Lateral Elements Research Articles

Evolutionary Origin of the Turtle Shell

The origin of the turtle shell has perplexed biologists for more than two centuries. It was not until Odontochelys semitestacea was discovered, however, that the fossil and developmental data could be synthesized into a model of shell assembly that makes predictions for the as-yet unestablished history of the turtle stem group. We build on this model by integrating novel data for Eunotosaurus africanus-a Late Guadalupian (∼260 mya) Permian reptile inferred to be an early stem turtle. Eunotosaurus expresses a number of relevant characters, including a reduced number of elongate trunk vertebrae (nine), nine pairs of T-shaped ribs, inferred loss of intercostal muscles, reorganization of respiratory muscles to the ventral side of the ribs, (sub)dermal outgrowth of bone from the developing perichondral collar of the ribs, and paired gastralia that lack both lateral and median elements. These features conform to the predicted sequence of character acquisition and provide further support that E. africanus, O. semitestacea, and Proganochelys quenstedti represent successive divergences from the turtle stem lineage. The initial transformations of the model thus occurred by the Middle Permian, which is congruent with molecular-based divergence estimates for the lineage, and remain viable whether turtles originated inside or outside crown Diapsida.

Hoxa-5 acts in segmented somites to regulate cervical vertebral morphology

The vertebrate axial skeleton (vertebral column and ribs) is derived from embryonic structures called somites. Mechanisms of somite formation and patterning are largely conserved along the length of the body axis, but segments acquire different morphologies in part through the action of Hox transcription factors. Although Hox genes’ roles in axial skeletal patterning have been extensively characterized, it is still not well understood how they interact with somite patterning pathways to regulate different vertebral morphologies. Here, we investigated the role of Hoxa-5 in after somite segmentation in chick. Hoxa-5 mRNA is expressed in posterior cervical somites, and within them is restricted mainly to a sub-domain of lateral sclerotome. RNAi-based knockdown leads to cartilage defects in lateral vertebral elements (rib homologous structures) whose morphologies vary within and outside of the Hoxa-5 expression domain. Both knockdown and misexpression suggest that Hoxa-5 acts via negative regulation of Sox-9. Further, Hoxa-5 misexpression suggests that spatial and/or temporal restriction of Hoxa-5 expression is necessary for proper vertebral morphology. Finally, the restriction of Hoxa-5 expression to lateral sclerotome, which we hypothesize is important for its patterning function, involves regulation by signaling pathways that pattern somites, Fgf-8 and Shh.

Expression of Pannexin1 in the outer plexiform layer of the mouse retina and physiological impact of its knockout

Pannexin1 (Panx1) belongs to a class of vertebrate proteins that exhibits sequence homology to innexins, the invertebrate gap junction proteins, and which also shares topological similarities with vertebrate gap junction proteins, the connexins. Unlike gap junctional channels, Panx1 forms single-membrane channels, whose functional role in neuronal circuits is still unsettled. We therefore investigated the subcellular distribution of Panx1 in the mouse retina of wildtype and Panx1-null mice by reverse-transcription polymerase chain reaction (RT-PCR), immunohistochemistry, and electron microscopy. Use of Panx1-deficient mice served as a model to assess the physiological role of Panx1 by electroretinographic recordings and also to ensure the specificity of the anti-Panx1 antibody labeling. Expression of Panx1 was found in type 3a OFF bipolar cells and in dendrites and axonal processes of horizontal cells. Panx1 was also found in horizontal cell dendrites representing the lateral elements of the triad synapse at cone and rod terminals. In vivo electroretinography of Panx1 knockout mice indicated an increased a- and b-wave compared to Panx1 wildtype mice under scotopic conditions. The effect on the b-wave was confirmed by in vitro electroretinograms from the inner retina. These results suggest that Panx1 channels serve as sinks for extracellular current flow making them possible candidates for the mediation of feedback from horizontal cells to photoreceptors.

An evaluation of surgical outcome of bilateral cleft lip surgery using a modified Millard′s (Fork Flap) technique

The central third of the face is distorted by the bilateral cleft of the lip and palate and restoring the normal facial form is one of the primary goals for the reconstructive surgeons. The history of bilateral cleft lip repair has evolved from discarding the premaxilla and prolabium and approximating the lateral lip elements to a definitive lip and primary cleft nasal repair utilising the underlying musculature. The aim of this study was to review surgical outcome of bilateral cleft lip surgery (BCLS) done at the Lagos University Teaching Hospital. A review of all cases of BCLS done between January 2007 and December 2012 at the Lagos University Teaching Hospital was done. Data analysis included age and sex of patients, type of cleft deformity and type of surgery (primary or secondary) and whether the cleft deformity was syndromic and non-syndromic. Techniques of repair, surgical outcome and complications were also recorded. A total of 39 cases of BCLS involving 21 males and 18 females were done during the period. This constituted 10% (39/390) of all cases of cleft surgery done during the period. There were 5 syndromic and 34 non-syndromic cases. Age of patients at time of surgery ranged between 3 months and 32 years. There were 24 bilateral cleft lip and palate deformities and 15 bilateral cleft lip deformities. Thirty-one of the cases were primary surgery, while 8 were secondary (revision) surgery. The most common surgical technique employed was modified Fork flap (Millard) technique, which was employed in 37 (95%) cases. Bilateral cleft lip deformity is a common cleft deformity seen in clinical practice, surgical repair of which can be a challenge to an experienced surgeon. A modified Fork flap technique for repair of bilateral cleft lip is a reliable and versatile technique associated with excellent surgical outcome.

Earthquake Resistance of Modular Building Units Using Load-Bearing Steel Stud Panels

본 연구에서는 스터드패널과 경량철골골조로 구성된 모듈러건물 유닛의 강성, 하중재하능력, 연성능력, 에너지소산능력 등 내진성능을 평가하기 위하여 주기실험을 수행하였다. 모듈러건물 유닛의 횡력저항요소로서 스트랩브레이스 및 시트강판으로 보강된 스터드패널을 사용하였다. 실험 결과, 스트랩브레이스 및 시트강판 보강 스터드패널을 사용한 모듈러건물유닛은 우수한 연성거동을 보였다. 최대변위비는 5.37% 이상을 보였고, 변위연성도는 5.76 이상인 것으로 나타났다. 그러나 주기거동 동안 핀칭이 크게 발생하여 주기당 에너지소산량은 좋지 않은 것으로 나타났다. 모듈러건물유닛의 소성메커니즘을 바탕으로 내진설계를 위한 강도, 항복변위, 탄성강성 등 설계식을 제안하였고 실험 결과와 비교를 통하여 제안된 설계식을 검증하였다. 제안된 방법은 모듈러건물유닛의 하중재하능력, 강성 등 내진성능을 합리적으로 예측하였다. 그러나 스트랩브레이스 보강 스터드패널의 탄성강성은 크게 과대평가되었으므로, 안전한 내진설계를 위해서는 구조해석 시 탄성강성을 50%로 줄이는 것이 필요하다. Cyclic tests on modular building units for low-rise buildings composed of stud panels and a light-weight steel perimeter frame, were performed to evaluate the earthquake resistance such as stiffness, load-carrying capacity, ductility, and energy dissipation per load cycle. The strap-braced and sheeted stud panels were used as the primary lateral load-resistant element of the modular building units. Test results showed that the modular building units using the strap-braced and sheeted stud panels exhibited excellent post-yield ductile behaviors. The maximum drift ratios were greater than 5.37% and the displacement ductility ratios were greater than 5.76. However, the energy dissipation per load cycle was poor due to severe pinching during cyclic loading. Nominal strength, stiffness, and yield displacement of the modular building units were predicted based on plastic mechanisms. The predictions reasonably and conservatively correlated with the test results. However, the elastic stiffness of the strap-braced stud panel was significantly overestimated. For conservative design, the elastic stiffness of the strap-braced stud panel needs be decreased to 50% of the nominal value.

Damage to synaptonemal complex structure and peculiarities of selection of mouse spermatocytes I at response to drug administration

Using immunocytochemistry methods, the structure of synaptonemal complexes (SC) of chromosomes in spread nuclei of primary spermatocytes of mice at 1, 10, and 36 days after the 10-day intraperitoneal administration of antibacterial preparations of three pharmacological groups: furacilin, an antiseptic derivative of nitrofuran; cifran, an antibiotic from the group of fluoroquinolones; and sextaphage, a polyvalent piobacteriophage was investigated. The maximal number of disturbances in the structure and behavior of synaptonemal complex was revealed on the first day after the end of preparation administration. On days 10 and 36, the total number of disturbances in SC structure decreased gradually. On the first day after the end of the administration of cifran and sextaphage in 41.8 and 25% of nuclei, respectively, the fragmentation of synaptonemal complexes was revealed and, in males to whom furacilin had been administered, the fragmentation of synaptonemal complexes was identified in 100% of nuclei. Multiple chromosome fragmentation is a meiotic disaster and results in the degeneration of cells without enabling the mechanism ofpachytene arrest. The features of pachytene arrest were revealed in the nuclei of primary spermatocytes with the disturbances of chromosomes pairing. After the administration of sextaphage, circle structures released from the lateral elements of SC and are dyed with antibodies to SCP3 protein.

Assessment of cracks in lateral supports of the magnet system of Wendelstein 7-X

The superconducting coils of the magnet system of Wendelstein 7-X (W7-X) are bolted onto a central support ring and interconnected with five so-called lateral support elements (LSEs) per half module. After welding of the LSE hollow boxes to the coil cases cracks were found in the vicinity of the welds that could potentially limit the allowed number N of electromagnetic (EM) load cycles of the machine.In response to the appearance of first cracks during assembly, the stress intensity factor (SIF) of theoretical cracks of various sizes in potentially critical position and orientation were predicted in a fast approach. For each crack size, N was based on the SIF, derived from beam theory, and on Paris’ law parameters determined in fatigue crack growth rate (FCGR) tests, thus leading to tolerable maximal crack sizes and distances between cracks. It was proved that the actual crack dimensions remained below these values or turned out to be only superficial. Afterwards, (extended) finite element method (XFEM and FEM) and boundary element method (BEM) models were developed to project the SIF of most critical tolerated cracks, considering new FCGR tests and the local stress state in more detail. N appeared highly sensitive to the assumptions which were therefore critically reviewed.Finally, the limit for load combinations of different amplitudes was determined using Miner's rule. As a result it was shown that the predefined number of W7-X operation cycles is not jeopardized by any of the detected cracks.

Antagonistic Gene Activities Determine the Formation of Pattern Elements along the Mediolateral Axis of the Arabidopsis Fruit

The Arabidopsis fruit mainly consists of a mature ovary that shows three well defined territories that are pattern elements along the mediolateral axis: the replum, located at the medial plane of the flower, and the valve and the valve margin, both of lateral nature. JAG/FIL activity, which includes the combined functions of JAGGED (JAG), FILAMENTOUS FLOWER (FIL), and YABBY3 (YAB3), contributes to the formation of the two lateral pattern elements, whereas the cooperating genes BREVIPEDICELLUS (BP) and REPLUMLESS (RPL) promote replum development. A recent model to explain pattern formation along the mediolateral axis hypothesizes that JAG/FIL activity and BP/RPL function as antagonistic lateral and medial factors, respectively, which tend to repress each other. In this work, we demonstrate the existence of mutual exclusion mechanisms between both kinds of factors, and how this determines the formation and size of the three territories. Medial factors autonomously constrain lateral factors so that they only express outside the replum, and lateral factors negatively regulate the medially expressed BP gene in a non-autonomous fashion to ensure correct replum development. We also have found that ASYMMETRIC LEAVES1 (AS1), previously shown to repress BP both in leaves and ovaries, collaborates with JAG/FIL activity, preventing its repression by BP and showing synergistic interactions with JAG/FIL activity genes. Therefore AS gene function (the function of the interacting genes AS1 and AS2) has been incorporated in the model as a new lateral factor. Our model of antagonistic factors provides explanation for mutant fruit phenotypes in Arabidopsis and also may help to understand natural variation of fruit shape in Brassicaceae and other species, since subtle changes in gene expression may cause conspicuous changes in the size of the different tissue types.

Morphological manifestation of unique DNA segments in human meiotic prophase I

The commercial sample of human DNA fragment from the choromosome 17 was used as the probe for FISH to study of the mode of its attachment to the lateral elements of synaptonemal complex (SC) in human spermatocytes. It was a 160 kb probe from the band 17p1.2, containing RAI1 gene with D17S620 marker (the probe for deletion causing Smith-Magenis syndrome). The probe made lateral chromatin protrusions, contacting with SC stained with anty-SYCP3. Different morphological configuration of lateral chromatin protrusions where observed. They depended on substages of meiotic prophase I. At zygotene, FISH probe form two sticks, c. a. 6 micro long, which was perpendicular to SC longitudinal axe, one stick at each SC side. At early pachytene, each stick transforms into a globule, one globule at each SC side again. At late pachytene each globule transformed into two crumbly globules containing short threads and clumps. At diplotene, globules finally transformed into thin DNA (chromatin) loops up to 10 micro long from the base to top with periodical thickenings (beads) along their length. As the result of this dynamics of transformation, two chromatin loops with beads were found on each side of SC of the chromosome 17. These loops most probably were the loops of sister chromatides, the full set of chromatide loops at the particular SC (bivalent) site being four in number, i. e. representing of two pair of chromatides. This study is the first one in which lateral chromatin loops in human mail meiotic prophase I are visualized as true open loop instead of that usually postulated "loops" after observation of condensed road-like or brush-like chromatin protrusion attached to the lateral elements of synaptonemal complexes. Open configuration of the loops, presumably, depends on activation of transcription during late pachytene-early diplotene. They resemble lateral loops of mini lampbrush chromosomes.

Paleontological and developmental evidence resolve the homology and dual embryonic origin of a mammalian skull bone, the interparietal

The homologies of mammalian skull elements are now fairly well established, except for the controversial interparietal bone. A previous experimental study reported an intriguing mixed origin of the interparietal: the medial portion being derived from the neural crest cells, whereas the lateral portion from the mesoderm. The evolutionary history of such mixed origin remains unresolved, and contradictory reports on the presence or absence and developmental patterns of the interparietal among mammals have complicated the question of its homology. Here we provide an alternative perspective on the evolutionary identity of the interparietal, based on a comprehensive study across more than 300 extinct and extant taxa, integrating embryological and paleontological data. Although the interparietal has been regarded as being lost in various lineages, our investigation on embryos demonstrates its presence in all extant mammalian "orders." The generally accepted paradigm has regarded the interparietal as consisting of two elements that are homologized to the postparietals of basal amniotes. The tabular bones have been postulated as being lost during the rise of modern mammals. However, our results demonstrate that the interparietal consists not of two but of four elements. We propose that the tabulars of basal amniotes are conserved as the lateral interparietal elements, which quickly fuse to the medial elements at the embryonic stage, and that the postparietals are homologous to the medial elements. Hence, the dual developmental origin of the mammalian interparietal can be explained as the evolutionary consequence of the fusion between the crest-derived "postparietals" and the mesoderm-derived "tabulars."

Rumors of Its Disassembly Have Been Greatly Exaggerated: The Secret Life of the Synaptonemal Complex at the Centromeres

Rumors of Its Disassembly Have Been Greatly Exaggerated: The Secret Life of the Synaptonemal Complex at the Centromeres

Synaptonemal Complex Components Persist at Centromeres and Are Required for Homologous Centromere Pairing in Mouse Spermatocytes

Recent studies in simple model organisms have shown that centromere pairing is important for ensuring high-fidelity meiotic chromosome segregation. However, this process and the mechanisms regulating it in higher eukaryotes are unknown. Here we present the first detailed study of meiotic centromere pairing in mouse spermatogenesis and link it with key events of the G2/metaphase I transition. In mouse we observed no evidence of the persistent coupling of centromeres that has been observed in several model organisms. We do however find that telomeres associate in non-homologous pairs or small groups in B type spermatogonia and pre-leptotene spermatocytes, and this association is disrupted by deletion of the synaptonemal complex component SYCP3. Intriguingly, we found that, in mid prophase, chromosome synapsis is not initiated at centromeres, and centromeric regions are the last to pair in the zygotene-pachytene transition. In late prophase, we first identified the proteins that reside at paired centromeres. We found that components of the central and lateral element and transverse filaments of the synaptonemal complex are retained at paired centromeres after disassembly of the synaptonemal complex along diplotene chromosome arms. The absence of SYCP1 prevents centromere pairing in knockout mouse spermatocytes. The localization dynamics of SYCP1 and SYCP3 suggest that they play different roles in promoting homologous centromere pairing. SYCP1 remains only at paired centromeres coincident with the time at which some kinetochore proteins begin loading at centromeres, consistent with a role in assembly of meiosis-specific kinetochores. After removal of SYCP1 from centromeres, SYCP3 then accumulates at paired centromeres where it may promote bi-orientation of homologous centromeres. We propose that, in addition to their roles as synaptonemal complex components, SYCP1 and SYCP3 act at the centromeres to promote the establishment and/or maintenance of centromere pairing and, by doing so, improve the segregation fidelity of mammalian meiotic chromosomes.

Class 1 Integrons in Environments with Different Degrees of Urbanization

BackgroundClass 1 integrons are one of the most successful elements in the acquisition, expression and spread of antimicrobial resistance genes (ARG) among clinical isolates. Little is known about the gene flow of the components of the genetic platforms of class 1 integrons within and between bacterial communities. Thus it is important to better understand the interactions among “environmental” intI1, its genetic platforms and its distribution with human activities.Methodology/Principal FindingsAn evaluation of two types of genetic determinants, ARG (sul1 and qacE1/qacEΔ1 genes) and lateral genetic elements (LGE) (intI1, ISCR1 and tniC genes) in a model of a culture-based method without antibiotic selection was conducted in a gradient of anthropogenic disturbances in a Patagonian island recognized as being one of the last regions containing wild areas. The intI1, ISCR1 genes and intI1 pseudogenes that were found widespread throughout natural communities were not associated with urbanization (p>0.05). Each ARG that is embedded in the most common genetic platform of clinical class 1 integrons, showed different ecological and molecular behaviours in environmental samples. While the sul1 gene frequency was associated with urbanization, the qacE1/qacEΔ1 gene showed an adaptive role to several habitats.Conclusions/SignificanceThe high frequency of intI1 pseudogenes suggests that, although intI1 has a deleterious impact within several genomes, it can easily be disseminated among natural bacterial communities. The widespread occurrence of ISCR1 and intI1 throughout Patagonian sites with different degree of urbanization, and within different taxa, could be one of the causes of the increasing frequency of multidrug-resistant isolates that have characterized Argentina for decades. The flow of ARG and LGE between natural and clinical communities cannot be explained with a single general process but is a direct consequence of the interaction of multiple factors operating at molecular, ecological, phylogenetic and historical levels.

Response of Reinforced Concrete Buildings in Concepción during the Maule Earthquake

Detailed observations are reported for eight shear wall buildings from the Concepción region that experienced severe damage during the 27 February 2010 Chile earthquake. The repetitive nature of some of the damage suggests that these field observations may be applicable to similar buildings elsewhere, whereas other damage may be unique. Several shear walls experienced failures that apparently started at the boundaries due to the high compression in these unconfined edges, and propagated into the wall web. Other walls, including horizontal and vertical wall segments in perforated walls, experienced shear failure. Damage also was observed in columns, beams, and coupling slabs. In most cases, the percentage of damaged elements was less than 10% of the lateral force-resisting elements of the building, suggesting that these structures were not capable of distributing damage. Several building indices are calculated, including vibration periods and regularity indices, for comparison with observed behavior.

Synthesis and Characterization of (Na0.5K0.5)NbO3 (NKN) Thin Films Formed by a Diol-based Sol-gel Process

Lead-free (Na 0.5 K 0.5 )NbO 3 (NKN) thin films were fabricated by spin coating on Pt/Ti/SiO 2 /Si substrates by a diol-based sol-gel process. Na-acetate, K-acetate, Nb-pentaethoxide and 1,3 propanediol were used to prepare the NKN precursor solution. Thermal analysis showed two characteristic temperatures of 360 and 600 °C. Based on these temperatures, a heat treatment program with pyrolysis at 360 °C and calcination at 600 °C after every layer was used. To avoid inhomogeneities and secondary phases, an excess of sodium and potassium was necessary. To evaluate the proper excess amount of sodium and potassium secondary ion mass spectrometry (SIMS) lateral element maps and X-ray diffraction (XRD) patterns were recorded. An excess amount of 20% led to homogeneous distribution of the elements and to single phase perovskite NKN films with random crystal orientation. Scanning electron microscopy (SEM) images showed a pore free surface with 100 nm grains. The leakage current measurements showed a current of 1×10 −3 A/cm 2 at 150 kV/cm.

A Simple PCR Cloning Step to Utilize a Binary Vector for Identification and Characterization of Enhancer Element in Plant Promoters

Identification and characterization of regulatory elements, such as enhancers, require designing of specialized plant transformation vector to testify its unique features of position and orientation-independent core-promoter activation. Very few plant transformation vectors are available till date for identification of enhancer elements in plant promoters. Hence, we need to utilize the existing binary plant transformation vectors, such as pBI101, through tedious and time-consuming restriction enzyme-based cloning steps. We describe here a simple one-step PCR-based cloning strategy to introduce a 35S CaMV minimal promoter in a binary plant transformation vector pBI101, and utilize the unique features to identify a lateral organ junction tissue-specific enhancer element in a plant promoter from Arabidopsis thaliana.

Altered distribution of MLH1 foci is associated with changes in cohesins and chromosome axis compaction in an asynaptic mutant of tomato

In most multicellular eukaryotes, synapsis [synaptonemal complex (SC) formation] between pairs of homologous chromosomes during prophase I of meiosis is closely linked with crossing over. Asynaptic mutants in plants have reduced synapsis and increased univalent frequency, often resulting in genetically unbalanced gametes and reduced fertility. Surprisingly, some asynaptic mutants (like as1 in tomato) have wild-type or increased levels of crossing over. To investigate, we examined SC spreads from as1/as1 microsporocytes using both light and electron microscopic immunolocalization. We observed increased numbers of MLH1 foci (a crossover marker) per unit length of SC in as1 mutants compared to wild-type. These changes are associated with reduced levels of detectable cohesin proteins in the axial and lateral elements (AE/LEs) of SCs, and the AE/LEs of as1 mutants are also significantly longer than those of wild-type or another asynaptic mutant. These results indicate that chromosome axis structure, synapsis, and crossover control are all closely linked in plants.

A Comparative Analysis of the Mole Vole Sibling Species Ellobius tancrei and E. talpinus (Cricetidae, Rodentia) through Chromosome Painting and Examination of Synaptonemal Complex Structures in Hybrids

A comparative genomic analysis was carried out in the mole vole sibling species Ellobius tancrei and E. talpinus. Performing fluorescent in situ hybridisation (Zoo-FISH) using chromosome paints from the field vole Microtus agrestis showed no differences in the allocation of syntenic groups in the karyotypes of these sibling species. The only difference between their karyotypes was the position of the centromere in one pair of chromosomes, which is assumed to be the result of an inversion. To verify this hypothesis, we analysed chromosome synapsis in prophase I of meiosis. We utilised a synaptonemal complex (SC) surface-spreading technique to visualise the process of chromosome synapsis in the spermatocytes and oocytes of first-generation hybrids and back-crosses of these sibling species. In prophase I of meiosis, immunocytochemical and electron microscopy analyses revealed that all bivalents had been fully adjusted. Even in the case of a submetacentric-acrocentric bivalent with different centromere locations, synapsis of SC lateral elements was fulfilled along the entire length of the chromosomes and the formation of an inversion loop was not observed. We hypothesise that a possible mechanism leading to the change in centromere position is the repositioning and/or generation of a neocentromere. Despite the great similarity in the karyotypes of these sibling species, they exhibited significant genomic diversification, which manifested as hybrid sterility and parous female death.

Two New Species of Halicyclops (Crustacea: Copepoda: Cyclopidae) from Honshu with a Key to the Species of the Genus in Japan

Two new species of cyclopoid copepod, Halicyclops setiformis and H. itohi, are described based on female specimens from the Tama River estuary in Tokyo Bay, central Japan. One of the defining characteristics of H. setiformis is the presence of a setiform lateral element on the third exopodal segment of legs 3 and 4. Among its congeners, only H. blachei Lindberg, 1952 has similar elements; H. setiformis differs from it in the shape of the third endopodal segment of leg 4. This new species is considered carnivorous based on the occurrence of a specimen biting a nematode and the presence of stout, claw-like spines on the oral appendages. The second new species, Halicyclops itohi, is distinguishable from most of its congeners in the absence of lateral protuberances on the genital double-somite, the absence of a terminal accessory seta on the caudal ramus, and a 3.4.4.3 spine formula on the third exopodal segment of legs 1-4. Congeners sharing these three morphological characteristics differ variously from H. itohi in their caudal ramus shape, urosomal frills, antennal setal number, medial elements of the third endopodal segment of leg 4, and/or the shape of leg 5. The hitherto known Halicyclops species from Japan are briefly reviewed, and a key to the Japanese species is presented to resolve previous questionable identifications.

Synaptonemal Complex-Dependent Centromeric Clustering and the Initiation of Synapsis in Drosophila Oocytes

The pairing of homologous chromosomes and the intimate synapsis of the paired homologs by the synaptonemal complex (SC) are essential for subsequent meiotic processes including recombination and chromosome segregation. Here we show that the centromere clustering plays an important role in initiating homolog synapsis during meiosis in Drosophila females. Although centromeres are not clustered prior to the onset of meiosis, all four pairs of centromeres are actively clustered into one or two masses during early meiotic prophase. Within the 16-cell cyst, centromeric clustering appears to define the first step in the initiation of synapsis. Clustering is restricted to the nuclei that form the SC and is dependent on all known SC proteins. Surprisingly, both centromeric clusters and the SC components associated with them persist long after the disassembly of the euchromatic SC at the end of pachytene. The initiation of homologous recombination through the formation of programmed double-strand breaks (DSBs) is not required for either the formation or the maintenance of the centromeric clusters. Our data support a view in which the SC-mediated clustering at the centromeres is the initiating event for meiotic synapsis.