Achiasmate male meiosis in two Cymatia species (Hemiptera,Heteroptera, Corixidae)

The karyotype and male meiosis, with a particular focus on the presence or absence of chiasmata between the homologs, were studied in the water boatman species Cymatia rogenhoferi (Fieber) and Cymatia coleoptrata (Fabricius) (Corixidae, Cymatiainae). It is shown that the species have 2n = 33 (28A+2m+X₁X₂Y) and 2n = 24 (20A+2m+XY) respectively, post-reduction of sex chromosomes, and achiasmate meiosis of an alignment type in males. Cytogenetic and some morphological diagnostic characters separating Cymatia Flor from the rest of Corixidae are discussed.     

Evolutionary history and positional shift of a rice centromere

Rice centromere 8 was previously proposed to be an "immature" centromere that recently arose from a genic region. Our comparative genomics analysis indicates that Cen8 was formed at its current location at least 7-9 million years ago and was physically shifted by a more recent inversion of a segment spanning centromeric and pericentromeric regions.     

Changing partners: moving from non-homologous to homologous centromere pairing in meiosis

Reports of centromere pairing in early meiotic cells have appeared sporadically over the past thirty years. Recent experiments demonstrate that early centromere pairing occurs between non-homologous centromeres. As meiosis proceeds, centromeres change partners, becoming arranged in homologous pairs. Investigations of these later centromere pairs indicate that paired homologous centromeres are actively associated rather than positioned passively, side-by-side. Meiotic centromere pairing has been observed in organisms as diverse as mice, wheat and yeast, indicating that non-homologous centromere pairing in early meiosis and active homologous centromere pairing in later meiosis might be themes in meiotic chromosome behavior. Moreover, such pairing could have previously unrecognized roles in mediating chromosome organization or architecture that impact meiotic segregation fidelity.     

Couples,pairs, and clusters: mechanisms and implications of centromere associations in meiosis

Observations of a wide range of organisms show that the centromeres form associations of pairs or small groups at different stages of meiotic prophase. Little is known about the functions or mechanisms of these associations, but in many cases, synaptonemal complex elements seem to play a fundamental role. Two main associations are observed: homology-independent associations very early in the meiotic program—sometimes referred to as centromere coupling—and a later association of homologous centromeres, referred to as centromere pairing or tethering. The later centromere pairing initiates during synaptonemal complex assembly, then persists after the dissolution of the synaptonemal complex. While the function of the homology-independent centromere coupling remains a mystery, centromere pairing appears to have a direct impact on the chromosome segregation fidelity of achiasmatic chromosomes. Recent work in yeast, Drosophila, and mice suggest that centromere pairing is a previously unappreciated, general meiotic feature that may promote meiotic segregation fidelity of the exchange and non-exchange chromosomes.     

Chromosomal polymorphisms involving telomere fusion,centromeric inactivation and centromere shift in the ant Myrmecia (pilosula) n=1

Detailed karyological surveys of the ant Myrmecia pilosula species group, which is characterized by the lowest chromosome number in higher organisms (2n=2), were attempted. We revealed that this species has developed highly complicated chromosomal polymorphisms. Their chromosome numbers are in the range 2n=2, 3, and 4, and six polymorphic chromosomes are involved, i.e., two for chromosome 1 (denoted as SM₁ and ST₁), three for chromosome 2 (A₂, A₂, and M₂), and M₍₁₊₂₎ for the 2n=2 karyotype. We suggested that these chromosomes were induced from a pseudo-acrocentric (A ₁ ^M ) and A₂ as follows: (1) A ₁ ^M SM₁ or ST₁ by two independent pericentric inversions; (2) A₂A₂M₂ by chromosomal gap insertion and centromere shift; and (3) ST₁+A₂M₍₁₊₂₎ by telomere fusion, where (3) means that the 2n=2 karyotype was derived secondarily from a 2n=4 karyotype. It is a noteworthy finding that active nucleolus organizer (NOR) sites, in terms of silver staining, are tightly linked with the centromere in this species, and that both the centromere and NOR of A₂ were inactivated after the telomere fusion.     

Phosphorylation of histone H2A is associated with centromere function and maintenance in meiosis

Histone phosphorylation is dynamically regulated during cell division, for example phosphorylation of histone H3 (H3)-Ser10, H3-Thr11 and H3-Ser28. Here we analyzed maize (Zea mays L) for Thr133-phosphorylated histone H2A, which is important for spindle checkpoint control and localization of the centromere cohesion protector Shugoshin in mammals and yeast. Immunostaining results indicate that phosphorylated H2A-Thr133 signals bridged those of the centromeric H3 histone variant CENH3 by using a plant displaying yellow fluorescent protein-CENH3 signals and H2A-Thr133 is phosphorylated in different cell types. During mitosis, H2A-Thr133 phosphorylation becomes strong in metaphase and is specific to centromere regions but drops during later anaphase and telophase. Immunostaining for several maize dicentric chromosomes revealed that the inactive centromeres have lost phosphorylation of H2A-Thr133. During meiosis in maize meiocytes, H2A phosphorylation becomes strong in the early pachytene stage and increases to a maximum at metaphase I. In the maize meiotic mutant afd1 (absence of first division), sister chromatids show equational separation at metaphase I, but there are no changes in H2A-Thr-133 phosphorylation during meiosis compared with the wild type. In sgo1 mutants, sister chromatids segregate randomly during meiosis II, and phosphorylation of H2A-Thr-133 is observed on the centromere regions during meiosis II. The availability of such mutants in maize that lack sister cohesion and Shugoshin indicate that the signals for phosphorylation are not dependent on cohesion but on centromere activity.     

The role of centromere alignment in meiosis I segregation of homologous chromosomes in Saccharomyces cerevisiae

During meiosis, homologous chromosomes pair and then segregate from each other at the first meiotic division. Homologous centromeres appear to be aligned when chromosomes are paired. The role of centromere alignment in meiotic chromosome segregation was investigated in Saccharomyces cerevisiae diploids that contained one intact copy of chromosome I and one copy bisected into two functional centromere-containing fragments. The centromere on one fragment was aligned with the centromere on the intact chromosome while the centromere on the other fragment was either aligned or misaligned. Fragments containing aligned centromeres segregated efficiently from the intact chromosome, while fragments containing misaligned centromeres segregated much less efficiently from the intact chromosome. Less efficient segregation was correlated with crossing over in the region between the misaligned centromeres. Models that suggest that these crossovers impede proper segregation by preventing either a segregation-promoting chromosome alignment on the meiotic spindle or some physical interaction between homologous centromeres are proposed.     

Pat(ting) boosts meiosis

Comment on: Cipak L, et al. Cell Cycle 2012; 11:1626-33     

An unpaired mouse centromere passes consistently through male meiosis and does not significantly compromise spermatogenesis

ST1 is an artificial mini-chromosome approximately 4.5 Mb in size containing mouse minor and major satellite DNA, human alphoid DNA and sequences derived from interval 5 of the human Y chromosome. Here we have measured the mitotic and meiotic transmission of ST1 and have used the mini-chromosome to define the ability of mice to monitor the presence of unpaired centromeres during meiosis. ST1 is mitotically stable, remaining intact and autonomous in mice for many generations. Female mice efficiently transmit ST1 to their offspring at a frequency approaching 50%. Male mice also reliably transmit the mini-chromosome, though to only 20% of their offspring. Presence of ST1 in males is not associated with any compromise in the output of the seminiferous epithelium nor with histological or immunocytochemical evidence of increased apoptosis, outcomes predicted for a synapsis checkpoint. These data indicate that the presence of an unpaired centromere is not sufficient to arrest male meiosis, implying that univalents are normally eliminated by a mechanism other than a tension-sensitive spindle checkpoint.P.J. Mee and M.M. Shen contributed equally to this article     

Lignans,phenylpropanoids and polyacetylenes from Chaerophyllum aureum L. (Apiaceae)

Sub-aerial parts of Chaerophyllum aureum L. yielded two polyacetylenes, falcarinol (1), falcarindiol (2), three lignans, namely nemerosin (3), deoxypodorhizone (4), deoxypodo-phyllotoxin (5), two phenylpropanoids, 1'-hydroxymyristicin (6) and its angeloyl ester (7). Compounds 6 and 7 were isolated for the first time from plant material and their structures were elucidated by means of extensive 1- and 2-dimensional NMR spectroscopy and high resolution mass spectrometry. In bioautographic tests on TLC plates the dichloromethane extract showed a significant antimicrobial activity. Falcarindiol was identified as the main active principle whereas the phenylpropanoids and lignans showed no activity.     

The pericentromeric heterochromatin of homologous chromosomes remains associated after centromere pairing dissolves in mouse spermatocyte meiosis

Chromosoma - In meiosis, crossovers between homologous chromosomes link them together. This enables them to attach to microtubules of the meiotic spindle as a unit, such that the homologs will be...     

Male meiosis in camel-flies (Raphidioptera; Neuropteroidea)

Male meiosis in 3 species of the raphidioptera genus Agulla-- A. bicolor Banks, A. astuta (Banks), and A. bractea Carpenter-- closely parallels that of Neuroptera. The diploid complement in each comprises 12 pairs of autosomes plus X and Y; all are mediokinetic. One male of A. bicolor carried an extra pair of autosomes indistinguishable from the shortest member of the usual set: these formed a normal bivalent and segregated synchronously with the other autosomes. The spindle is formed by the collocation of individual units which envelope each chromosomal mass. The sex chromsomes are spatially separate on emergence from the joint vesicle of early prophase; oriented toward opposite poles they move into this interpolar axis and a central spindle unit forms about them. This unit elongates disproportionately in early premetaphase, and its subsequent contraction is not synchronous with that of the other units. Distance segregation of X and Y is completed in early premetaphase. Autosomal bivalents are chiasmate; their congressional maneuvers involve, in addition to the usual interpolar oscillations, a lateral movement to the periphery of the spindle to form a variably complete ring at the equator. Autosomal univalents occurs with a frequency of 13% in A. bicolor, 2% in A. astuta, and 1% in A. bractea; they undergo distance segregation with the sex chromosomes in the central spindle unit. The phylogenetic significance of the data is considered.     

SGO1 but not SGO2 is required for maintenance of centromere cohesion in Arabidopsis thaliana meiosis

Shugoshin is a protein conserved in eukaryotes and protects sister chromatid cohesion at centromeres in meiosis. In our study, we identified the homologs of SGO1 and SGO2 in Arabidopsis thaliana. We show that AtSGO1 is necessary for the maintenance of centromere cohesion in meiosis I since atsgo1 mutants display premature separation of sister chromatids starting from anaphase I. Furthermore, we show that the localization of the specific centromeric cohesin AtSYN1 is not affected in atsgo1, suggesting that SGO1 centromere cohesion maintenance is not mediated by protection of SYN1 from cleavage. Finally, we show that AtSGO2 is dispensable for both meiotic and mitotic cell progression in Arabidopsis.     

Mutational analysis of the central centromere targeting domain of human centromere protein C, (CENP-C)

Human centromere protein C (CENP-C) is an essential component of the inner kinetochore plate. A central region of CENP-C can bind DNA in vitro and is sufficient for targeting the protein to centromeres in vivo, raising the possibility that this domain mediates centromere localization via direct DNA binding. We performed a detailed molecular dissection of this domain to understand the mechanism by which CENP-C assembles at centromeres. By a combination of PCR mutagenesis and transient expression of GFP-tagged proteins in HeLa cells, we identified mutations that disrupt centromere localization of CENP-C in vivo. These cluster in a 12 amino acid region adjacent to the core domain required for in vitro DNA binding. This region is conserved between human and mouse, but is divergent or absent in invertebrate and plant CENP-C homologues. We suggest that these 12 amino acids are essential to confer specificity to DNA binding by CENP-C in vivo, or to mediate interaction with another as yet unidentified centromere component. A differential yeast two-hybrid screen failed to identify interactions specific to this sequence, but nonetheless identified 14 candidate proteins that interact with the central region of CENP-C. This collection of mutations and interacting proteins comprise a useful resource for further elucidating centromere assembly.     

Distribution of kinetochore (centromere) antigen in mammalian cell nuclei

Antigens associated with mammalian centromeres were localized at the high and electron microscopic levels using the peroxidase-labeled antibody method. The antibody used was of a type naturally occurring in the sera of patients with scleroderma. At the light microscopic level, it reacts specifically with the centromere regions of chromosomes in a variety of mammalian species and strains in discrete foci in interphase nuclei. We find that the number of foci approximates the number of chromosomes present in the various cell types. At the ultrastructural level, the antigenic foci are confirmed to lie in the kinetochore regions of each chromosome. In interphase nuclei, the antigenic foci were usually associated either with the inner surfaces of the nuclear envelope or with the nucleoli. These observations indicate that the centromere regions of the chromosomes in interphase are not randomly distributed within the nucleus but are usually fixed either to the inner surface of the nuclear envelope or to nucleoli.     

Araucnephia iberaensis n. sp., a neotropical black fly with a peculiar distribution (Diptera-Simuliidae)

Araucnephia Wygodzinsky & Coscarón is a Neotropical black fly genus in which only one species from Central Chile is known. Another species has now been found in Corrientes province on the eastern side of the Iberá tropical swamps of Argentina, on the western border of the mountainous region of southern Brazil. This new species, A. iberaensis, is herein described and illustrated and information on its bionomics is recorded. It is an interesting species because previous to its discovery no black fly genus or subgenus from Central Chile region has been found in tropical areas, because these two regions are separated by the Monte and Pampas realms. Similarly, no Brazilian genus or subgenus has crossed the Andes mountains to Chile. A comparison with other Neotropical, Nearctic, Ethiopian (Afrotropical) and Australian Prosimuliini (sensu Crosskey & Howard) showed Araucnephia to be a valid taxon most closely related to Araucnephioides (sympatric in Chile). Araucnephia also shows great affinities with Lutzsimulium from Southeast Brazil and Argentina and Paracnephia from South Africa.     

Stretching it: putting the CEN(P-A) in centromere

The centromere is the locus responsible for the segregation of chromosomes during mitosis and meiosis. The number of newly characterised centromere-associated proteins continues to increase. The kinetochore complex assembles at this site and in many organisms is visible as the primary constriction. In several systems the location of the site of kinetochore assembly is known to vary and the site is not specified by a strict cis-acting primary sequence. It is proposed that tension between bioriented sister centromeres may act to imprint the site.     

Pushing the (nuclear) envelope into meiosis

A recent study shows that a short isoform of a mammalian nuclear lamin is important for homologous chromosome interactions during meiotic prophase in mice.Meiosis is the specialized cell division process required for sexual reproduction. As cells enter meiotic prophase, a relatively long period preceding the two chromosome divisions, nuclei and chromosomes undergo remodeling to promote interactions between homologous chromosomes. Each chromosome must find and identify its unique partner within the volume of the nucleus, a process that obviously involves large-scale chromosome movements.Over 100 years ago, cytological analysis of meiotic cells revealed a unique chromosome configuration termed the meiotic ''bouquet'', in which chromosome ends seem to be attached to the nuclear periphery, frequently in a tight cluster. The presence of the bouquet was found to coincide with the stage during which homologous chromosomes undergo pairing and synapsis. This was the first indication that interactions between the chromosomes and the nuclear envelope might be important for meiotic pairing. More recent analysis in diverse model systems has revealed that the bouquet is a consequence of interactions between chromosomes and cytoskeletal elements - microtubules or actin cables - via a protein bridge that spans the nuclear envelope. A study recently published in PLOS Genetics [1] has shed further light on the role of the nuclear lamina in meiotic progression by studying the role of a meiosis-specific isoform of a nuclear lamin protein.In metazoans the nuclear envelope is fortified by the nuclear lamina, a meshwork of intermediate filament proteins (lamins) and associated proteins that underlies the inner nuclear membrane. The lamina confers structural rigidity to nuclei and also interacts with a wide variety of nucleoplasmic, transmembrane and chromosome-associated proteins. The composition of the lamina in metazoans shows tissue-specific variability and developmental regulation. Most differentiated mammalian cells express both A-type lamins (lamins A and C, which are generated by alternative splicing of the LMNA gene) and B-type lamins (encoded by two different genes), whereas some invertebrates express only a single lamin protein. Stem cells typically lack A-type lamins, which are also dispensable for early development in mice.Among the nuclear envelope components that interact with lamins are LINC (linker of nucleoskeleton and cytoskeleton) complexes. These versatile networks involve a pair of SUN/KASH proteins that bridge both membranes of the nuclear envelope. SUN domain proteins traverse the inner membrane, with their amino termini projecting into the nucleus and their SUN domains in the lumen between the two membranes. Their partners have membrane-spanning regions adjacent to their carboxy-terminal KASH domains, short peptides that bind to the SUN domains. Using a variety of interaction modules, LINC complexes create connections between nuclear structures such as the lamina or chromosomes and cytoskeletal elements such as actin filaments or microtubules. Throughout the eukaryotes, they have essential roles in diverse processes, including the positioning and migration of nuclei within cells and anchorage of centrosomes to the nuclear envelope. During meiosis, specific LINC complexes are recruited to interact with chromosomes through the expression of meiosis-specific proteins that bind to telomeres or, less frequently, to other specialized loci [2]. These connections, probably in conjunction with meiosis-specific modifications to the cytoskeleton and motor proteins, lead to large-scale chromosome motions that facilitate homologous chromosome pairing. These movements involve dramatic motion of the LINC proteins within the nuclear membrane, sometimes involving movements of up to several micrometers that occur within a few seconds [3]. This stands in sharp contrast to the behavior of some of the same protein complexes in somatic or premeiotic cells, in which they show highly constrained motion and minimal turnover [3].In the new PLOS Genetics study [1], groups led by Manfred Alsheimer and Ricardo Benavente, both of the University of Würzburg, have now engineered a disruption of an exon in the mouse LMNA gene that is specific to the meiotic isoform lamin C2 to generate C2-deficient mice (C2^-/- mice). These collaborators have previously provided important insights into the regulation and functions of cell-type specific lamin isoforms, particularly during meiosis. Using antibodies, they characterized the lamin isoforms present in rat spermatocytes [4]. Immunolocalization revealed that a truncated isoform of lamin C (lamin C2) was localized in a patchy pattern along the nuclear envelope, along with a short B-type lamin (lamin B3) [4]. Because these short isoforms lack domains implicated in interactions between lamin subunits, they and others proposed that these proteins might form a more flexible network. This idea was supported by experiments in which meiosis-specific lamin C2 was ectopically expressed in fibroblasts and found to be more mobile within the nuclear envelope than full-length lamin C [5]. Expression of lamin C2 also resulted in aberrant localization of Sun1 in these cells. The collaborators also demonstrated that spermatogenesis was disrupted in Lmna^-/- mice, although oocyte meiosis was not obviously perturbed [6]. Although defects in meiosis-specific processes were observed in the knockout mice, it was not possible to rule out an indirect effect of lamin depletion in somatic cells on meiosis in spermatocytes, prior to the new study.An important feature of the new research [1] is that the C2^-/- mice show normal expression of all other A-type lamins. The C2^-/- males recapitulate the meiotic failure seen in Lmna^-/- mice. Nevertheless, their chromosomes frequently fail to synapse and they engage in heterologous associations or show aberrant telomere-telomere interactions; all of these defects are rare in wild-type spermatocytes. As a result of extensive apoptosis and failure of sperm maturation, the males are completely infertile. However, females are fertile, despite some evidence for pairing defects in C2^-/- oocytes.These sex-specific differences in the effects of lamin C2 loss are somewhat surprising. They could in part reflect differential implementation of meiotic checkpoints, which cull defective spermatocytes more ruthlessly than oocytes [7]. However, analysis of homologous pairing and synapsis in the C2^-/- mutant mice also revealed more severe defects in males. Both male and female mice lacking Sun1 protein are completely sterile and show synaptic failure during meiotic prophase [8]. This suggests that LINC-mediated chromosome dynamics are essential for homolog interactions during meiosis in both sexes. The milder defects caused by loss of lamin C2 in both male and female meiosis suggest that it has a less direct role in mediating chromosome movement than Sun1. This is consistent with the idea that expression of short lamin isoforms during meiosis acts primarily to increase the mobility of proteins within the nuclear envelope, relative to somatic cells. It seems likely that the dynamics of pairing, synapsis and recombination differ dramatically between spermatocytes, which are produced continually during the adult life of the male, and oocytes, which undergo meiotic prophase during fetal development. Such differences might render male meiosis more sensitive to changes in nuclear envelope organization or dynamics.The modifications made to the mouse nuclear envelope during meiosis are likely to be conserved in concept, if not in detail, in other taxa. As mentioned above, the isoforms and expression patterns of lamin proteins have diverged rapidly among the metazoa, as have the structures and functions of LINC complexes. For example, amphibians lack lamin C (and lamin C2), suggesting that its meiotic role in mammals is a recent innovation. Furthermore, the mouse Sun1 protein has a C2H2 zinc finger lacking in primate orthologs, which might suggest that it has evolved a distinct way to connect with meiotic chromosomes. It is thus not currently clear which aspects of meiotic lamina remodeling in mice can be extrapolated to other species.In Caenorhabditis elegans, meiotic chromosome dynamics are probably mediated by post-translational modification of the amino-terminal (nucleoplasmic) domain of sun-1 [9]. It is not yet known how this modification contributes to the function of the meiotic LINC complex. Direct observation has indicated that the motion of LINC complexes within the nuclear envelope becomes much less constrained as cells enter meiosis [3]. Phosphorylation of sun-1 may weaken interactions between the LINC complexes and the lamina to increase their mobility within the nuclear envelope, and/or promote interactions between LINC complexes to create high load-bearing aggregates of these proteins necessary to drive chromosome movement. It is not currently known whether the lamina itself is modified in C. elegans meiotic nuclei, but it is easy to imagine that phosphorylation could also be used to tweak protein-protein interactions within the lamina to optimize its properties during meiosis and other specialized cellular processes. It is likely that metazoans have evolved a wide range of mechanisms to modify their nuclear envelopes to meet the special demands of meiotic prophase.Homologous chromosome pairing remains one of the most mysterious aspects of meiosis. This new work in mice [1] adds an important piece of the puzzle by illuminating how the nuclear lamina can be modified to facilitate meiotic chromosome dynamics. To understand this process will clearly require looking beyond the chromosomes, and even beyond the nucleus, to the cellular networks connected by LINC complexes.