Chromosoma

The region between the kinetochores of syntelically oriented autosomes and the pole in meta- and anaphase of Pales ferruginea spermatocytes was studied by means of serial sections. Microtubule (MT) were counted and measured, and the spindle region was reconstructed by superimposition of successive micrographs. Kinetochoric (kMTs) and non-kinetochoric microtubules (nkMTs) interdigitate with one another forming a bundle which is often arrow-shaped due to an inclination of nkMTs (skew nkMTs) with respect to the kinetochore-pole axis. The average length of MT in the bundle decreases towards anaphase while the average number increases. The extent of MT disorder in anaphase half-spindles is higher than in metaphase. The number of kMTs inserted in the kinetochore was found to remain unchanged from meta- to early anaphase. Some of the kMTs become divergent in anaphase. The relative proportion of skew nkMTs within the kMT/nkMT bundle is higher in anaphase. It is proposed that the morphological changes observed to occur from meta- to anaphase are due to fragmentation of kMTs followed by disorientation of the MTs pieces. Some aspects of the physical properties of the half-spindles are discussed.

The ultrastructure of polytene chromosomes of Drosophila and Stylonychia were compared in whole-mount spread preparations. In Drosophila the chromomeres appear as dense, unresolvable structures interconnected by 10-nm interband fibers. In contrast, chromomeres of Stylonychia polytene chromosomes are formed by aggregates of 30-nm loops laterally attached to 10-nm interband fibers. It is suggested that the polytene chromosomes in these two species are analogous rather than homologous structures.

Es wurden die Chromosomenverhältnisse des pädogenetisch-parthenogenetischen Entwicklungszyklus der heterogonen GallmückeOligarces paradoxus Mein, untersucht. Die Äquatorialplatten der einzigen, äquationellen Reifeteilung, bei der keine Tetraden auftreten, sowie der 1., 2. und 3. Furchungsteilung bestehen aus 66 Chromosomen. Im 3. Furchungsschritt spielt sich an den drei vorderen Abkömmlingen des primären Furchungskerns während der Anaphase ein Vorgang ab, denKahle (1908) bei der mitOligarces nahe verwandten CecidomyideMiastor metraloas Mein, als „Chromatindiminution“ beschrieben hat. Er besteht beiOligarces darin, daß 11 Chromosomen einer Spindel normal geteilt werden, ihre Chromatiden an die ihnen entsprechenden Spindelpole wandern und sich dort zu Tochterkernen umbilden („11er Kerne“), daß dagegen alle übrigen 55 Chromatidenpaare im Spindeläquator liegen bleiben und später im Cytoplasma resorbiert werden. Die Chromatindiminution ist also beiOligarces eine Chromosomenelimination. — Der vierte, hinterste Kern teilt sich in typischer Weise. Sein vorderer Toehterkern vollzieht im 4. Furchungsschritt die gleiche Chromosomenelimination, sein hinterer Tochterkern jedoch, der Urgeschlechtskern, der im „polaren Plasma“ (Keimbahnkörper) liegt, sowie alle seine Abkömmlinge, also sämtliche Keimbahnkerne, teilen sich normal und enthalten demnach 66 Chromosomen. Durch eine zweite, letzte Chromosomenelimination, die in der gleichen Weise wie die 1. Elimination vor sich geht, wird aus den 11er Kernen wiederum ein Chromosom, und zwar, wie alle analysierbaren Fälle zeigen, immer eines von der gleichen Struktur, ausgeschieden und allmählich im Cytoplasma aufgelöst. Die 2. Elimination erfolgt entweder in der 6. oder in der 7., oder in der 6.und 7., oder 7.und 8., oder schließlich in der 6., 7.und 8. Furchungsteilung. Nach der 2. Elimination besitzen die 10er Kerne fünf untereinander verschiedene, bei allen Individuen aber gleiche Chromosomenpaare, sind also diploid. In ungefähr 10% der Fälle werden in die Tochterkerne der Spindeln der 1. Elimination neben den 11 Chromosomen noch weitere Chromosomen oder Chromosomenfragmente einbezogen; sie werden dann während der Anaphase der 4.–8. Furchungsteilung ausgestoßen. Im 4. bis ungefähr im 10. Furchungsschritt beginnen zahlreiche (bis über 100) somatische Kerne in der Prophase bis in der Telophase zu degenerieren und bilden so der Resorption verfallende Chromatinklumpen. Die somatischen Kerne teilen sich nach dem 11. Furchungsschritt nicht mehr synchron. Die Oogonienkerne besitzen einen anderen Teilungsrhythmus als die somatischen Kerne. In gewissen ruhenden und in Prophase befindlichen Oogonienkernen ist eine periphere Schicht von 55 und eine mehr oder weniger zentral gelegene Gruppe von 11 Chromosomen vorhanden; diese verhalten sich anders als die 55 Chromosomen. Manches spricht dafür, daß die „66er Kerne“ in bezug auf die „Her Kerne“ hexaploid sind. Mit den Chromosomen der letzteren Kerne sind in den Oogonienkernen die Chromosomen der 11er Gruppe möglicherweise identisch. Es ist nicht ausgeschlossen, daß in den Geschlechtszellkernen zwischen der 55er und 11er Gruppe eine Chromosomenauswechslung vorkommt. Deren mögliche genetische Folgen werden kurz besprochen. Der Richtungskern stirbt in den meisten Eiern ab. Er kann sich daneben auch teilen; Abkömmlinge des Richtungskerns verhalten sich dann, soweit sie nicht abgestorben sind, genau so wie die Abkömmlinge des primären Furchungskerns, vollziehen also ebenfalls die 1. und 2. Chromosomenelimination und werden wie jene am Aufbau des Embryos verwendet.

The in situ distribution of phosphorus was studied in unstained ultrathin sections of salivary glands ofChironomus tentans andCh. thummi larvae using elemental mapping by means of an energy-filtering transmission electron microscope. This distribution was related to the structures observed using contrast enhancement with inelastically scattered electrons at 250 eV. This procedure demonstrated that a phosphorus-containing fibril about 2 nm thick is the common substructure of the following nuclear ribonucleoprotein structural constituents: the Balbiani ring granules, their precursor fibrils seen at the sites of transcription, especially at the Balbiani rings, and the fibres traversing the pore of the nuclear envelope. These phosphorus-containing thin fibrils are sensitive to RNase. Thicker substructural features of the Balbiani ring granules, occurring as a curved ribbon on a dense particle, appear to be formed by the dense packing of the fine fibrils. The Balbiani ring granules located near the nuclear envelope are often linked to it by fine filaments.

Karyotypes were compared in 48 species, including 6 subspecies, of birds from 12 orders: Casuariiformes, Rheiformes, Sphenisciformes, Pelecaniformes, Ciconiiformes, Anseriformes, Phoenicopteriformes, Gruiformes, Galliformes, Columbiformes, Falconiformes and Strigiformes. — With the exception of the family Accipitridae, all the species studied are characterized by typical bird karyotypes with several pairs of macrochromosomes and a number of microchromosomes, though the boundary between the two is not necessarily sharp. The comparative study of complements revealed that a karyotype with 3 morphologically distinct pairs of chromosomes is frequently encountered in all orders except the Strigiformes. Those 3 pairs, submetacentric nos. 1 and 2, and a subtelocentric or telocentric no. 3, are not only morphologically alike but also have conspicuous homology revealed by the G-banding patterns. Furthermore, G-banding analysis provided evidence for the derivation of the owl karyotype from a typical bird karyotype.—The above cytogenetic features led to the assumption that the 3 pairs of marker chromosomes had been incorporated into an ancestral bird karyotype. It seems probable that those chromosomes have been transmitted without much structural changes from a common ancestor of birds and turtles, since the presence of the same marker chromosomes in the fresh water turtle Geoclemys reevesii is ascertained by G-banding patterns. — A profile of a primitive bird karyotype emerged through the present findings. Hence, it has become possible to elucidate mechanisms involved in certain structural changes of macrochromosomes observed in birds. It was concluded that a major role had been played by centric fission as well as fusion, translocation, and pericentric inversion.

Ultrastructural studies of cereal anthers found intranuclear bundles of microfilaments in pollen mother cells (PMCs) but not elsewhere. The ultrastructure, distribution, and behaviour of this fibrillar material (FM) are described. FM was seen in all 19 genotypes studied comprising Aegilops, Triticum, Secale, Hordeum and Avena species, which together included haploid, diploid and allo-and autopolyploid, and natural and synthetic polyploid examples. Detailed studies in diploid S. cereale, and hexaploid T. aestivum and Triticale showed that FM was present in PMC nuclei during premeiotic interphase, leptotene and zygotene but not at pachytene and later meiotic stages. Moreover, it was most abundant at late premeiotic interphase in T. aestivum, and at leptotene in S. cereale and Triticale, when it occurred in up to 100% of sampled PMC nuclei in an anther. Although FM and synaptonemal complex (SC) occurred together in some PMC nuclei at later stages, FM was present long before SC, and reached its peak of abundance before SC did. Bundles of FM often formed links at their ends between either two masses of chromatin, or more rarely, between chromatin and the nuclear membrane. Individual bundles of FM varied in length but showed roughly similar ranges of lengths and widths in these three species. They were up to about 0.2 μm in diameter and about 3 μm in length, equivalent to about 20% of the maximum diameter of the nuclei containing them. Reconstructions of PMC nuclei indicated that FM was never associated with centromeres but was sometimes, and perhaps usually, associated with telomeric or sub-telomeric chromosome segments. The function of FM is unknown but its possible role is discussed in relation to (1) previously described intranuclear inclusions in meiocytes and (2) the cytogenetics and developmental behaviour of meiotic nuclei in the wheat comparium. As FM was a constant and characteristic structural component of PMC nuclei, its presence is probably of functional significance to the meiotic process. If so, it may function before, and over greater distances, than SC in establishing or maintaining the coorientation of chromosomes prerequisite for normal chromosome pairing. As FM was most abundant at stages when major chromosome movements occur, yet its distribution was non-centromeric, it is suggested that it may function in the attachment and movement of telomeres at the nuclear membrane formed after premeiotic mitosis. The possibility that a bundle of FM normally links corresponding sites on two homologues is considered.

Pairing failure in pachytene chromosomes was studied against a genetically constant background. Very significant negative correlations were found between total chromosome length per cell on the one hand and number of terminal pairing failures and their total extent (per cent length) on the other, and between number of terminal failures and their average extent (per cent length). No significant correlation was found, however, between total chromosome length per cell and the average extent (per cent length) of pairing failure. If the pairing failures are dissociations increasing in extent and number during the pachytene stage studied, the simplest reconciliation of the results requires that the average rate of their extension be roughly proportional to total chromosome length or that certain constants be related to each other as described. An alternate interpretation, that the pairing failures were present before pachytene and that chromosomes are shortening faster in cells containing them, is favored by the findings that heterogeneity is low between anthers in number of failures per cell and that no significant correlation was found between anthers in total chromosome length and number of pairing failures. The possibility is also discussed that the pairing failures are a complex combination of both initial synaptic failure and later dissociation. Distributions of chromosomes containing pairing failure at both ends and of those containing both terminal and intercalary pairing failures generally follow expectations of randomness.