Kathleen Church

Centromere behavior during interphase and meiotic prophase in Allium fistulosum from 3-D, E.M. reconstruction

Centromeres at premeiotic interphase are clustered and situated in a small area of the nucleus opposite to the nuclear envelope associated heterochromatic masses. The centromeres may occur singly or they may associate to form a structure composed of 2 or more centromeres. Many centromere associations are nonhomologous. Interphase centromeres are not attached to the nuclear envelope. — At zygotene and pachytene centromeres are no longer clustered at one pole of the nucleus but rather are distributed throughout the nucleus. Premeiotic associations appear to be resolved prior to meiotic pairing. Only homologous centromere associations occur during zygotene and pachytene. There is no indication that premeiotic centromere associations are involved in prezygotene alignment of homologous chromosomes.

Meiosis in Drosophila melanogaster

Individual bivalents or chromosomes have been identified in Drosophila melanogaster spermatocytes at metaphase I, anaphase I, metaphase II and anaphase II in electron micrographs of serial sections. Identification was based on a combination of chromosome volume analysis, bivalent topology, and kinetochore position. — Kinetochore microtubule numbers have been obtained for the identified chromosomes at all four meiotic stages. Average numbers in D. melanogaster are relatively low compared to reported numbers of other higher eukaryotes. There are no differences in kinetochore microtubule numbers within a stage despite a large (approximately tenfold) difference in chromosome volume between the largest and the smallest chromosome. A comparison between the two meiotic metaphases (metaphase I and metaphase II) reveals that metaphase I kinetochores possess twice as many microtubules as metaphase II kinetochores. — Other microtubules in addition to those that end on or penetrate the kinetochore are found in the vicinity of the kinetochore. These microtubules penetrate the chromosome rather than the kinetochore proper and are more numerous at metaphase I than at the other division stages.

Kinetochore microtubules and chromosome movement during prometaphase in Drosophila melanogaster spermatocytes studied in life and with the electron microscope.

Prometaphase I chromosome behavior was examined in wild-type Drosophila melanogaster primary spermatocytes. Cine analysis of live cells reveals that bivalents exhibit complex motions that include (1) transient bipolar orientations, (2) simultaneous reorientation of homologous kinetochores, (3) movements not parallel to the spindle axis, and (4) movement along the nuclear membrane. — Kinetochores and kinetochore microtubule have been analyzed for bivalents previously studied in life. The results suggest that most chromosome motions (complex though they may be) can be explained by poleward forces acting on or through kinetochore microtubules that span the distance between the kinetochore and the vicinity of a pole. The results also suggest that the majority of short kinetochore microtubules may be remnants of previous microtubule-mediated associations between a kinetochore and a pole.

The distribution of synaptonemal complex material in metaphase I bivalents of Locusta and Chloealtis (Orthoptera: Acrididae)

At metaphase I synaptonemal complex (SC) material is located in a continuous but irregularly shaped bundle between sister chromatids. Only at the site of a chiasma is it present between homologous chromosomes. When the chromosomes pull apart at anaphase I the SC material becomes rearranged into poly-SCs which dissociate from the chromosomes. The observations agree with previous reports that modified SCs may function in meiotic chromosome disjunction.

Centromere sizes, positions, and movements in the interphase nucleus

Each microspore of the onion Allium fistulosum (n=8) has 8 chromosomes. It is shown that in the microspore the 8 centromeres aggregate to form 2 or 3 centromeric structures. Subsequently, at early mitotic prophase, these aggregates are resolved into 8 separate centromeres and each becomes structurally double. After mitosis the pollen grain contains 2 nuclei, each with 8 separate and distinct centromeres, clustered at the nuclear envelope. As interphase progresses the centromeres of the vegetative nucleus are no longer at the nuclear envelope and they aggregate into 3 or 4 centromeric masses. In the generative nucleus there is less movement. The interphase centromere movements occur in the absence of microtubules. The centromeres range in size from about 0.10 to 0.17 μm3 with an average of 0.14 μm3 per centromere.

Chromosome ends and the nuclear envelope at premeiotic interphase in the male grasshopper Brachystola magna by 3-D, E.M. reconstruction

During premeiotic interphase in the male grasshopper Brachystola magna the nucleus is divided into two nuclear envelope bound compartments, one containing the X chromosome and one the autosomes. — The autosomal compartment is characterized by an invaginated nuclear envelope with nuclear pores distributed throughout the envelope. In a polarized region of the cell the pericentric heterochromatic chromocenters are associated with the inner membrane of the envelope invaginations. In this species the chromosomes are telocentric (acrocentric?) and the pericentric heterochromatin marks the proximal chromosome ends. It is concluded that the chromosome ends are attached to the nuclear envelope at premeiotic interphase. — Comparisons are made between the present observations on chromosome arrangements and the nuclear envelope at premeiotic interphase to earlier observations at early meiotic prophase in the same species (Church, 1976). It is concluded that a rearrangement of both the proximal chromosome ends and the nuclear envelope occurs as cells enter meiotic prophase.

The grasshopper X chromosome

The sub-stages of spermatocyte interphase (G1, S and G2) have been identified in the grasshopper Brachystola magna using E.M. autoradiography and serial thin sectioning techniques. The X chromosome occupies a nuclear envelope bound compartment separate from an autosome compartment during G1 and S. At G2 the X compartment is resolved by coalescence of the membranes enveloping the X chromosome and autosome compartments. — At G1 and S, the compartmentalized X chromatin is laced with nuclear membrane material. This X chromatin associated membrane decreases in amount as the cell passes through G2 and enters early meiotic prophase. There are at least 2 and possibly 3 states of condensation of the heterochromatic X during premeiotic interphase and early meiotic prophase correlated with the presence or absence of membrane material associated with the chromatin. — The X chromatin replicates asynchronously with autosomal euchromatin and synchronously with autosomal heterochromatin associated with nucleoli. The X chromatin replication appears to be associated with the nuclear membrane. — The observations indicate that the nuclear membrane is involved with X chromosome condensation and may be implicated in asynchronous X chromosome replication as well.

Presynaptic chromosome behavior in Lilium

The orientation and movement of chromosomes throughout premeiotic interphase in Lilium speciosum has been studied through three-dimensional reconstruction of electron micrographs of serial thin sections through microsporocyte nuclei. Anthers were chosen based upon the correlation between their length and the stage of the microsporocytes within, and were fixed for light and electron microscopy. A light microscopic survey of both squash preparations and thick sections was done to select the material for electron microscopic analysis. Microsporocytes from the selected anthers were serially sectioned (200–300 consecutive gold sections), stained for electron microscopy, and alternate sections of entire nuclei were photographed. Prints were traced, and these tracings were compiled to produce a composite of each nucleus in which the locations of the centromeres were indicated. The position of the centromeric structures (CeS) in each nucleus was characterized by the average distance between CeSs, the average distance between CeSs and the nuclear envelope, and the coefficients of variation of these distances. A test was made to determine if CeSs were positioned evenly throughout the nucleus. — The results indicate that centromeres do not exhibit extensive movement during PMI in Lilium speciosum cv. Rosemede and that homologous chromosomes do not undergo a prealignment during PMI which facilitates their pairing during later meiotic stages. A model of centromere movement in the interphase nucleus is proposed.

Meiosis in the grasshopper

SummaryThe effect of high temperature on chiasma formation during oogenesis has been studied in the grasshopper Melanoplus femur-rubrum. Prolonged heat treatment (40° C.) during mid-prophase of meiosis causes a reduction in the mean chiasma frequency per cell. Only those bivalents in which more than one chiasma occurs are affected by the heat. The pattern of chiasma frequency response to heat is similar to that which occurs in males of the same species.

The effect of supernumerary heterochromatic chromosome segments on non-homologous chromosome associations in the grasshopper Camula pellucida

SummaryThe effect of a supernumerary heterochromatic segment on non-homologous chromosome associations has been investigated in the grasshopper Camula pellucida. A supernumerary segment located in one of the smallest chromosomes of the complement is associated with a substantial increase in the frequency of non-homologous chromosome associations at diplotene of meiosis. Bivalents that are segmented enter into more non-homologous chromosome associations than non-segmented bivalents. The non-homologous associations are completely resolved prior to metaphase II of meiosis. Both segmented and non-segmented chromosomes are randomly arranged in the radial metaphase II configuration.