[Electron microscopic studies on the form change of kinetochores during spermatocyte divisions in Pales ferruginea (Nematocera)]

Kinetochore rearrangement in meiosis II requires attachment to the spindle

The distinctive behaviors of chromosomes in mitosis and meiosis depend upon differences in kinetochore position. Kinetochore position is well established except for a critical transition between meiosis I and meiosis II. We examined kinetochore position during the transition and compared it with the position of kinetochores in mitosis. Immunofluorescence staining using the 3F3/2 antibody showed that in mitosis in grasshopper cells, as in other organisms, kinetochores are positioned on opposite sides of the two sister chromatids. In meiosis I, sister kinetochores are positioned side by side. At nuclear envelope breakdown in meiosis II, sister kinetochores are still side by side, but are separated by the time all chromosomes have fully attached in metaphase II. Micromanipulation experiments reveal that this switch from side-by-side to separated sister kinetochores requires attachment to the spindle. Moreover, it is irreversible, as chromosomes detached from a metaphase II spindle retain separate kinetochores. How this critical separation of sister kinetochores occurs in meiosis is uncertain, but clearly it is not built into the chromosome before nuclear envelope breakdown, as it is in mitosis.

Fission yeast Bub1 is essential in setting up the meiotic pattern of chromosome segregation

In meiosis, sister-chromatids move to the same spindle pole during the first division (MI) and to opposite poles during the second division (MII). This requires that MI sister kinetochores are co-orientated and form an apparent single functional unit that only interacts with microtubules from one pole, and that sister-chromatids remain associated through their centromeres until anaphase II. Here we investigate the function of Bub1 and Mad2, which are components of the mitotic-spindle checkpoint, on chromosome segregation during meiosis. Both proteins are required to prevent the occurrence of non-disjunction events in MI, which is consistent with recent findings that components of the mitotic-spindle checkpoint also operate during meiosis. However, Bub1 has several functions that are not shared with Mad2. When the bub1 gene is deleted, sister chromatids often move to opposite spindle poles during MI, indicating that sister kinetochores are disunited. Furthermore, the cohesin Rec8 is never retained at centromeres at anaphase I and sister-chromatid cohesion is lost. Our results show that Bub1, besides its functions in monitoring chromosome attachment, is essential for two other significant aspects of MI - unification of sister kinetochores and retention of centromeric cohesion.

The reduction of chromosome number in meiosis is determined by properties built into the chromosomes

In meiosis I, two chromatids move to each spindle pole. Then, in meiosis II, the two are distributed, one to each future gamete. This requires that meiosis I chromosomes attach to the spindle differently than meiosis II chromosomes and that they regulate chromosome cohesion differently. We investigated whether the information that dictates the division type of the chromosome comes from the whole cell, the spindle, or the chromosome itself. Also, we determined when chromosomes can switch from meiosis I behavior to meiosis II behavior. We used a micromanipulation needle to fuse grasshopper spermatocytes in meiosis I to spermatocytes in meiosis II, and to move chromosomes from one spindle to the other. Chromosomes placed on spindles of a different meiotic division always behaved as they would have on their native spindle; e.g., a meiosis I chromosome attached to a meiosis II spindle in its normal fashion and sister chromatids moved together to the same spindle pole. We also showed that meiosis I chromosomes become competent meiosis II chromosomes in anaphase of meiosis I, but not before. The patterns for attachment to the spindle and regulation of cohesion are built into the chromosome itself. These results suggest that regulation of chromosome cohesion may be linked to differences in the arrangement of kinetochores in the two meiotic divisions.

Chromosome movement during meiotic prophase in crane-fly spermatocytes: IV. Actin and the effects of cytochalasin D

Cytochalasin D (CD) was applied to crane-fly spermatocytes at late diakinesis with the aim of perturbing actin structure and actin function, thereby testing the hypothesis that intranuclear chromosome movement during late diakinesis is actin-based. Isolated tests were incubated in a range of CD concentrations (2-100 microM) for 1 or 2 h. None of those treatments resulted in cessation of prophase movements in living cells. An immediate effect of 10-100 microM CD at late diakinesis was the formation of highly refractile, actin-containing cables within the nonchromosomal nucleoplasm. No such cables were observed in vehicle-treated control cells. CD treatments caused autosomal bivalents in unusually large numbers of spermatocytes to become aggregated into densely-packed clusters; for example, with 40 microM CD about 80% of late diakinesis spermatocytes had clustered autosomes, vs. about 25% clustering in untreated cells. We conclude from these data that the mechanism of chromosome positioning at the nuclear envelope is CD-sensitive. Rhodamine-conjugates of phalloidin and DNase I were used to assess the status of actin in untreated cells as well as the effect of CD on actin distribution. Differences in nucleoplasmic staining with phalloidin and DNase I conjugates suggest that nucleoplasm at late diakinesis contains actin in a nonfilamentous form.

Visualization of kinetochores and analysis of their refractility in crane-fly spermatocytes after aldehyde fixation

Glutaraldehyde and formaldehyde were used to fix crane-fly spermatocytes for observation with differential interference contrast (DIC) microscopy. In aldehyde-fixed cells, kinetochores exhibit contrast not normally observed in living cells. Although the mechanism underlying this result is not understood, the visualization of kinetochores as distinct refractile objects opens the way for analysis of unstained kinetochores with the light microscope. The analysis of kinetochore refractility reported in this paper is made possible by the finding that the refractility of chromosomes in formaldehyde-fixed cells decreases as the concentration of formaldehyde is increased. In 4% formaldehyde, the refractility of chromosomes is matched with that of its surround, chromosomes appear invisible, and kinetochores may be analyzed as if chromosomes were not present. Kinetochores were imaged with DIC optics, and then digital image analysis was performed. Gray-level scans through the highlight and shadow of an individual kinetochore parallel to the axis of shear resulted in a curve having a slope proportional to the DIC optical path gradient. Curves from autosomal kinetochores imaged in anaphase had slopes approximately one-half those recorded at metaphase under identical optical conditions. By contrast, kinetochore thicknesses (defined as the distance between the peak and the valley of a gray-level scan) at those two stages were not significantly different. These data suggest a loss of dry mass from autosomal kinetochores during anaphase. Neither the refractility nor thickness of lagging sex kinetochores varied as autosomes went through anaphase. The conclusion drawn from these findings is that the decreased refractility of autosomal kinetochores in anaphase is movement-related.

Specific regulation of CENP-E and kinetochores during meiosis I/meiosis II transition in pig oocytes

To understand the mechanisms which regulate meiosis-specific cell cycle and chromosome distribution in mammalian oocytes, the level and the localization of CENP-E and the kinetochore number and direction on a half bivalent were examined during pig oocyte maturation. CENP-E is a kinetochore motor protein whose intracellular level and localization are strictly regulated in the somatic cell cycle. The localizations of CENP-E on meiotic chromosomes from diakinesis stage to anaphase I and at the spindle midzone at telophase I were shown by immunofluorescent confocal microscopy to be similar to those in somatic cells of pig and other species. Further, ultrastructural analysis revealed the presence of CENP-E on fibrous corona and outer plate of kinetochores of the meiotic chromosomes. However, unlike mitosis, CENP-E staining was continuously detected either at the spindle midzone or on the kinetochores of segregated chromosomes during the first polar body emission. Consistent with this, immunoblot analysis revealed that CENP-E level remained high during meiosis I/meiosis II (MI/MII) transition and that some of CENP-E survived through the transition even in cycloheximide-treated oocytes in which cyclin B1 was completely degraded. Furthermore, examinations of CENP-E signals in confocal microscopy and kinetochores in electron microscopy in MI and MII oocytes provide the cytological evidence in mammalian oocytes which suggests that each sister chromatid in a pair has its own kinetochore which localizes side-by-side so that two sister chromatids on a half bivalent are oriented toward and connected to the same pole in MI.

Orientation and segregation of a micromanipulated multivalent: familiar principles, divergent outcomes

We studied the orientation and segregation of a particular quadrivalent in living grasshopper spermatocytes. Quadrivalents were detached from the spindle by micromanipulation, then placed and bent as desired. The detached quadrivalents reattach and orient on the spindle. Their orientation is determined by the same principles that apply to ordinary chromosomes in mitosis and meiosis, but the outcome is different. Certain characteristics of the quadrivalent lead to a variety of orientations rather than the single one typical of ordinary chromosomes. Two kinetochores in the quadrivalent are linked to the others by unusually long, flexible chromosome arms. These kinetochores may face either the same pole or opposite poles and tend to orient initially to the pole toward which they face. Consequently, the initial orientation of the flexibly linked kinetochores is variable, and, moreover, they frequently reorient. In contrast, the other two kinetochores are as rigidly connected as those in a small bivalent and so display the typical back-to-back arrangement. Usually, this arrangement leads quickly to a stable orientation of the two kinetochores to opposite poles. Sometimes, however, the back-to-back arrangement changes to a side-by-side arrangement so that the orientation of both kinetochores to the same pole is favored. The combined effect of this diverse behavior is that the quadrivalent has four stable orientations, each leading to a different distribution of chromosomes in anaphase. The result is genetic chaos. Ironically, this chaos is produced by the same mechanisms that, in ordinary bivalents and mitotic chromosomes, produce a single stable orientation and genetically appropriate chromosome distribution.

The evolution of meiosis

Meiosis is too complex to have arisen at once full blown and a stepwise scheme is proposed for its evolution, where each step is believed to have provided an immediate selective advantage: (1) The first step in this tentative sequence is the development of a haploidization process by means of a rapid series of mitotic non-disjunctions, turned on under conditions where haploidy is favored. The non-disjunctions may have resulted from a conditional mutation which caused sister centromere cohesiveness in the past mitotic metaphase. (2) Next probably came the formation of rudimentary synaptonemal complex type structures, first at Holliday-type configurations and later extending from these along chromosome pairs. These structures between homologues, though costly to produce and maintain, may have directly served the disjunctive function by setting the stage for the production of haploidy in one division, under conditions where it was advantageous. (3) Then secondarily acquired functions of the synaptonemal complex or structures associated with it may have promoted greatly increased crossover frequency, in part at least by increasing the frequency of the isomerization-type reaction. The resulting recombination of linked genes could have been advantageous under some conditions. (4) Finally, it is proposed that the capability was acquired for enhanced association of sister chromatids during the period between pachytene and anaphase I to give rise to chiasma-mediated disjunction, so that the relatively costly synaptonemal complex maintenance until anaphase I could be abandoned without losing disjunctive capability. It is implied that the modern synaptonemal complex is a structure which embodies a number of separately encoded proteins and that secondary structures and functions are associated with close homologue pairing. This scheme is based upon observable cytological and molecular characteristics of modern organisms.

Centromeric dots in crane-fly spermatocytes: meiotic maturation and malorientation

During the first meiotic division in crane-fly spermatocytes, the two homologs of a metaphase bivalent each bear two sister kinetochores oriented toward the same pole. We have previously reported treatments that increase the percentage of metaphase bivalents in which one or both homologs have bipolar malorientations: kinetochore microtubules extending from a homolog toward both poles. The maloriented homologs lag at anaphase. Treatments that induce this behavior include: (a) recoverey from exposure to low temperatures or Colcemid or Nocodazole concentrations that prevent spindle formation but allow nuclear membrane breakdown, and (b) exposure to 6 degrees C, a temperature that permits spindle assembly but slows progression through meiosis. Giemsa staining methods reveal two 0.5 micron diameter dots at the centromeric region of each metaphase homolog; these often are more separated in maloriented homologs. This investigation was undertaken to assess whether this separation precedes the establishment of bipolar malorientation, and hence may be a cause of it, or is only a consequence of forces resulting from bipolar malorientation. Analysis showed that, in untreated cells, the average center-to-center distance between sister centromeric dots increases during the course of meiosis I. After the above-mentioned treatments, center-to-center distances similar to those normally seen in untreated half-bivalents at anaphase I were seen in bivalents, both after and before nuclear membrane breakdown. Longer exposure to temperatures that arrested meiosis increased the degree of dot separation. Based on our data, we conclude that normal orientation during the first meiotic division is aided by the close apposition of centromeric dots, and that a time-dependent maturation occurs causing centromeric dots to separate for the second meiotic division and facilitating orientation of sister kinetochores to opposite poles. If centromeric maturation occurs either prior to or during early stages of the first meiotic division, then it may contribute to persisting bipolar malorientation.

Bivalent orientation and behavior in crane-fly spermatocytes recovering from cold exposure

At metaphase in crane-fly primary spermatocytes, the two sister kinetochores at the centromere of each homologue in a bivalent normally are adjacent and face the same pole; one homologue has all its kinetochore microtubules (kMTs) extending toward one pole and its partner has all its kMTs extending toward the opposite pole. In contrast, during recovery from exposure to 2 degrees C, one or both homologues in many metaphase bivalents had bipolar malorientations: all kMTs of one kinetochore extended toward one pole and some or all those of its sister extended toward the other. Metaphase sister kinetochores that had most of their kMTs extending toward the same pole were adjacent, and those with most extending toward opposite poles were separated from each other. Distances between homologous centromeres were similar to those in properly oriented bivalents. Maloriented bivalents were tilted relative to the spindle axis, and analysis of living cells showed that tilted configurations were rare during prometaphase in untreated cells but frequently arose in cold-recovering cells as initial configurations, then persisted through metaphase. This was in contrast to unipolar configurations of bivalents (configurations suggesting orientation of both homologous centromeres toward the same pole), which always reoriented shortly after the configuration arose. We conclude that in cold-recovering cells, bipolar malorientations are more stable than unipolar malorientations, and the orientation process is affected such that bipolar malorientations arise in bivalents upon initial interaction with the spindle and persist through metaphase.

Cell division in two large pennate diatoms Hantzschia and Nitzschia III. A new proposal for kinetochore function during prometaphase

Prometaphase in two large species of diatoms is examined, using the following techniques: (a) time-lapse cinematography of chromosome movements in vivo; (b) electron microscopy of corresponding stages: (c) reconstruction of the microtubules (MTs) in the kinetochore fiber of chromosomes attached to the spindle. In vivo, the chromosomes independently commence oscillations back and forth to one pole. The kinetochore is usually at the leading edge of such chromosome movements; a variable time later both kinetochores undergo such oscillations but toward opposite poles and soon stretch poleward to establish stable bipolar attachment. Electron microscopy of early prometaphase shows that the kinetochores usually laterally associate with MTs that have one end attached to the spindle pole. At late prometaphase, most chromosomes are fully attached to the spindle, but the kinetochores on unattached chromosomes are bare of MTs. Reconstruction of the kinetochore fiber demonstrates that most of its MTs (96%) extend past the kinetochore and are thus apparently not nucleated there. At least one MT terminates at each kinetochore analyzed. Our interpretation is that the conventional view of kinetochore function cannot apply to diatoms. The kinetochore fiber in diatoms appears to be primarily composed of MTs from the poles, in contrast to the conventional view that many MTs of the kinetochore fiber are nucleated by the kinetochore. Similarly, chromosomes appear to initially orient their kinetochores to opposite poles by moving along MTs attached to the poles, instead of orientation effected by kinetochore MTs laterally associating with other MTs in the spindle. The function of the kinetochore in diatoms and other cell types is discussed.

Mechanisms of chromosome orientation revealed by two meiotic mutants in Drosophila melanogaster

Two disjunction defective meiotic mutants, ord and mei-S332, each of which disrupts meiosis in both male and female Drosophila melanogaster, were analyzed cytologically and genetically in the male germ-line. It was observed that sister-chromatids are frequently associated abnormally during prophase I and metaphase I in ord. Sister chromatid associations in mei-S332 are generally normal during prophase I and metaphare I. By telophase I, sister chromatids have frequently precociously separated in both mutants. During the first division sister chromatids disjoin from one another frequently in ord and rarely in mei-S332. It is argued that the simplest interpretation of the observations is that each mutant is defective in sister chromatid cohesiveness and that the defect in ord manifests itself earlier than does the defect in mei-S332. In addition, based on these mutant effects, several conclusions regarding normal meiotic processes are drawn. (1) The phenotype of these mutants support the proposition that the second meiotic metaphase (mitotic-type) position of chromosomes and their equational orientation is a consequence of the equilibrium, at the metaphase plate, of pulling forces acting at the kinetochores and directed towards the poles. (2) Chromosomes which lag during the second meiotic division tend to be lost. (3) Sister chromatid cohesiveness, or some function necessary for sister chromatid cohesivenss, is required for the normal reductional orientation of sister kinetochores during the first meiotic division. (4) The kinetochores of a half-bivalent are double at the time of chromosome orientation during the first meiotic division. Finally, functions which are required throughout meiosis in both sexes must be considered in the pathways of meiotic control.

MICROTUBULE DISTRIBUTION IN THE ANAPHASE SPINDLE OF PRIMARY SPERMATOCYTES OF A CRANE FLY (NEPHROTOMA SUTURALIS)

Microtubules are described in crane fly spermatocytes (Nephrotoma suturalis Loew). The numbers of microtubules as determined from serial transverse sections of the spindles were compared in different regions of anaphase spindles: the interzonal regions had 60-80% of the number of microtubules between the chromosomes and the poles. The microtubule densities per micrometer-square cross-sectional area of the spindle were determined: the density of interzonal microtubules (i.e., those between the separating chromosomes in anaphase) was two to three times less than that of the kinetochore-associated microtubules, and the density of interzonal microtubules was about that of microtubules between the chromosomes and the poles once the kinetochore-associated microtubules were subtracted from the total. Microfilaments [Formula: see text] were also seen in the spindles. Sex-chromosomes in crane fly spermatocytes are amphitelically oriented, and tubular "bridging" material was seen extending between the two kinetochores of each sex-chromosome. The tubular "bridging" material is presumably related to the amphitely.

Ultrastructure of cytoplasmic nucleolus-like bodies and nuclear RNP particles in late prophase of tipulid spermatocytes

Late prophase stages of Pales ferruginea (Tipulidae) spermatocytes were examined by means of conventional electron microscopic section technique, combined with cytochemical methods. The cytoplasm of cells in diakinesis contains nucleolus-like bodies (NLB) 1 mum in diameter which are formed in diplotene at the pores of the nuclear membrane. They are compound structures consisting of fibro-granular RNP material which is associated wth one or two electron-dense gobules. The RNP material has a hollow core which contains polyribosomes. The NLBs possibly indicate rRNA gene amplification. At diakinesis the nucleus contains numerous electron-dense RNP particles scattered throughout the chromatin-free karyoplasm, and associated with the condensed chromosomes. The diameter of the chromation associated particles is markedly higher (mean 630 A) than that of the free particles (mean 540 A). The RNP particles seem to be aggregates of 200 A subunits. They are regarded as transcription products of chromosomal genes.

Light and electron microscopy of rat kangaroo cells in mitosis. III. Patterns of chromosome behavior during prometaphase

Chromosome orientation and behavior during prometaphase of mitosis in PtK1 rat kangaroo cells were investigated by cinémicrography and electron microscopy. The first chromosome movements occur soon after the nuclear envelope begins to break down in the region near each pole. Initial chromosome behavior is primarily determined by the distance from the kinetochore region to the spindle poles. The predominant pattern is a movement to and/or association with the proximal pole. Movement to and association with the more distant pole, or direct alignment at or near the spindle equator (direct congression) are less frequent patterns. Except for rare cases, pole-associated chromosomes congress sooner or later and most congressed chromosomes oscillate about the equator.--Ultrastructural observations suggest that pole-associated chromosomes are oriented only to the poximal pole (monotelic or syntelic orientation) and they demonstrate that the sister-kinetochores of congressing or oscillating chromosomes are oriented to opposite poles (amphitelic orientation).--Based on the structure of the early prometaphase spindle and four assumptions concerning the formation of kinetochore fibers and their force-producing interaction with complementary elements, the different patterns of chromosome behavior observed can be explained as a result of synchronous or asynchronous formation of sister-kinetochore fibers. The few chromosomes whose kinetochore region is approximately equidistant from the poles amphi-orient immediately because their sister-kinetochores form fibers synchronously and they congress directly because of the bidirectional forces to which they are subjected. The kinetochore region of most chromosomes is not equidistant from the poles. Therefore, they form a functional fiber first to the nearer pole and move to, or associate with it, it because of the unidirectional force. Eventually, however, these chromosomes achieve amphitelic orientation and congress. Once established, amphitelic orientation is stable. Re-orientations do not occur during congression or oscillatory movements.

Presence of actin during chromosomal movement

Actin has been shown to be present in the nucleoli, kinetochore and centriolar regions, and in the mitotic spindle of rat kangaroo cells which have been stained with fluorescently labeled heavy meromyosin. The actin in the spindle is confined to the fibers that connect the chromosomes with the centriolar region. Actin was not present in astral fibers, in the continuous spindle fibers that connect the poles, or in non-kinetochore regions of the chromosomes. The specific localization of actin in chromosomal spindle fibers suggests an actin-mvosin interaction as the force-producing mechanism for chromosomal movement.