Kinetochores of grasshoppers with Robertsonian chromosome fusions

The pachytene karyotypes of three grasshopper species with 2 and 3 Robertsonian fusions were constructed from electron micrographs of serially sectioned spermatocyte nuclei. Tracings of the synaptonemal complexes permitted identification of each bivalent and its centromeric region. Chromosomes with the centromere in a terminal position have a knob of centric heterochromatin on the synaptonemal complex where it ends at the nuclear envelope. In Chorthippus and in Chloealtis the submetacentric Robertsonian fusion chromosomes each have a single centric knob in the appropriate place. In Neopodismopsis each of the 2 submetacentric chromosomes have a centric knob which is double in size and structure. In spermatogonial metaphases the submetacentric chromosomes of Neopodismopsis have 70–80 microtubules per kinetochore while the telocentric chromosomes have 30–40 tubules per kinetochore. These observations are correlated with evidence from light microscopy that Robertsonian fusions may produce mono- or dicentric chromosomes.     

The behaviour of chromosomal axes during diplotene in mouse spermatocytes

The fate of the synaptonemal complex and its elements after pachytene has been studied by serial sectioning of diplotene nuclei in mouse spermatocytes. The lateral elements of the synaptonemal complex separate from each other during diplotene, and they form single axes, 300 Å wide, surrounded by chromatin fibrils. The single axes are continuous and end on the nuclear membrane by two different ends: the basal knob and the simple end. The single axes do not cross-over each other, but they remain approached at the convergence regions. In these regions a modified piece of synaptonemal complex is found. This piece changes into a chromatin bridge during diplotene. It has been inferred that the convergence regions represent chiasmata and that the single axes do not represent axial structures of chromatids.     

Reconstruction of the Neurospora crassa pachytene karyotype from serial sections of synaptonemal complexes

Serial sections from isolated asci were used to reconstruct the seven pachytene bivalents of Neurospora crassa. The synaptonemal complex could be traced for its whole length in each bivalent, being attached to the nuclear envelope at both ends in six. The satellite end of the nucleolar chromosome did not appear to be attached to the nuclear envelope. The estimated lengths of the bivalents ranged from 10.7 to 5.1 microns in one nucleus, from 11.5 to 4.2 microns in another, and from 8.5 to 4.4 microns in a third, with total haploid complement lengths of 45.5 microns, 47.3 microns, and 43.9 microns respectively. These values are considerably smaller than published light microscopical measurements.—The synaptonemal complex in N. crassa, as in other ascomycetes, has two banded ca. 400 Å wide lateral components held about 1200 Å apart by a central region containing the ca. 200 Å wide central component. With normal glutaraldehyde/OsO₄-phosphate buffered fixation the chromatin of the pachytene bivalents is poorly contrasted. Occasional local thickenings of the central component into electron dense nodes ca. 1000 × 500 Å in longitudinal section are characteristic of the complex.     

The meiotic behaviour of the XY pair in Lutreolina crassicaudata (Marsupialia: Didelphoidea)

The meiotic behaviour of the XY pair of the didelphid Lutreolina crassicaudata is analyzed by microspreading of spermatocytes for visualization of chromosomal axes and by three-dimensional reconstruction of spermatocyte nuclei from EM thin sections. The delay in pairing of sex chromosomes compared to autosomes and the absence of a synaptonemal complex between the axes of the X and Y chromosomes, already described for South American marsupials by three-dimensional reconstruction and for Australian species with synaptonemal complex microspreadings, is confirmed for this species. Sections demonstrate that at the diffuse stage and diplotene the dense plate occupies the region of the inner face of the nuclear envelope in contact with the XY body. Spreads show an structure similar in staining to the axes that becomes apparent simultaneously with the dense plate, called a balloon. The mechanism of XY pairing during meiotic prophase appears to be common to American and Australian marsupials as the same morphological pattern is found in all the species described. This mechanism is different from the way of pairing and segregation known for eutherian sex chromosomes.     

Number and Nuclear Localisation of Nucleoli in Mammalian Spermatocytes

In seven mammalian species, including man, the position and number of nucleoli in pachytene spermatocyte nuclei were studied from electron microscope (EM) nuclear sections or bivalent microspreads. The number and position of the nucleolar organiser regions (NORs) in mitotic and meiotic chromosomes were also analysed, using silver staining techniques and in situ hybridisation protocols. The general organisation of pachytene spermatocyte nucleoli was almost the same, with only minor morphological differences between species. The terminal NORs of Thylamys elegans (Didelphoidea, Marsupialia), Dromiciops gliroides (Microbiotheridae, Marsupialia), Phyllotys osgoodi (Rodentia, Muridae) and man, always gave rise to peripheral nucleoli in the spermatocyte nucleus. In turn, the intercalated NORs from Octodon degus, Ctenomys opimus (Rodentia, Octodontidae) and Chinchilla lanigera (Rodentia, Cavidae), gave rise to central nucleoli. In species with a single nucleolar bivalent, just one nucleolus is formed, while in those with multiple nucleolar bivalents a variable number of nucleoli are formed by association of different nucleolar bivalents or NORs that occupy the same nuclear peripheral space (Phyllotis and man). It can be concluded that the position of each nucleolus within the spermatocyte nucleus is mainly dependent upon: (1) the position of the NOR in the nucleolar bivalent synaptonemal complex (SC), (2) the nuclear pathway of the nucleolar bivalent SC, being both telomeric ends attached to the nuclear envelope, and (3) the association between nucleolar bivalents by means of their NOR-nucleolar domains that occupy the same nuclear space. Thus, the distribution of nucleoli within the nuclear space of spermatocytes is non-random and it is consistent with the existence of a species-specific meiotic nuclear architecture.     

The meiotic prophase in Bombyx mori females analyzed by three-dimensional reconstructions of synaptonemal complexes

Serial sectioning followed by three dimensional reconstruction of lateral components of the synaptonemal complex have been used to follow chromosome pairing during the prophase of the achiasmatic meiotic division in the silkworm, Bombyx mori. During leptotene and early zygotene, the lateral components become attached to the nuclear envelope at a specific region, thus forming a chromosome bouquet. The attachment of lateral components to the nuclear envelope precedes the completion of the components between their attachment points. Synapsis and synaptonemal complex formation start during the period of lateral component organization in the individual nucleus. Telomeric movements on the nuclear envelope occur at two stages of the prophase: the chromosome pairing appears to be initiated by an association of unpaired ends of homologous chromosomes, the nature of this primary attraction and recognition being unknown. Secondly, the paired chromosomes become dispersed in the nucleus by shifting of attachment sites of completed synaptonemal complexes at the end of zygotene. This movement is possibly related to a membrane flow occurring during this stage. Membrane material is synthesized at the region of synaptonemal complex attachment. Later, the excess membrane material is shifted to the opposite pole where it protrudes into the lumen of the nuclei thus forming vacuoles. — Two previously undescribed features of chromosome pairing were revealed. In late zygotene, chromosome pairing and synaptonemal complex formation were frequently observed to be delayed or even prevented over a short distance by interlocking of two bivalents, both being attached to the nuclear envelope. Such interlocking of bivalents was not found in pachytene. Secondly, one nucleus was found in which two homologous chromosomes were totally unpaired while the remaining 27 bivalents were completed or in a progressed state of pairing. The lateral components of the two unpaired chromosomes had the same length and were located several microns apart, thus eliminating the possibility of a permanent association of homologous chromosomes before the onset of meiosis in Bombyx mori females. — During pachytene, one of the 8 cells belonging to the syncytial cell cluster characteristic of oogenesis continues the meiotic prophase whereas the remaining 7 cells, the nurse cells, enter a different developmental sequence, finally resulting in their degeneration. The synaptonemal complex of the oocyte develops into a sausage-like structure after pachytene by a deposition of dense material onto the lateral components, thus filling out most of the central region. The diameter of this modified synaptonemal complex reaches at least 300 nm, as compaired to a pachytene width of approximately 130 nm. Also, the length of synaptonemal complexes increases from 212 at zygotene/pachytene to at least 300 at the modified pachytene stage. In nurse cells, synaptonemal complexes are shed from the bivalents shortly after pachytene simultaneously with a condensation of the chromatin. These free synaptonemal complex fragments associate and form various aggregates, either more or less normal looking polycomplexes or various complex figures formed by reorganized synaptonemal complex subunits. Later stages have not been included in the present investigation.     

Evolution conservatively of SCAR DNA localization in genome isochores of warm-blooded vertebrates

In meiotic prophase I, chromatin fibrils attached to the lateral elements of the synaptonemal complexes form loops. Synaptonemal complex associated regions of DNA (SCARs DNA) are a family of genomic DNA sequences tightly associated with the synaptonemal complex; they are located at the chromatin loop basements. Isochore compositional fractions of the human and chicken genomes were used as 32P labeled probes for hybridization with SCAR DNA isolated previously from the spermatocyte nuclei of the golden hamster Mesocricetus auratus. Nucleotide sequences similar to the golden hamster’s SCAR DNA were found in human and chicken genome isochores. The localization of SCAR DNA in isochore compartments of the examined genomes was established to be evolutionary conservative.     

MORPHOGENESIS OF THE AXONEME IN THE SPERMATOCYTE OF COCCINELLA SEPTEMPUNCTATA LINNAEUS

Abstract  Using cell whole mount preparation, early morphogenesis and ultrastructure of the axoneme of Coccinella septempunctata L. spermatocyte were investigated by transmission electron microscope. During spermatogenesis two pairs of basal body-axoneme complexes originated from centrioles are found in the spermatocyte and they are separated completely from each other at interkinesis. The centriolar adjunct begins to generate while a basal body-axoneme complex is attached to the nuclear envelope of a spermatid nucleus, and it, on the proximal end of a growing axoneme, reaches a maximum before chromatin condensation. The growing axoneme is accompanied by the condensable nucleus elongation. The early axoneme of a basal body-axoneme complex consists nine doublets with only inner and outer dynein arms, no central microtubules.     

Meiosis in Bombyx mori females.

Crossing over is absent in oocytes of the silkworm, Bombyx mori. Synaptonemal complexes are present during pachytene between the paired chromosomes. At leptotene, lateral components of the synaptonemal complex are attached in a bouquet to a limited region of the nuclear envelope. Before completion of lateral components, synaptonemal complex formation begins at the nuclear envelope. With synaptonemal complex formation proceeding from both ends bivalents occasionally become interlocked. After pairing is completed, the bouquet arrangement is dissolved possibly as a result of a flow of the inner membrane of the nuclear envelope thereby separating the telomeres. After the telomeres are released from the nuclear envelope, material is deposited onto the lateral components of the synaptonemal complex. The modified synaptonemal complexes are retained by the bivalents until metaphase I. It is suggested that these modified synaptonemal complexes substitute for chiasmata in order to ensure regular disjunction of homologous chromosomes in the absence of crossing over.     

Intranuclear destruction of heterochromatin in two species of armored scale insects

Spermatogenesis is described in two species of armored scale insects,Parlatoria proteus andParlatoria ziziphus. In the males of both species, a haploid set of four chromosomes becomes heterochromatic during early embryogeny. The heterochromatic chromosomes are lost later by two different mechanisms during spermatogenesis. Just before meiosis begins one or more heterochromatic chromosomes disappear from each primary spermatocyte as a consequence of a rapid intranuclear chromosome destruction. Meiosis consists of a single achiasmatic division. At prophase four euchromatic and from one to three heterochromatic chromosomes are present in each cell. Although both the euchromatic and remaining heterochromatic chromosomes divide, the heterochromatic chromosomes are later eliminated by posttelophase ejection; the eliminated chromosomes then disintegrate slowly in the cytoplasm. Each of the two species displays a species specific level of heterochromatin retention and both differ in this regard from the previously describedParlatoria oleae. The evolution of a chromosome system involving intranuclear chromosome destruction is discussed.     

Precocious formation of synaptonemal-like polycomplexes and their subsequent fate in female Ascaris lumbricoides var. suum

During meiotic interphase, before leptotene, synaptonemal-like polycomplexes are seen in the cytoplasm of the Ascaris lumbricoides oocytes and in the communal anucleate rachis. In some females short intranuclear synaptonemal complexes are present briefly at that early stage. The number of extranuclear complexes increases just before leptotene, some are attached to the pores of the nuclear membrane. During zygotene most polycomplexes disappear. At late pachytene they reappear in some females but not in others. The morphology, when first seen, is that of disorganized filamentous bodies, later lateral elements appear among the filaments. The dimensions of the lateral elements of the polycomplexes are variable. In the male the distribution of polycomplexes among the rachis, the cell cytoplasm, and at the nuclear envelope is similar to that observed in the female.These observations confirm the precocious occurrence of synaptonemal-like polycomplexes reported by Bogdanov (1977). Ascaris lumbricoides thus, uniquely, appears to manufacture synaptonemal complex-like material in the communal cytoplasm of the germ cells prior to the time that the full complement of synaptonemal complexes appears in the nucleus.     

Die intraspezifische Variabilität des heterochromatischen Armes eines Chromosoms bei der GattungCarabus L. (Coleoptera)

The chromosome complement ofC. auronitens Fabr. is 2n _♂=26+XY. One autosomal pair—called A-chromosomes—is relatively long.A-chromosomes consist of a euchromatic and a heterochromatic arm. Labelling of mitotic chromosomes with³H-thymidine shows that replication of the heterochromatic arm continues when it has ended in the euchromatic arm. In males and females the length of the heterochromatic arm varies intraindividually. In 47 of 99 males the heterochromatic arms were heteromorphic. Calculations of the quotient length of the euchromatic/length of the heterochromatic arm have shown that at least 6 different types of the A-chromosome exist. These types differ from each other in the number of heterochromatic sections separated by constrictions. The longest heterochromatic arm observed consisted of 8 such sections. The genetic significance of the heterochromatin in the genus ofCarabus is at present unknown (Zusammenfassung see p.305).      

七星瓢虫精母细胞内轴丝的形态发生(英文)

应用细胞整装技术研究了七星瓢虫精子轴丝的早期形态发生和超微结构。在精子发生期间,起源于中心粒的两对基体—轴丝复合体出现在精母细胞内,在分裂间期它们彼此完全分离。当基体—轴丝复合体附着于精细胞核的核膜上,中心粒附体开始发生于生长轴丝的近心端,在染色质凝聚前中心粒附体最大。生长着的轴丝伴随着凝聚细胞核伸长。一个早期基体—轴丝复合体的轴丝是由具有内、外动力蛋白臂的9个双微管组成,缺少中央微管。     

Unequal crossing over and heterochromatin exchange in the X-Y bivalents of the deer mouse,Peromyscus beatae

Differences in length of the heterochromatic short arms of the X and Y chromosomes in individuals ofPeromyscus beatae are hypothesized to result from unequal crossing over. To test this hypothesis, we examined patterns of synapsis, chiasma formation, and segregation for maleP. beatae which were either heterozygous or homozygous for the amount of short-arm sex heterochromatin. Synaptonemal complex analysis demonstrated that mitotic differences in heterochromatic shortarm lengths between the X and Y chromosomes were reflected in early pachynema as corresponding differences in axial element lengths within the pairing region of the sex bivalent. These length differences were subsequently eliminated by synaptic adjustment such that by late pachynema, the synaptonemal complex configurations of the XY bivalent of heterozygotes were not differentiable from those of homozygotes. Crossing over between the heterochromatic short arms of the XY bivalent was documented by the routine appearance of a single chiasma in this region during diakinesis/metaphase I. Sex heterochromatin heterozygotes were characterized by the presence of asymmetrical chiasma between the X and Y short arms at diakinesis/metaphase I and sex chromosomes with unequal chromatid lengths at metaphase II. These data corroborate our hypothesis on the role of unequal crossing over in the production and propagation of X and Y heterochromatin variation and suggest that, in some cases, crossing over can occur during the process of synaptic adjustment.     

Light and electron microscope studies of chromosome pairing in relation to chiasma localisation in Stethophyma grossum (Orthoptera: Acrididae)

The relationship of chromosome pairing to chiasma localisation in the grasshopper Stethophyma grossum was investigated by a combined light and electron microscope study. Observations on the extent of synaptonemal complex formation in spermatocytes suggest that pairing is complete in all chromosome regions and that localised chiasmata do not therefore follow from localised pairing of homologues. This study also revealed an unprecedented variant of synaptonemal complex structure. Each bivalent was found to contain an asymmetrical region within which one lateral element was enormously enlarged while the other lateral element retained normal dimensions. This asymmetrical region is probably confined to one end of each bivalent and seems to extend constantly about 4 from the nuclear membrane attachment site. The possible significance of this variant of synaptonemal complex structure is briefly discussed.     

Localised chiasmata due to partial pairing: A 3D reconstruction of synaptonemal complexes in male Stethophyma grossum

The possible role of localised pairing as a mechanism producing localised chiasmata in Stethophyma grossum spermatocytes has been examined ultrastructurally. Nuclei at four successive stages of meiosis from leptotene to pachytene were reconstructed from a series of ultrathin sections and the extent of synapsis as demonstrated by synaptonemal complex (SC) formation was calculated. On the basis of the relative lengths of SCs and condensed chromosomes it was reasoned that only the centromeric ends of the long and medium length bivalents paired, and only one end of these SCs was found attached to the nuclear envelope. Only the three shortest bivalents paired completely, and both their SC ends were attached to the nuclear envelope. Thus pairing was directly related to the distribution of chiasmata. The extent of pairing at different stages suggests that the shortest bivalent paired very quickly, the longer ones progressively slower, and that pairing proceeded zip-like from a point at or very close to the end attached to the nuclear envelope, since all stretches of SC were attached to the envelope, and there were never more than 11 pieces, one for each bivalent. Chromosome decondensation and axial core formation did not occur far in advance of SC formation, and synapsis appeared to be much slower in S. grossum than in other species with non-localised chiasmata, as a larger proportion of the meiotic cysts were in zygotene, compared to Stauroderus scalaris and Locusta migratoria, although this was not quantified.     

A radiation analysis of a comstockiella chromosome system: destruction of heterochromatic chromosomes during spermatogenesis in Parlatoria oleae (Coccoidea: Diaspididae)

In the males of the olive scale insect, Parlatoria oleae (2n=8), the paternal set of chromosomes becomes heterochromatic during late cleavage or early blastula and remains so until spermatogenesis. Immediately before the onset of meiosis in the males one or more heterochromatic chromosomes disappear from each primary spermatocyte. At prophase four euchromatic and from one to three heterochromatic chromosomes are present in each cell. The disappearance of the heterochromatic chromosomes before meiosis could be due either to the dehetero-chromatization of the heterochromatic chromosomes and their subsequent pairing with their euchromatic homologues, or to the destruction of the heterochromatic chromosomes. — The alternative interpretations of spermatogenesis in P. oleae were tested by using chromosome aberrations, which had been induced in the heterochromatic set by paternal X-irradiation, as genetic markers in breeding tests of about 400 X₁ males. Meiosis was examined in X₁ males which showed conspicuous chromosomal rearrangements in their somatic cells. The absence of either heteromorphic chromosome pairs or multivalents at spermatogenesis and the failure of the X₁ males to transmit any form of chromosome aberration induced by paternal irradiation is strong evidence that the heterochromatic chromosomes are destroyed in P. oleae. — The evolutionary relationships of the chromosome systems in the coccids are considered. Models are outlined for the derivation of a Comstockiella system involving chromosome destruction either from a lecanoid sequence or from a hypothetical Comstockiella sequence involving chromosome pairing. Problems concerning the control of chromosome destruction are discussed.From a dissertation submitted in partial fulfillment of the requirements of Doctor of Philosophy in Genetics.This work was supported by grant GB 8196 from the National Science Foundation to Dr. Spencer W. Brown, and by a National Institutes of Health Fellowship 1 F02 CA 44173-01 to the author from the National Cancer Institute.Dedicated to Dr. Sally Hughes-Schrader on the occasion of her seventy-fifth birthday.     

Synaptonemal complex spreading in Allium

A surface-spreading technique for synaptonemal complexes was applied to triploid Allium sphaerocephalon L. (Liliaceae). In early pachytene two of the three axial elements of each set of three homologues are synapsed, the third is intimately aligned with and accompanies them throughout their whole length. The unsynapsed axis is attached to the synaptonemal complex of the other 2 at up to 50 association sites per trivalent. The distribution of these sites within the trivalents is not even; they are under-represented in the proximal regions. From nought to eight switches (pairing partner exchanges), where the accompanying axis joins in synapsis in exchange for one of the two other strands, occur per trivalent. Very often the telomeres of the aligned axes are attached to their synapsed counterparts by dense spherules, which makes this type of association different from the interstitial ones. Frequently the unsynapsed axes show a double structure along short distances. In late pachytene the intercalary associations are abolished, allowing the unsynapsed axes to engage in various types of non-homologous pairing. Since the association sites involve homologous chromosomes and are less abundant in the pericentric regions (which are usually the last to synapse), it is conceivable that similar structures are responsible for the pre-synaptic alignment of homologues and provide the initiation sites for synaptonemal complex formation in diploids.     

The synaptonemal complexes of Caenorhabditis elegans: pachytene karyotype analysis of male and hermaphrodite wild-type and him mutants

Only five synaptonemal complexes (SC), representing the 5 autosomes, are present in wild-type, him-4 and him-8, Caenorhabditis elegans males, whereas there are six SCs, accounting for 5 autosomal bivalents and the XX bivalent, in the C. elegans hermaphrodite. The univalent X chromosome of the male is present as a heterochromatic X-body in spermatocyte pachytene nuclei. The XX bivalent in wild-type, him-4 and him-8 hermaphrodites (SC1, 2.5 m in length) represented 6% of the total karyotype length and a SC of this size is missing from the respective male karyotypes. This corresponds with the fact that the total male karyotype length is only approximately 94% that of the hermaphrodite. Associated with the central element of the SC are structures termed SC knobs that were first described in the wild-type hermaphrodite. The six SC knobs present in the wild-type hermaphrodite oocyte pachytene nuclei and the two SC knobs in the male spermatocyte pachytene nuclei are apparently randomly placed with the exception that they are never found at the ends of the SC. This is also true in him-4 and him-8 in which case there are 3 and zero SC knobs in the hermaphrodites, respectively, and one SC knob each in the male pachytene nuclei. The decrease in number of SC knobs in hermaphrodite to male represents a true sex difference. The presence or absence of the SC knobs may influence the X chromosome nondisjunction process and this effect is not localized to the region of the SC on which the SC knob is located.