Spermatogenesis in Sciara coprophila. I. Chromosome orientation on the monopolar spindle of meiosis I

Non-random chromosome segregation and chromosome eliminations in the fly Bradysia (Sciara)

Mendelian inheritance is based upon random segregation of homologous chromosomes during meiosis and perfect duplication and division of chromosomes in mitosis so that the entire genomic content is passed down to the daughter cells. The unusual chromosome mechanics of the fly Bradysia (previously called Sciara) presents many exceptions to the canonical processes. In male meiosis I, there is a monopolar spindle and non-random segregation such that all the paternal homologs move away from the single pole and are eliminated. In male meiosis II, there is a bipolar spindle and segregation of the sister chromatids except for the X dyad that undergoes non-disjunction. The daughter cell that is nullo-X degenerates, whereas the sperm has two copies of the X. Fertilization restores the diploid state, but there are three copies of the X chromosome, of which one or two of the paternally derived X chromosomes will be eliminated in an early cleavage division. Bradysia (Sciara) coprophila also has germ line limited L chromosomes that are eliminated from the soma. Current information and the molecular mechanisms for chromosome imprinting and eliminations, which are just beginning to be studied, will be reviewed here.

Cytogenetic analysis of Malpighian tubule and salivary gland polytene chromosomes of Bactrocera oleae (Dacus oleae) (Diptera: Tephritidae)

Photomaps of the Malpighian tubule and the salivary gland polytene chromosomes of Bactrocera oleae (Dacus oleae) are presented and compared with those of the fat body. Five polytene chromosomes (10 polytene arms) corresponding to the five autosomes of the mitotic nuclei, as well as a heterochromatic mass corresponding to the sex chromosomes, are observed in the nuclei of the three somatic tissues. The most prominent features of each polytene chromosome, the reverse tandem duplications, as well as the rather unusual ectopic pairing of the telomeric regions of different chromosome arms, are described. The constancy of the banding pattern based on the analysis of the three larval tissues is discussed.

Molecular and cytological characterization of repetitive DNA sequences from the centromeric heterochromatin of Sciara coprophila

Sciara coprophila (Diptera, Nematocera) constitutes a classic model to analyze unusual chromosome behavior such as the somatic elimination of paternal X chromosomes, the elimination of the whole paternal, plus non-disjunction of the maternal X chromosome at male meiosis. The molecular organization of the heterochromatin in S. coprophila is mostly unknown except for the ribosomal DNA located in the X chromosome pericentromeric heterochromatin. The characterization of the centromeric regions, thus, is an essential and required step for the establishment of S. coprophila as a model system to study fundamental mechanisms of chromosome segregation. To accomplish such a study, heterochromatic sections of the X chromosome centromeric region from salivary glands polytene chromosomes were microdissected and microcloned. Here, we report the identification and characterization of two tandem repeated DNA sequences from the pericentromeric region of the X chromosome, a pericentromeric RTE element and an AT-rich centromeric satellite. These sequences will be important tools for the cloning of S. coprophila centromeric heterochromatin using libraries of large genomic clones.

Genome size and determination of DNA content of the X chromosomes, autosomes, and germ line-limited chromosomes of Sciara coprophila

The unique chromosome biology of the fungus fly Sciara coprophila has fascinated investigators for over 80 years. Male meiosis exhibits a monopolar spindle, nonrandom segregation of imprinted chromosomes and nondisjunction of the X chromosome. The unusual mechanism of sex determination requires selective elimination of X chromosomes in embryogenesis. Supernumerary (L) chromosomes are also eliminated from the soma during early cleavage divisions. Distinctive DNA puffs on the larval salivary gland chromosomes are sites of DNA amplification. As a foundation for future genome studies to explore these many unusual phenomena, we have used DNA-Feulgen cytophotometry to determine genome size from hemocyte nuclei of male (X0) and female (XX) larvae and adults. The DNA content of the X chromosome is approximately 0.05 pg DNA and the autosomal complement is approximately 0.45 pg DNA. Measurements of DNA levels for individual sperm from adults showed that the DNA contribution of the germ line-limited (L) chromosomes constitutes as much as 35% of the DNA of the male gamete. A parallel study using Sciara ocellaris, a related species lacking L chromosomes, confirmed the presence of two X chromosomes in the sperm of this species.

Chromosome elimination in sciarid flies

The programmed elimination of part of the genome through chromosome loss or chromatin diminution constitutes an exceptional biological process found to be present in several diverse groups of organisms. The occurrence of this phenomenon during early embryogenesis is generally correlated to somatic versus germ-line differentiation. A most outstanding example of chromosome elimination and genomic imprinting is found in sciarid flies, where whole chromosomes of exclusive parental origin are selectively eliminated at different developmental stages. Three types of tissue-specific chromosome elimination events occur in sciarids. During early cleavages, one or two X paternal chromosomes is/are discarded from somatic cells of embryos which then develop as females or males respectively. Thus, the sex of the embryo is determined by the number of eliminated paternal X chromosomes. In germ cells, instead, a single paternal X chromosome is eliminated in embryos of both sexes. In addition, while female meiosis is orthodox, male meiosis is highly unusual as the whole paternal chromosome set is discarded from spermatocytes. As a consequence, only maternally derived chromosomes are included in the functional sperm. This paper reviews current cytological and molecular knowledge on the tissue-specific cell mechanisms evolved to achieve chromosome elimination in sciarids.

Role of microtubules and microtubule organizing centers on meiotic chromosome elimination in Sciara ocellaris

Spindle formation and chromosome elimination during male meiosis in Sciara ocellaris (Diptera, Sciaridae) has been studied by immunofluorescence techniques. During meiosis I a monopolar spindle is formed from a single polar complex (centrosome-like structure). This single centrosomal structure persists during meiosis II and is responsible for the non-disjunction of the maternal X chromatids. During meiosis I and II non-spindle microtubules are assembled in the cytoplasmic bud regions of the spermatocytes. The chromosomes undergoing elimination during both meiotic divisions are segregated to the bud region where they associate with bundles of microtubules. The presence and distribution of centrosomal antigens in S. ocellaris meiotic spindles and bud regions has been investigated using different antibodies. gamma-Tubulin and centrin are present in the bud as well as in the single polar complex of first meiotic spindle. The results suggest that spermatocyte bud regions contain microtubule-organizing centres (MTOCs) that nucleate cytoplasmic microtubules that are involved in capturing chromosomes in the bud regions. The distribution of actin and myosin in the spermatocytes during meiosis is also reported.

Nonrandom chromosome segregation in male meiosis of a sciarid fly: elimination of paternal chromosomes in first division is mediated by non-kinetochore microtubules

The mechanism of nonrandom chromosome segregation and elimination in male first meiotic division of sciarid flies is one of the enigmas of classical cytogenetics. Interpretations of the available data regarding this unusual chromosome transport are contradictory [reviewed by Gerbi, 1986: "Germline-Soma Differentiations. Results and Problems of Cell Differentiation," Vol. 13:71-104]. In the present study of spermatogenesis in Trichosia pubescens the process is reinvestigated by indirect anti-tubulin immunofluorescence labeling of spindle microtubules and DAPI chromosome staining, partly in combination with ultrathin sectioning and electron microscopy. While the maternal homologues and the sex-limited (L) chromosomes seem to be transported very quickly toward the pole of the monopolar spindle, where they stay throughout first meiotic division, paternal homologues remain some distance from the pole, stick together, and, as an undifferentiated cluster of chromatin, withdraw from the pole in the course of division. Finally the paternal chromatin becomes eliminated in a cytoplasmic bud which is cast off. The different behavior of maternal homologues (and L chromosomes) and paternal homologues may be caused by differences in kinetochore structure and function. In contrast to maternal and L chromosomes, the paternal homologues do not display structurally defined kinetochore-like regions and seem to be unable to orient poleward. During the process of elimination, a prominent bundle of pole-oriented microtubules is associated with the paternal chromatin cluster. It is suggested that transport away from the pole is brought about by these microtubules. Thus, meiotic chromosome elimination in sciarids may be related to mechanisms involving "polar ejection forces" [Rieder et al., 1986: J. Cell Biol. 103:581-591].

Incomplete sister chromatid separation is the mechanism of programmed chromosome elimination during early Sciara coprophila embryogenesis

Sex in Sciara coprophila is determined by maternally supplied factors that control the number of paternal X chromosomes eliminated during the syncytial embryonic divisions. Confocal microscopy and FISH demonstrate that the centromeres of the X chromosomes separate at anaphase and remain functional during the cycle in which the X chromosomes are eliminated. However, a region of the sister chromatids fails to separate and the X chromosomes remain at the metaphase plate. This indicates that failure of sister chromatid separation is the mechanism of chromosome elimination. Elimination of the X chromosomes requires the presence of a previously discovered Controlling Element that acts in cis during male meiosis. Using an X-autosome translocation, we demonstrate that the Controlling Element acts at-a-distance to prevent sister chromatid separation in the arm of an autosome. This indicates that the region in which sister chromatid separation fails is chromosome-independent. Although chromosome elimination occurs in all somatic nuclei and is independent of location of the nuclei within the embryo, the decision to eliminate is made at the level of the individual nucleus. Programmed X chromosome elimination occurs at different cycles in male and female embryos. These observations support a model in which elements on the X chromosome are titrating maternally supplied factors controlling the separation of sister X chromatids.

Unorthodox male meiosis in Trichosia pubescens (Sciaridae). Chromosome elimination involves polar organelle degeneration and monocentric spindles in first and second division

Male meiosis in Trichosia pubescens (Sciaridae) was investigated by means of serial section electron microscopy and immunofluorescence light microscopy. From earlier studies of another sciarid fly, Sciara coprophila (Phillips (1967) J. Cell. Biol. 33, 73–92), it is known that the spindle poles in sciarid spermatogonia are characterized by pairs of ‘giant centrioles’, ring-shaped organelles composed of large numbers of singlet microtubules. In the present study spermatocytes in early prophase of Trichosia were found to possess single giant centrioles at opposite sides of the nucleus. The obvious reduction in centriole number from the spermatogonial to the spermatocyte stage is suggested to be the result of a suppression of daughter centriole formation. In late prophase, a large aster is developed around the centriole at one pole. At the opposite pole no comparable aster is formed. Instead, a number of irregular centriolar components appear in this region, a process that is understood to be a degeneration of the polar organelle. The components of the degenerate pole migrate into a cytoplasmic protrusion (‘bud’), which later is also utilized for the elimination of paternal chromosomes. The existence of only one functional polar centre is the reason for the formation of a monopolar monocentric spindle in first meiotic division, which in turn is one of the prerequisites for the elimination of paternal chromosomes. While the set of maternal and L chromosomes orientates and probably moves towards the pole, paternal chromosomes seem to be unable to contact the pole, possibly due to an inactivation of their kinetochores. Retrograde (‘away from the pole’) chromosome motion not involving kinetochores is assumed. Eventually, paternal chromosomes move into the pole-distal bud and are eliminated by casting off, together with the components of the degenerate polar organelle. Chromosome elimination can be delayed until the second meiotic division. The spindle of the second meiotic division is bipolar and monocentric. One spindle pole is marked by the polar centre of first division. The opposite spindle apex is devoid of a polar centre. It is assumed that spindle bipolarity in the second division is induced by the amphi-orientated chromosomes themselves. The maternal and L chromosome set (except the non-disjunctional X chromosome, which is found near the polar centre) congress in a metaphase plate, divide and segregate. Of the two daughter nuclei resulting from the second meiotic division, the one containing the X chromatids is retained as the nucleus of the future spermatozoon. The other nucleus becomes again eliminated within a second cytoplasmic bud.

Oscillatory movements of monooriented chromosomes and their position relative to the spindle pole result from the ejection properties of the aster and half-spindle

During mitosis a monooriented chromosome oscillates toward and away from its associated spindle pole and may be positioned many micrometers from the pole at the time of anaphase. We tested the hypothesis of Pickett-Heaps et al. (Pickett-Heaps, J. D., D. H. Tippit, and K. R. Porter, 1982, Cell, 29:729-744) that this behavior is generated by the sister kinetochores of a chromosome interacting with, and moving in opposite direction along, the same set of polar microtubules. When the sister chromatids of a monooriented chromosome split at the onset of anaphase in newt lung cells, the proximal chromatid remains stationary or moves closer to the pole, with the kinetochore leading. During this time the distal chromatid moves a variable distance radially away from the pole, with one or both chromatid arms leading. Subsequent electron microscopy of these cells revealed that the kinetochore on the distal chromatid is free of microtubules. These results suggest that the distal kinetochore is not involved in the positioning of a monooriented chromosome relative to the spindle pole or in its oscillatory movements. To test this conclusion we used laser microsurgery to create monooriented chromosomes containing one kinetochore. Correlative light and electron microscopy revealed that chromosomes containing one kinetochore continue to undergo normal oscillations. Additional observations on normal and laser-irradiated monooriented chromosomes indicated that the chromosome does not change shape, and that the kinetochore region is not deformed, during movement away from the pole. Thus movement away from the pole during an oscillation does not appear to arise from a push generated by the single pole-facing kinetochore fiber, as postulated (Bajer, A. S., 1982, J. Cell Biol., 93:33-48). When the chromatid arms of a monooriented chromosome are cut free of the kinetochore, they are immediately ejected radially outward from the spindle pole at a constant velocity of 2 micron/min. This ejection velocity is similar to that of the outward movement of an oscillating chromosome. We conclude that the oscillations of a monooriented chromosome and its position relative to the spindle pole result from an imbalance between poleward pulling forces acting at the proximal kinetochore and an ejection force acting along the chromosome, which is generated within the aster and half-spindle.

Inertial force as a possible factor in mitosis

Some previous studies of cell division have suggested that chromosome movements in mitosis involve two distinct forces: one which pulls chromosomes poleward by means of attached fibers, and another which tends to push chromosome arms away from the pole. The latter force may also be a factor in non-chromosomal spindle transport, by which objects other than chromosomes are transported toward or away from spindle poles. Based on a survey of previous literature, this paper makes a prima facie case for describing this latter force as "inertial", since in some respects it can be simulated by centrifugation. A theoretical analysis demonstrates that an inertial force could arise in the spindle from postulated high-frequency, small-amplitude oscillations, which could be caused by changes in coherently processing electron spin alignments at the spindle poles. Some possible experimental approaches to the problem are briefly outlined.

Meiotic chromatin diminution in a vertebrate, the holocephalan fish Hydrolagus collie (Chondrichthyes, Holocephali)

A histochemical, microdensitometric, and electron microscopic study of testes of the ratfish Hydrolagus colliei shows that an instance of the rare phenomenon of germ line chromatin diminution occurs in this vertebrate species. In primary spermatocytes at metaphase I a spherical mass of heterochromatin accumulates at one side of the metaphase plate. At anaphase I the heterochromatic mass is left in the equatorial cytoplasm and is passed into one of the two secondary spermatocytes formed during cytokinesis. As nuclear membranes are being restored, a double membrane envelope is also formed around the heterochromatic mass, which is then termed the 'chromatin diminution body' (CDB). At second meiotic division the CDB is included in the cytoplasm of one of the four spermatids and retained there, apparently unchanged, until mid-spermiogenesis. At that time the CDB becomes adherent to the spermatid plasma membrane and is pinched off from the spermatid by a process of apocrine exocytosis, taking a layer of spermatid plasma membrane along with it. Simultaneously this tri-membrane CDB is taken into the adjacent Sertoli cell by endocytosis, thereby acquiring a fourth membrane layer, a part of the Sertoli cell plasma membrane. The CDBs are subsequently phagocytized, possibly first fusing with dense, multilaminate bodies in the Sertoli cell cytoplasm. The CDB chromatin mass is strongly positive with the Feulgen method for DNA and the alkaline fast green method for histones. Microdensitometric analysis shows that the discarded chromatin amounts to about 10% of the diploid nuclear content and that it appears to be part of the normal diploid complement rather than DNA amplified during meiosis.

Spermatogenesis in Sciara coprophila. II. Precocious chromosome orientation in meiosis II

In the second meiotic division of spermatogenesis in Sciara coprophila the X dyad undergoes a directed nondisjunction appearing precociously at one pole. All other chromosomes behave in a normal fashion aligning on the metaphase plate and dividing. Crouse has determined that this directed nondisjunction is governed by a region of the X centromere heterochromatin known to contain the rDNA (Crouse et al., 1977; Crouse, 1979). In order to further characterize this system we have utilized DAPI c-banding and rDNA in situ hybridization to demonstrate that the precocious chromosome (X or translocation chromosome) orients at metaphase II with its centromere end juxtaposed to the pole. Even when the controlling region is not near the centromere as in the case of translocations T1 and T32, the precocious chromosome orients with the centromere end rather than the controlling region end adjacent to the pole. These data may argue that precocious positioning is established at telophase I and maintained throughout the second meiotic division. - We have examined the hypothesis that the controlling region is transcriptionally active at metaphase II and can find no evidence for this speculation. This argues that if an RNA product is related to precocious positioning it must be synthesized earlier in spermatogenesis. - An analysis of naturally occurring tetraploid spermatocytes demonstrates that the two independent precocious chromosomes of such cells are always associated with the same pole. This datum in conjunction with the observation that tetraploid primary spermatocytes have only one monopolar spindle and not two, further supports the notion that a precocious chromosome-pole interaction may be established in meiosis I and maintained throughout meiosis II.