C-heterochromatin and extra (B) chromosome distribution in six species of the Nabis (Heteroptera, Nabidae) with the modal male karyotype 2n = 16 + XY

The basic male karyotype of the six Nabis species (Heteroptera, Nabidae) is confirmed as being 2n=16+XY. The chromosomes are holokinetic while male meiosis is achiasmatic. The sex chromosomes undergo postreduction and in second metaphase show distance pairing, registered in all nabid species examined so far. Using C-banding technique for the first time in the family Nabidae, the heterochromatin was revealed on chromosomes of six species. The species showed different amount and distribution of C-heterochromatin. Only in Nabis (Dolichonabis) limbatus did the C-bands distribution make possible the identification of every chromosome pair in the karyotype. In other species, C-bands were found in some of the autosomes and the X, localized either interstitially or at telomeres. Only the Y usually showed relative stability ofthe C-banding pattern. In four of six species, extra (B) chromosomes were observed and their behaviour in meiosis described.     

Karyotype and male pre-reductional meiosis of the sharpshooter Tapajosa rubromarginata (Hemiptera: Cicadellidae)

Cicadellidae in one of the best represented families in the Neotropical Region, and the tribe Proconiini comprises most of the xylem-feeding insects, including the majority of the known vectors of xylem-born phytopathogenic organisms. The cytogenetics of the Proconiini remains largely unexplored. We studied males of Tapajosa rubromarginata (Signoret) collected at El Manantial (Tucumán, Argentina) on native spontaneous vegetation where Sorghum halepense predominates. Conventional cytogenetic techniques were used in order to describe the karyotype and male meiosis of this sharpshooter. T. rubromarginata has a male karyological formula of 2n = 21 and a sex chromosome system XO:XX (male:female). The chromosomes do not have a primary constriction, being holokinetic and the meiosis is pre-reductional, showing similar behavior both for autosomes and sex chromosomes during anaphase I. For this stage, chromosomes are parallel to the acromatic spindle with kinetic activities in the telomeres. They segregate reductionally in the anaphase I, and towards the equator during the second division of the meiosis. This is the first contribution to cytogenetic aspects on proconines sharpshooters, particularly on this economic relevant Auchenorrhyncha species.     

First evidence of sex chromosome pre-reduction in male meiosis in the Miridae bugs (Heteroptera)

The karyotype and male meiosis of Macrolophus costalis Fieber (Insecta, Heteroptera, Miridae) were studied using C-banding, AgNOR-banding and DNA sequence specific fluorochrome staining. The chromosome formula of the species is 2n = 28(24+X1X2X3Y). Male meiotic prophase is characterized by a prominent condensation stage. At this stage, two sex chromosomes, "X" and Y are positively heteropycnotic and always appeared together, while in autosomal bivalents homologous chromosomes were aligned side by side along their entire length, that is, meiosis is achiasmatic. At metaphase I, "X" and Y form a pseudobivalent and orient to the opposite poles. At early anaphase I, the "X" chromosome disintegrates into three separate small chromosomes, X1, X2, and X3. Hence both the autosomes and sex chromosomes segregate reductionally in the first anaphase, and separate equationally in the second anaphase. This is the first evidence of sex chromosome pre-reduction in the family Miridae. Data on C-heterochromatin distribution and its composition in the chromosomes of this species are discussed.     

Mitotic and meiotic chromosomes in Somatochlora metallica (Cordulidae,Odonata). The absence of localized centromeres and inverted meiosis

Spermatogonial metaphase chromosomes were examined in two dragonfly species, Somatochlora metallica (Cordulidae) and Aeshna grandis (Aeshnidae), and the behaviour of male meiotic chromosomes was studied in S. metallica. Both in S. metallica and A. grandis the male mitotic metaphase chromosomes from cells treated with colchicine consisted of two equidistantly aligned chromatids, showing no primary constriction. In meiosis the chromosomes of S. metallica males showed telokinetic activity during the first meiotic division, and kinetic activity was restricted in the middle parts of chromosomes during the second division. The kinetic behaviour of the chromosomes both in mitosis and meiosis showed that they were holocentric. One chiasma arises interstitially in each bivalent in S. metallica male meiosis. The chiasmata retain their interstitial position at metaphase I and do not terminalize. At metaphase I bivalents co-orient with homologous telomere regions towards the opposite poles. Thus genuine dyads segregate at the first anaphase. Meiosis in these male dragonflies is thus pre-reductional or conventional, not post-reductional or inverted, as has been previously proposed.     

Differential immunolocalization of a putative Rec8p in meiotic autosomes and sex chromosomes of triatomine bugs

Hemipteran chromosomes are holocentric and show regular, special behavior at meiosis. While the autosomes pair at pachytene, have synaptonemal complexes (SCs) and recombination nodules (RNs) and segregate at anaphase I, the sex chromosomes do not form an SC or RNs, divide equationally at anaphase I, and their chromatids segregate at anaphase II. Here we show that this behavior is shared by the X and Y chromosomes of Triatoma infestans and the X(1)X(2)Y chromosomes of Triatoma pallidipennis. As Rec8p is a widely occurring component of meiotic cohesin, involved in meiotic homolog segregation, we used an antibody against Rec8p of Caenorhabditis elegans for immunolocalization in these triatomines. We show that while Rec8p is colocalized with SCs in the autosomes, no Rec8p can be found by immunolabeling in the sex chromosomes at any stage of meiosis. Furthermore, Rec8p labeling is lost from autosomal bivalents prior to metaphase I. In both triatomine species the sex chromosomes conjoin with each other during prophase I, and lack any SC, but they form "fuzzy cores", which are observed with silver staining and with light and electron microscopy during pachytene. Thin, serial sectioning and electron microscopy of spermatocytes at metaphases I and II reveals differential behavior of the sex chromosomes. At metaphase I the sex chromosomes form separate entities, each surrounded by a membranous sheath. On the other hand, at metaphase II the sex chromatids are closely tied and surrounded by a shared membranous sheath. The peculiar features of meiosis in these hemipterans suggest that they depart from the standard meiotic mechanisms proposed for other organisms.     

Segregation of the amphitelically attached univalent X chromosome in the spittlebug <Emphasis Type="Italic">Philaenus spumarius</Emphasis>

In meiosis I, homologous chromosomes combine to form bivalents, which align on the metaphase plate. Homologous chromosomes then separate in anaphase I. Univalent sex chromosomes, on the other hand, are unable to segregate in the same way as homologous chromosomes of bivalents due to their lack of a homologous pairing partner in meiosis I. Here, we studied univalent segregation in a Hemipteran insect: the spittlebug Philaenus spumarius. We determined the chromosome number and sex determination mechanism in our population of P. spumarius and showed that, in male meiosis I, there is a univalent X chromosome. We discovered that the univalent X chromosome in primary spermatocytes forms an amphitelic attachment to the spindle and aligns on the metaphase plate with the autosomes. Interestingly, the X chromosome remains at spindle midzone long after the autosomes have separated. In late anaphase I, the X chromosome initiates movement towards one spindle pole. This movement appears to be correlated with a loss of microtubule connections between the kinetochore of one chromatid and its associated spindle pole.     

Meiosis in four trisomic and one double trisomic males of the newt Pleurodeles waltlii

In the newt Pleurodeles waltlii, meiosis was studied in four trisomic and one double trisomic males. Study of first prophase shows that trivalent frequencies and trivalent configurations are correlated with chromosome length; moreover, trivalent configurations indicate that long chromosomes have multiple points of initiation of synapsis whereas two points might be adequate to secure synapsis of short chromosomes. From the study of metaphase II it appears that the extra chromosomes segregate in half of the spermatocytes II. Some abnormal spermatocytes, resulting from nondisjunction of chromosomes at mitosis or at first division of meiosis, or from precocious division of chromosomes at first division of meiosis were observed. In the male double trisomic meiosis fails at anaphase of second division; this accounts for the sterility of the individual.     

Nonrandom chromosome segregation in Neocurtilla (Gryllotalpa) hexadactyla: an ultrastructural study

During meiosis I in males of the mole cricket Neocurtilla (Gryllotalpa) hexadactyla, the univalent X1 chromosome and the heteromorphic X2Y chromosome pair segregate nonrandomly; the X1 and X2 chromosomes move to the same pole in anaphase. By means of ultrastructural analysis of serial sections of cells in several stages of meiosis I, metaphase of meiosis II, and mitosis, we found that the kinetochore region of two of the three nonrandomly segregating chromosomes differ from autosomal kinetochores only during meiosis I. The distinction is most pronounced at metaphase I when massive aggregates of electron-dense substance mark the kinetochores of X1 and Y chromosomes. The lateral position of the kinetochores of X1 and Y chromosomes and the association of these chromosomes with microtubules running toward both poles are also characteristic of meiosis I and further distinguish X1 and Y from the autosomes. Nonrandomly segregating chromosomes are typically positioned within the spindle so that the kinetochoric sides of the X2Y pair and the X1 chromosome are both turned toward the same interpolar spindle axis. This spatial relationship may be a result of a linkage of X1 and Y chromosomes lying in opposite half spindles via a small bundle of microtubules that runs between their unusual kinetochores. Thus, nonrandom segregation in Neocurtilla hexadactyla involves a unique modification at the kinetochores of particular chromosomes, which presumably affects the manner in which these chromosomes are integrated within the spindle.     

Evidence of B-chromosomes in the karyotype of Barypeithes pellucidus Boheman 1834 (Coleoptera, Curculionidae, Entiminae) from Central Europe

B-chromosomes were observed in spermatogonial mitotic metaphases, meiotic metaphases I and II of Barpeithes pellucidus from one population in Slovakia. The number of B-chromosomes ranged from one to six per cell and they paired with the sex heterochromosomes in the first meiotic metaphase and rarely with the autosomes. In metaphase I one B-chromosome was always associated with X chromosome forming a tripartite complex. The XyBp was easily recognizable as a complex of three chromosomes in a parachute association The size of the B-chromosomes was approximately the same or a little smaller than that of the y heterochromosome which was the smallest element of the regular chromosome set. Their staining intensity seems to be similar to that of the autosomes and sex chromosomes, respectively. The behaviour of B-chromosomes during mitosis and meiosis in weevils is briefly discussed.     

Cytological studies of the two-spotted spider mite Tetranychus urticae Koch (Tetranychidae,trombidiformes). I: Meiosis in eggs

Meiosis in eggs of Tetranychus urticae Koch is described. The two maturation divisions result in (a) a haploid female pronucleus consisting of three karyomeres; — (b) a divided first polar body in which the chromosomes change into karyomeres; — (c) a second polar body, entering a new mitosis which is blocked in metaphase. Irradiation of adult females produced chromosome fragments in the meiotic divisions. The fragments behave as intact chromosomes which proves that during meiosis a diffuse kinetochore is present. The meiotic divisions show the cytologically characteristic features of an inverted meiosis. The presence of such a meiosis is corroborated by observations on eggs heterozygous for chromosome mutations. In both maturation divisions the chromosomes are orientated equatorially. It is suggested that the equatorial orientation is brought about by chiasmata having terminalized to both ends of the dyads. It is argued that in organisms with holokinetic chromosomes during meiosis an axial orientation of the bivalents does not necessarily imply a normal meiosis but can also imply an inverted meiosis.This work was carried out with financial aid of the Institute for Atomic Sciences in Agriculture and the Ministry of Public Health and Environment.     

Co-segregation of sex chromosomes in the male black widow spider <Emphasis Type="Italic">Latrodectus mactans</Emphasis> (Araneae,Theridiidae)

During meiosis I, homologous chromosomes join together to form bivalents. Through trial and error, bivalents achieve stable bipolar orientations (attachments) on the spindle that eventually allow the segregation of homologous chromosomes to opposite poles. Bipolar orientations are stable through tension generated by poleward forces to opposite poles. Unipolar orientations lack tension and are stereotypically not stable. The behavior of sex chromosomes during meiosis I in the male black widow spider Latrodectus mactans (Araneae, Theridiidae) challenges the principles governing such a scenario. We found that male L. mactans has two distinct X chromosomes, X₁ and X₂. The X chromosomes join together to form a connection that is present in prometaphase I but is lost during metaphase I, before the autosomes disjoin at anaphase I. We found that both X chromosomes form stable unipolar orientations to the same pole that assure their co-segregation at anaphase I. Using micromanipulation, immunofluorescence microscopy, and electron microscopy, we studied this unusual chromosome behavior to explain how it may fit the current dogma of chromosome distribution during cell division.     

Microinjected centromere [corrected] kinetochore antibodies interfere with chromosome movement in meiotic and mitotic mouse oocytes [published erratum appears in J Cell Biol 1990 Dec;111(6 Pt 1):following 2800]

Kinetochores may perform several functions at mitosis and meiosis including: (a) directing anaphase chromosome separation, (b) regulating prometaphase alignment of the chromosomes at the spindle equator (congression), and/or (c) capturing and stabilizing microtubules. To explore these functions in vivo, autoimmune sera against the centromere/kinetochore complex are microinjected into mouse oocytes during specific phases of first or second meiosis, or first mitosis. Serum E.K. crossreacts with an 80-kD protein in mouse cells and detects the centromere/kinetochore complex in permeabilized cells or when microinjected into living oocytes. Chromosome separation at anaphase is not blocked when these antibodies are microinjected into unfertilized oocytes naturally arrested at second meiotic metaphase, into eggs at first mitotic metaphase, or into immature oocytes at first meiotic metaphase. Microtubule capture and spindle reformation occur normally in microinjected unfertilized oocytes recovering from cold or microtubule disrupting drugs; the chromosomes segregate correctly after parthenogenetic activation. Prometaphase congression is dramatically influenced when antikinetochore/centromere antibodies are introduced during interphase or in prometaphase-stage meiotic or mitotic eggs. At metaphase, these oocytes have unaligned chromosomes scattered throughout the spindle with several remaining at the poles; anaphase is aberrant and, after division, karyomeres are found in the polar body and oocyte or daughter blastomeres. Neither nonimmune sera, diffuse scleroderma sera, nor sham microinjections affect either meiosis or mitosis. These results suggest that antikinetochore/centromere antibodies produced by CREST patients interfere with chromosome congression at prometaphase in vivo.     

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.     

Cytological studies of the two-spotted spider mite Tetranychus urticae Koch (tetranychidae,trombidiformes) II. Meiosis in growing oocytes

The morphology and behaviour of the holokinetic chromosomes of Tetranychus urticae Koch were studied in maturing oocytes from diplotene up to and including metaphase. Each of the three diplotene bivalents contains 2–3 chiasmata.During congression the dyads of the axially orientated bivalents make turning movements in opposite directions till they have reached an equatorial orientation which leads to an equational first meiotic division.It was argued that chiasmata terminalise towards both ends of the dyads and that each dyad contains two sister chromatids.It has been supposed that the inverted meiosis of organisms with holokinetic chromosomes can be brought about by the way in which chiasmata terminalise and the kinetochores are assembled subsequently on each chromatid.During congression vesicular strands form a Feulgen-negative zone round the bright plasm in which the bivalents are embedded. The question was raised whether these strands originate from the dense plasm surrounding each of the diplotene bivalents and bear some relation to the elimination chromatin present during the meiotic telophases.This investigation was supported by research grants from the Ministry of Public Health and Environment and from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.).     

Meiotic chromosome structure. Kinetochores and chromatid cores in standard and B chromosomes of Arcyptera fusca (Orthoptera) revealed by silver staining.

The behaviour of two chromosome structures in silver-stained chromosomes was analyzed through the first meiotic division in spermatocytes of the acridoid species Arcyptera fusca. Results showed that at diakinesis kinetochores and chromatid cores are individualized while they associate in bivalents of metaphase I; only kinetochores and distal core spots associate in the sex chromosome. Metaphase I is characterized by morphological and localization changes of both kinetochores and cores which define the onset of anaphase I. These changes analyzed in both autosomes and in the sex chromosome allow us to distinguish among three different substages in metaphase I spermatocytes. B chromosomes may be present as univalents, bivalents, or trivalents. Metaphase I B univalents are characterized by separated cores except at their distal ends and individualized and flat sister kinetochores. At anaphase I sister kinetochores of lagging B chromatids remain connected through a silver-stained strand. The behaviour of cores and kinetochores of B bivalents is identical with that found in the autosomal bivalents. The differences in the morphology of kinetochores of every chromosome shown by B trivalents at metaphase I may be related to the balanced forces acting on the multivalent. The results show dramatic changes in chromosome organization of bivalents during metaphase I. These changes suggest that chromatid cores are not involved in the maintenance of bivalents. Moreover, the changes in morphology of kinetochores are independent of the stage of meiosis but correlate with the kind of division (amphitelic-syntelic) that chromosomes undergo.     

Meiotic association and segregation of the achiasmatic giant sex chromosomes in the male field vole (Microtus agrestis)

Controversy exists regarding the meiotic behaviour of the giant sex chromosomes during spermatogenesis in the field vole, Microtus agrestis. Both univalents and bivalents have been observed between diakinesis and metaphase I. These differences seem to be dependent on the technique used. The present study employs electron microscopy of serially sectioned testes tubules and light microscopy of microspread preparations to re-examine the behaviour of sex chromosomes during meiosis. In microspreads, about one-third of the early pachytene nuclei examined showed end joining of the X and Y axes. The longitudinal heterogeneity of the chromosomes in the form of axial thickenings allowed the detection of two different end-joining patterns. In the remaining early pachytene cells as well as in all mid to late pachytene cells seen, the X and Y axes had, though near to each other, no contact in the form of a synaptonemal complex. If a synaptonemal complex is a prerequisite for genetic exchange, the sex chromosomes in M. agrestis males must be achiasmatic. The analysis of serial sections through an early pachytene and a late prophase I nucleus with the electron microscope revealed that the sex chromosomes occupied a common area. By metaphase I, the centromeres of the X and Y were oriented towards opposite spindle poles while the chromosomes remained attached to one another by their distal segments at the level of the metaphase I plate. As a consequence of the large size of the sex chromosomes their centromeres lay close to the spindle poles. In anaphase I the sex chromosomes maintained their metaphase position until the autosomes approached the spindle poles. During autosomal migration a medial constriction developed where the sex chromosomes were mutually associated, the X and Y became separated, and joined the autosomes. In metaphase II the chromatids of the sex chromosomes lay side by side and exhibited a delayed separation in the subsequent anaphase. It is suggested that heterochromatin, which represents a major part of both sex chromosomes, plays a role in the association of the two achiasmatic sex chromosomes in metaphase I and in the delayed separation of the chromatids of the sex chromosomes in anaphase II.Dedicated to Prof. C.-G. Arnold (Erlangen) on the occasion of his 60th birthday     

Chromosome condensation in mitosis and meiosis of rye (Secale cereale L.)

Structural investigation and morphometry of meiotic chromosomes by scanning electron microscopy (in comparison to light microscopy) of all stages of condensation of meiosis I + II show remarkable differences during chromosome condensation in mitosis and meiosis I of rye (Secale cereale) with respect to initiation, mode and degree of condensation. Mitotic chromosomes condense in a linear fashion, shorten in length and increase moderately in diameter. In contrast, in meiosis I, condensation of chromosomes in length and diameter is a sigmoidal process with a retardation in zygotene and pachytene and an acceleration from diplotene to diakinesis. The basic structural components of mitotic chromosomes of rye are "parallel fibers" and "chromomeres" which become highly compacted in metaphase. Although chromosome architecture in early prophase of meiosis seems similar to mitosis in principle, there is no equivalent stage during transition to metaphase I when chromosomes condense to a much higher degree and show a characteristic "smooth" surface. No indication was found for helical winding of chromosomes either in mitosis or in meiosis. Based on measurements, we propose a mechanism for chromosome dynamics in mitosis and meiosis, which involves three individual processes: (i) aggregation of chromatin subdomains into a chromosome filament, (ii) condensation in length, which involves a progressive increase in diameter and (iii) separation of chromatids.     

Comparison of the size of paternal and maternal homologous chromosomes during the first 2 cleavage divisions in mouse embryos

The mouse metaphase chromosomes of the 1st and 2nd cleavage divisions were prepared without colchicine and stained with trypsin-Giemsa. Both the homologues had the same pattern of differential staining (position and number of bands and interbands) in all pairs of chromosomes. The measurements of homologues of the 1st, 2nd, 3rd, 4th and 5th pairs of autosomes have shown that at the first cleavage division metaphase the paternal chromosomes are 1.2 times, on the average longer than the maternal ones, whereas at the second division metaphase no reliable differences in the length of homologues were found. In mice, thus, the heterocyclic pattern of the paternal and maternal sets of chromosomes manifested itself during the 1st cleavage division only and disappeared fully beginning from the 2nd division. This appears to be due to the early functional activity of chromosomes, i.e. to the fact that already in the 2-cell embryos both the maternal and paternal genes are expressed.