically upon structural disorder and instability, which would more commonly be associated with imprecision, that is, with aneuploidy. My aim is to review what is known about chromosome distribution in eukaryotes and to relate disorder and instability both to the normal outcome and to the origin of aneuploidy. The discussion is limited to orthodox mitosis and meiosis in organisms with localized kinetochores (on unorthodox mechanisms, see, e.g., Ref. 19; for broader reviews of mitosis, see Ref. 16 and 24).

Holokinetic composite chromosomes with ?diffuse? kinetochores in the micronuclear mitosis of a heterotrichous ciliate

During micronuclear mitosis of the heterotrichous ciliate Nyctotherus ovalis Leidy rod-shaped composite chromosomes are formed by lateral association of telokinetic chromosomes. The formation of these composite chromosomes seems to be a highly ordered process since only nuclei with either 18 or 24 such chromosomes can be observed, and nuclei with the same chromosome number show a similar length distribution of their chromosomes. Further, these data indicate that we examined two otherwise indistinguishable races. During metaphase the composite chromosomes become arranged in the spindle equator in a holokinetic fashion, their entire poleward surfaces being covered by kinetochore material. These “diffuse” kinetochores have a trilaminar appearance comparable to those of monokinetic chromosomes. Their electron density after employing Bernhard's procedure revealed the same ribonucleoprotein distribution as reported for the localized kinetochores formed during the extranuclear mitosis in other cells. During early anaphase the outer kinetochore layer remains continuous while the individual chromosomes in the composite group show a tendency to separate leaving chromatin-free spaces of about 40 nm diameter. Kinetochore microtubules which are still anchored in the outer kinetochore layer seem to elongate and to extend into the interpolar spindle region predominantly through these “holes” in the chromatin. These observations suggest a like polarity of kinetochore and interpolar microtubules in the polar spindle region while microtubules in the interpolar space seem to interdigitate in an antiparallel fashion. The activity of the kinetochore to act as a microtubule-organizing center (MTOC) seems to be modulated by the chromatin underlying the outer kinetochore layer which may prevent further outgrowth of kinetochore microtubules.

Chromosome studies in Chionographis (Liliaceae). III. The mode of meiosis.

The mode of meiosis in PMCs was studied in two members of Chionographis. C. japonica was found to have n=12 chromosomes, and C. japonica var. hisauchiana n=21 chromosomes. No localized kinetochore was observed in the chromosomes at various stages of meiosis. The bivalents at mid diakinesis showed various characteristic configurations, due to the presence of chiasma or chiasmata. Each bivalent at prometaphase I and metaphase I consisted of 2 homologous chromosomes connected end to end. Somewhat rectangular bivalents (in side view) were oriented axially with regard to the spindle at metaphase I. This mode of orientation of bivalents (axial orientation) differs from the one reported in some species of Luzula and some other organisms with holocentric chromosomes. Each pollen tetrad cell developed into a single mature pollen grain.

Chromosome Studies in <i>Chionographis</i> (Liliaceae)

Four Japanese members of Chionographis were investigated karyologically. Both C. japonica and C. koidzumiana had 2n=24 chromosomes, while C. japonica var. kurohimensis and C. japonica var. hisauchiana had 2n=44 and 2n=42 chromosomes, respectively. In the interphase nuclei, many small heteropycnotic segments, the number of which surpassed frequently the somatic chromosome number of each species, were observed. The chromosomes at early prophase had also many small precociously condensed segments, showing the beaded configuration. The chromo-somes at metaphase lay flat on the equatorial plane, and had no particular structure suggesting the presence of a localized kinetochore. The sister chromatids at ana-phase underwent a parallel disjunction. These observations of the chromosomes at metaphase and anaphase have suggested that the somatic chromosomes of all the species studied are holokinetic (holocentric). Relationships between the species were also discussed briefly from a karyological standpoint.

Occurrence of monocentric chromosomes inPieris brassicae L. (Lepidoptera, Pieridae)

Monocentric chromosomes with localized kinetochores were obtained inPieris brassicae (Lepidoptera, Pieridae) by using colchicine treatment and air-drying method.

Spindle and kinetochores in mitosis and meiosis in Dysdercus intermedius (Heteroptera)

In spermatogonial mitoses of Dysdercus intermedius, chromosomal spindle microtubules insert in discrete kinetochores. The latter consist of two electron dense plates of about 0,3 μm diameter located at opposite sites at the poleward surfaces of the daughter chromatids. The evidence presented permits the interpretation that a localized kinetochore owes its layered substructure to successive binding sites for kinetochore material at the ends of bundled microtubules. A working hypothesis, based upon different distributions of genetic loci for kinetochore material in species with localized and diffuse kinetochores is proposed. In Dysdercus, holokinetic chromosome behavior in mitotic anaphase movement is not due to truly diffuse kinetochores but to chromosomal interconnections which are established at prometaphase. Such interconnections are absent in the first meiotic division when each bivalent is enclosed by a sheath of endoplasmic reticulum. Spindle microtubules penetrate through terminal openings in this sheath and end singly in dense spots of kinetochore material.

Centromeric organization in the chloromonadophycean alga Vacuolaria virescens

Observations with light and electron microscopes indicate the presence of a localized kinetochore in the phytoflagellate Vacuolaria virescens Cienk. (class Chloromonadophyceae). Thus, metaphase chromosomes have a clearly defined constriction which stains less densely than the remainder of the chromosome; at late metaphase the chromatids separate at definite points; acentric chromosome fragments may be formed. Ultrastructurally the kinetochore is recognizeable as a specialized area on the poleward surface of the chromosome into spindle microtubules are inserted.

The Ultrastructure of Luzula Kinetochores During Mitosis

In most eukaryotic organisms the attachment of each chromosome to the division spindle occurs at a specialized region, the kinetochore (centromere), which generally appears in light microscopy as a constriction. Ultrastructural descriptions of localized kinetochores may be classified into the following categories: (a) the kinetochore region appears morphologically undifferentiated from the rest of the chromatin; (b) the kinetochore appears more electron-dense than the rest of the chromatin; (c) the kinetochore appears less electron-dense than the rest of the chromatin; and (d) the kinetochore consists of several layers of varying complexity and electron-density (for reviews see 1, 3, and 6).

Evidence of a Diffuse Centromere in the European Corn Borer, Ostrinia nubilalis (Lepidoptera: Pyralidae)

In his studies of certain Lepidoptera, Federley (1943, 1945) suggested that the Lepidoptera do not have a restricted, localized kinetochore (centromere). In other words, they appeared to have a diffuse centromere of some type. On the basis of his studies of several of the Cidaria species Suomalainen (1953) concluded that the Lepidoptera have a diffuse centromere. Virkki (1963), in his work on the sugarcane borer moth, Diatraea saccharalis (F.), also inferred that the Lepidoptera have a diffuse centromere. However, only a relatively few species in the Order Lepidoptera have been investigated with respect to the type of centromere.

ON THE KARYOLOGY OF THE RADIOLARIAN AULACANTHA SCOLYMANTHA AND THE FINE STRUCTURE OF ITS CHROMOSOMES

1. Cachon-Enjumet's (1961) attempt to interpret the nuclear division of the radiolarian, Aulacantha scolymantha, as a typical, though superficially modified mitosis is subject to criticism. There is no evidence for differences between the numerous chromosomes of this species. The identification of nucleolus organizing chromosomes is inconclusive since supposed nucleoli contain DNA and are obviously due to the disintegration of chromosomes. No evidence for the existence of true nucleoli was found. Instead, all chromosomes contain unusually great amounts of RNA. Evidence for chromosomal differences on the basis of arm length and kinetochore position is equally inconclusive. No localized points of spindle fiber attachment could be identified in the electron microscope. Attention was drawn to a number of artefacts in this connection. It is concluded that Cachon-Enjumet did not succeed in bringing forth any new and conclusive arguments against Grell's (1953) hypothesis that all chromosomes of Aulacantha are homologous and genetically equivalent. 2. The basic ultrastructural component of the chromosomes of this radiolarian is a fibril about 80 A in thickness. 3. A new ultrastructural component, consisting of an axial complex demonstrable after treatment of the sections with both uranyl and lead ions, is described. So far, it is unknown if this represents a special feature of Aulacantha chromosomes which may have some bearing on cytogenetic peculiarities of this species. 4. Within a specialized „spindle plasma“ excluding formed elements like Golgi vesicles and mitochondria, tubular spindle fibers have been identified. Their attachment to the chromosomes is not confined to localized kinetochores. It seems to conform to a type of „diffuse“ spindle fiber attachment as described from various insect orders by technique of light microscopy.

Comportamento dos cromossômios na meiose de Euryophthalmus rufipennis Laporte (Hemiptera Pyrrhocoridae)

Comportamento dos cromossômios na meiose de Euryophthalmus rufipennis Laporte (Hemiptera Pyrrhocoridae)