Ultrastructure,meiotic behavior,and evolution of sex chromosomes of the genus Ellobius

The whole-mount SC preparations from males of three species of the genus Ellobius (Ellobius fuscocapillus, Ellobius lutescens), and Ellobius tancrei were studied by electron microscopy. In the males of Ellobius fuscocapillus, behavioral peculiarities of the sex bivalent (viz. the normal male heterozygosity) are characterized by early complete desynapsis of sex chromosomes (X, Y), occurring at late pachytene-early diplotene. The karyotype of species Ellobius lutescens is unique for mammals. In both sexes it is characterized by an odd number of chromosomes (2n=17). At prophase I the unpaired chromosome 9 is not involved in synapsis with other chromosomes and forms a sex body at the end of pachytene.The complete Robertsonian fan has been described for superspecies Ellobius tancrei. As shown on the basis of G-band patterns the male and female sex chromosomes are cytologically indistinguishable.Analysis of whole-mount SC preparations revealed the formation of a closed sex SC bivalent and showed some morphological differences in the axes of sex chromosomes at meiotic prophase I. A number of assumptions are made about the relationship between the behavior of sex chromosomes, their evolution and the sex determination system in the studied species of genus Ellobius.

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Fine structure of anaphase bridges in meiotic chromosomes of the crane fly Pales

Chromatin bridges of autosomal bivalents in anaphase I were observed in spermatocytes of Pales ferruginea. The bridges are formed without simultaneous production of akinetochoric (akinetic) fragments. A bridge consists of a single fiber up to approximately. 500 Å in diameter. Filamentous substructures of approximately 100 Å diameter can be visualized. It is suggested that these bridges represent a low order coiling of the chromatid, and may be caused by non-separation of the terminal segments of the chromatids (telomeres).     

Fine structure of meiotic chromosomes comparative study of nine species of insects

A comparative electron microscope study at magnifications ranging from about 80,000 up to 800,000 x was carried out in nine species of insects (Gryllus argentinus, Myogryllus verticalis, undetermined species of Gryllus; Blaptica dubia, Periplaneta americana, Blattella germanica; Laplatacris dispar, Aleua lineata and Omexechae servillei). Particular attention was paid: a) to the elementary components of the s. complexes and b) to the structure of their medial ribbon. - a) In all the species examined the basic element found is a curled filament some 15–20 Å thick. Filaments of this kind integrate: the 100 Å fibrils of the chromosome body, the compacts layers of the s. complexes (lateral arms) and the slender planes of the pairing space. The filaments are similar to those described in metaphase chromosomes and their kinetochores (Wettstein and Sotelo, 1965). A difference in density between the filaments of the lateral arms and those of the medial planes is sometimes noticed. - b) Three structural patterns were found in the pairing space. In crickets, the medial ribbon is composed of three parallel, longitudinal planes of filaments, interconnected and connected with the lateral arms by means of bridges. The latter are constituted by fibrils or by single filaments. In cockroaches only two longitudinal planes were found. The distribution of components in these planes follows a plan similar to the one found in crickets. In the electron-micrographs the medial component of both groups of Insecta appears as composed of three (crickets) or two (cockroaches) lines in the longitudinal frontal views, and ladder-like striated in lateral views. The latter striae correspond to filaments or groups of filaments running in antero-posterior direction. - The pattern of structure of grasshoppers differs completely from those mentioned above. Bridging between the homologues is made of regularly spaced transversal planes of filaments. No longitudinal array was observed.This investigation was supported by United States Public Health Service Research Grant GM 08337 from the Research Grants Branch, Division of Medical Sciences, and partly by Grant RF 61034 from The Rockefeller Foundation.     

The meiotic structure and behavior of the strongly heteromorphic X/Y sex chromosomes of neotropical plethodontid salamanders of the genus Oedipina

Plethodontid salamanders in the genus Oedipina are characterized by a strongly heteromorphic sex-determining pair of X/Y chromosomes. The telocentric X chromosome and the subtelocentric Y chromosome are clearly distinguished from the autosomes and their behavior during meiosis can be sequentially followed in squash preparations of spermatocytes. In Oedipina the sex chromosomes are not obscured by an opaque sex vesicle during early meiotic stages, making it possible to observe details of sex bivalent structure and behavior not directly visible in other vertebrate groups. The sex chromosomes can first be distinguished from autosomal bivalents at the conclusion of zygotene, with X and Y synapsed only along a short segment at their non-centromeric ends, forming a bivalent that contrasts sharply with the completely synapsed autosomes. During pachytene, the XY bivalent becomes progressively shortened and more compact, disappearing as a visible structure when pachytene progresses into the diffuse stage of male meiosis. Diplotene bivalents gradually emerge from the diffuse nuclei, presumably by the return of the loops of chromatin into their respective chromomeres. During early diplotene, the X/Y bivalent is clearly visible with a single chiasma within the synapsed segment. This chiasma is terminalized by first meiotic metaphase with the X and Y appearing either in end-to-end synaptic contact or as univalents separated at opposite poles relative to the equatorially distributed autosomal bivalents. In C-banded preparations, the Y is entirely heterochromatic while the X contains a large centromeric C-band and another block of heterochromatin located at the telomeric end, in the region of synapsis with the Y. We find no cytological evidence of dosage compensation, such as differential staining of the X chromosomes or Barr bodies, in mitotic or interphase cells from female animals.     

Regions of active chromatin conformation in ‘inactive’ male meiotic sex chromosomes of the mouse

The sensitivity to DNase I of the meiotic sex chromosomes of the male mouse was determined by in situ nick translation. At pachytene and diakinesis-metaphase I, six segments, four at the ends of the X and Y chromosomes and two at internal sites on the X chromosome, were found to be more sensitive than the other parts of these chromosomes. The sensitive segments presumably reflect an active or potentially active chromatin conformation which is maintained in the sex chromosomes despite the earlier reported, almost complete cessation of uridine incorporation. The distribution of regions which are sensitive to DNase I corresponds to that of early DNA replication bands. Active conformation patterns like those figured here, probably exist in the sex chromosomes of other mammals as well.     

The behaviour and structure of the sex chromosomes in spermatocytes of Microtus oeconomus

The meiotic behaviour and structure of the sex chromosomes of Microtus oeconomus (2n=30) in Giemsa stained preparations are described. The X-Y pair appears as a sex vesicle at late zygotene. At late pachytene an unfolded sex vesicle is visible. A condensed sex vesicle appears during pre-diffuse diplotene and starts to unfold again during post-diffuse diplotene. At diakinesis and metaphase I the X and Y chromosomes can be recognized in an end-to-end association. During anaphase I, interkinesis and metaphase II the sex chromosomes are heteropycnotic and can therefore easily be recognized during the final stages of meiosis. During spermiogenesis the X and Y chromosomes can be identified in Giemsa stained preparations until the stage of spermatid elongation.     

Specialization in the behaviour of chromosomes on the meiotic spindle

The kinetic activity of chromosomes is either distributed evenly along the length of the chromosome or localized in a single centromere. Within the group of organisms with evenly distributed (holokinetic) activity great variation occurs, which has its consequences for chromatid segregation. Three major types, with some overlap, can be distinguished. They form a gradient of increasing specialization. Arguments are presented for the hypothesis that the monokinetic chromosomes are nothing but the end point in the same gradient and do not form a fundamentally different category. These arguments include: concentration of kinetic activity at meiosis in some types of holokinetic chromosomes, and atavistic traits in monokinetic chromosomes pointing to a holokinetic origin. Possible reasons for the wide distribution of monokinetic chromosomes in the more evolved taxa are given.     

Differential associative behaviour of mitotic and meiotic acrocentric chromosomes

Summary A comparative study of the association of mitotic acrocentric chromosomes and acrocentric bivalents at the pachytene stage shows that at least two factors can act in the associative behaviour of these chromosomes: (1) Nor activity and (2) the presence of satellite DNA in the short arms of these chromosomes. These factors do not act with the same intensity in the two cell lines studied. In lymphocytes, Nor activity prevails, whereas satellite DNA plays the main role in the association of acrocentric chromosomes in germ cells at the pachytene stage.     

Effects of the male contraceptive agent gossypol on meiotic chromosomes of the male rat

Synaptonemal complexes (SCs) of rat spermatocytes were analyzed in silver-stained meiotic preparations 10-24 days after treatment with gossypol acetic acid, 30 mg/kg/day, for 70 days. Gossypol did not affect SC formation or function, as judged by the absence of pairing anomalies, SC fragmentation, or presynaptic arrest. The unpaired lateral axes could be seen at zygotene, and at pachytene normal SCs could be observed. The behavior of the XY axes also appeared to be normal.     

X and B chromosomes display similar meiotic characteristics in male grasshoppers

We have analysed the chromosome organisation and the location and temporal appearance of different proteins in X and B chromosomes in the grasshopper Eyprepocnemis plorans throughout the first meiotic prophase. We have used adult males that carry a B chromosome collected in natural Spanish populations. The scaffold organisation has been analysed by means of silver stained chromatid cores. In addition, we have detected by immunolabelling the presence of phosphoepitopes, the ensemble of cohesin axes, the location of histone gamma-H2AX, and recombinase Rad51. Our observations demonstrate that X and B chromosomes share similarities in chromatin organisation and in the expression of the tested proteins, which strongly differ from those of the autosomes. These results could be interpreted either as a support to the hypothesis that the Bs analysed here originated from the X chromosome, and/or that their chromatin composition and precocious condensation could determine their meiotic behaviour.     

The structure,meiotic behaviour and effects of B chromosomes in Briza humilis Bieb. (Gramineae)

A single population of Briza humilis contained two types of B chromosome, one a large (B^L) and the other a small (B^S) acrocentric. DNA measurements show that the B^L chromosome contains approximately twice as much DNA per unit length as the members of the regular complement. The meiotic pairing behaviour of the Bs is variable and B^L and B^S are seen to pair in some cells. The presence of B^L depresses the chiasma frequency of the regular complement but the chiasma frequency of A and B chromosomes does not appear to be related. The transmission rate of the B chromosomes is variable and the B^L shows a non-disjunction mechanism during microsporogenesis that is absent during megasporogenesis. For the B^S chromosome the transmission rate is very low and there is no evidence of a non-disjunction mechanism. In general seeds containing B^L chromosomes germinate more slowly than those without B chromosomes.     

The effects of ageing on the meiotic chromosomes of male and female mice

The effects of age on the chiasma frequencies, chiasma position and numbers of univalents at MI in males and females of three strains of mouse were examined. Males showed a slight but non significant rise in chiasma frequency in age due to an increase in bivalents with two chiasmata at the expense of single chiasmata bivalents. In contrast, females exhibited a significant decrease in chiasma frequency with age due to the loss of two chiasma bivalents with a corresponding increase in single terminal chiasmata bivalents. In both males and females there was no significant increase in univalents with age in the strains studied. Of interest was the finding of a greater degree of contraction of the MI chromosomes in the oocytes of old relative to young females, a differential contraction that was independent of culture time. This finding is discussed with regard to the production line theory and non disjunction at Anaphase in other strains of mice.