Abnormalities of spindle and cytokine behavior leading to the formation of meiotic restitution nuclei in intergeneric cereal hybrids

Mobile stages of meiosis have been analysed by visualizing the spindle in fertile cereal F1 hybrids. We describe four different mechanisms of the formation of restitution nuclei in meiotic division: (1) centripetal migration of telophase chromosome groups from the poles of a curved spindle at early telophase; (2) centripetal migration of the chromosome groups at late telophase when cell plate formation has failed; (3) preferable migration of univalents to one of the poles although spindle appearance is morphologically normal; and (4) in the absence of chromosome segregation where kinetochore fibers have failed to form.     

普通小麦与无芒雀麦不对称体细胞杂交的研究

普通小麦（ＴｒｉｔｉｃｕｍａｅｓｔｉｖｕｍＬ．）９９Ｐ原生质体与经紫外线照射的无芒雀麦（ＢｒｏｍｕｓｉｎｅｒｍｉｓＬｅｙｓ）愈伤组织来源的原生质体用ＰＥＧ法诱导融合。融合再生的３个单细胞克隆（Ｎｏ．１～Ｎｏ．３）经染色体、同工酶和ＲＡＰＤ分析表明,它们均为杂种细胞系。Ｎｏ．１克隆形成的愈伤组织分化出大量白化苗,其叶片同工酶和ＲＡＰＤ分析均出现双亲特征谱带及新带,白化苗的染色体数目为４２～５４,大于亲本小麦,并具有无芒雀麦的小染色体和染色体断片,确认它们为不对称体细胞杂种。     

Tree-shrub interactions in a subtropical savanna parkland: competition or facilitation?

Abstract. Prosopis glandulosa var. glandulosa has played a central role in the encroachment of woody plants in southern Texas, grasslands and savannas by acting as a nurse plant for various shrubs that establish in its understory. To test for continued facilitation of established understory shrubs by Prosopis and to determine if established shrubs compete with the Prosopis nucleus, selective removal experiments were conducted and monitored over a 2–5 yr period. Short-term (1–3 days) and long-term (2 yr) growth and physiological activities (midday net photosynthesis and leaf/shoot water potential) of two common understory shrubs, Zanthoxylum fagara and Berberis trifoliolata, growing with Prosopis, were generally comparable to those of individuals occurring in clusters where Prosopis was removed. Shrubs growing with an intact Prosopis occasionally showed significantly higher leaf-[N] and pre-dawn water potentials than those in clusters lacking a live Prosopis, especially under drought conditions; however, these differences did not translate into greater midday leaf gas exchange or shoot growth. By comparison, removal of understory shrubs elicited large increases in Prosopis net photosynthesis, annual trunk growth in each of the 5 yr monitored, and seed pod production in three of the four years monitored. Seven of 26 Prosopis plants in experimental clusters with an intact understory died over a 5-yr period, compared to only two of the 26 plants in clusters with the cleared understory. Results indicate that (1) the founding overstory Prosopis plant may continue to facilitate understory shrubs following their establishment, but these beneficial effects appear to be small and transitory, and (2) the understory shrubs have a pronounced negative effect on Prosopis, such that competition between overstory and understory woody plants is strongly asymmetrical. These findings suggest that understory shrubs will likely persist despite changes in microclimate and soils (potentially) that occur after the Prosopis plant, which facilitated their ingress or establishment, has died. Soil resource depletion by shallow-rooted understory shrubs appears to be a primary factor contributing to the demise of the deeply rooted, overstory Prosopis plants, especially on upland sites with duplex soils where below-ground competition is accentuated.     

Vitrification of calf oocytes: effects of maturation stage and prematuration treatment on the nuclear and cytoskeletal components of oocytes and their subsequent development

This study was designed to establish the effects of the meiotic stage of bovine oocytes and of a prematuration treatment with roscovitine (ROS) on their resistance to cryopreservation. Oocytes from prepubertal calves at the stages of germinal vesicle breakdown (GVBD) or at metaphase II (MII) were vitrified by the open pulled straw (OPS) method. In another experiment, oocytes were kept under meiotic arrest with 50 microM ROS for 24 hr and vitrified at the GVBD stage. After warming, some oocyte samples were fixed, stained using specific fluorescent probes and examined under a confocal microscope. The remaining oocytes were fertilized, and cleavage and blastocyst rates recorded. Significantly lower cleavage rates were obtained for the vitrified GVBD and MII oocytes (9.9% and 12.6%, respectively) compared to control oocytes (73.9%). Significantly worse results in terms of cleavage rates were obtained when GVBD calf oocytes were exposed to cryoprotectants (CPAs: ethylene glycol plus dimethyl sulfoxide, DMSO) (13.1%) or vitrified (1.6%) after a prematuration treatment with ROS, when compared to untreated control oocytes (68.7%) or ROS-control oocytes (56.6%). None of the vitrification procedures yielded blastocysts, irrespective of the initial meiotic stage or previous prematuration treatment. Compared to the control oocytes, significantly fewer oocytes exhibited normal spindle configuration after being exposed to CPAs or after vitrification of either GVBD or MII calf oocytes. These results indicate that the vitrification protocol has a deleterious effect on the meiotic spindle organization of calf oocytes cryopreserved at both the GVBD and MII stage, which impairs the capacity for further development of the embryos derived from these vitrified oocytes. Prematuration treatment with ROS has no beneficial effect on the outcome of vitrification by the OPS method.     

The roles of microtubule-based motor proteins in mitosis: comprehensive RNAi analysis in the Drosophila S2 cell line

Kinesins and dyneins play important roles during cell division. Using RNA interference (RNAi) to deplete individual (or combinations of) motors followed by immunofluorescence and time-lapse microscopy, we have examined the mitotic functions of cytoplasmic dynein and all 25 kinesins in Drosophila S2 cells. We show that four kinesins are involved in bipolar spindle assembly, four kinesins are involved in metaphase chromosome alignment, dynein plays a role in the metaphase-to-anaphase transition, and one kinesin is needed for cytokinesis. Functional redundancy and alternative pathways for completing mitosis were observed for many single RNAi knockdowns, and failure to complete mitosis was observed for only three kinesins. As an example, inhibition of two microtubule-depolymerizing kinesins initially produced monopolar spindles with abnormally long microtubules, but cells eventually formed bipolar spindles by an acentrosomal pole-focusing mechanism. From our phenotypic data, we construct a model for the distinct roles of molecular motors during mitosis in a single metazoan cell type.     

Distribution of SERCA isoforms in human intrafusal fibers

The sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) is a membrane protein that plays a crucial role in muscle relaxation by transporting cytosolic Ca²⁺ into the lumen of the sarco/endoplasmic reticulum. In this study, the presence of SERCA1 and SERCA2 was investigated in human intrafusal fibers by immunocytochemistry. Nuclear bag₁ fibers contained both SERCA1 and SERCA2 isoforms, with predominant staining seen with SERCA2 in the A and B regions. Most nuclear bag₂ fibers also contained SERCA1 and SERCA2 isoforms and their coexistence frequently occurred in the A region. SERCA1 was present whereas SERCA2 was generally absent in the nuclear chain fibers. The staining intensity seen with the SERCA1 monoclonal antibody varied in the order of chain>bag₁>bag₂. The expression of SERCA1 isoform was found to correlate with the presence of fast myosin heavy chain (MyHC) isoform in nuclear chain fibers, whereas for nuclear bag fibers there was no such apparent correlation between patterns of expression of SERCA and MyHC isoforms. The phenotype revealed for the human bag fibers was very sophisticated and adapted to attain a very wide range of contraction and relaxation velocities.     

Prospero and Snail expression during spider neurogenesis

Analysis of early neurogenesis in the spider Cupiennius salei (Chelicerata, Aranea, Ctenidae) has shown that the cells of the central nervous system are recruited from clusters of cells that invaginate from the neuroectoderm. This is in contrast to Drosophila, where only single cells delaminate and become neuroblasts, the stem cells of the nervous system. In order to compare the processes further, we have cloned homologues of the pan-neural Drosophila genes prospero and snail from the spider and have analysed their RNA and protein expression pattern. We find that snail expression is transient and only a subset of neural cells expresses Snail protein at any given time, making it difficult to assess whether it is indeed a pan-neural gene in the spider. Prospero protein expression, on the other hand, is seen in all invaginating cells and continues throughout differentiation of the neurons. In contrast to Drosophila, asymmetric localization cannot be detected, even in cells that still divide. Our results provide no evidence for neuroblasts or stem cells in the spider, although there are a limited number of mitoses in the cells that are derived from the invaginating clusters. These aspects of spider neurogenesis are more similar to the neurogenesis process known from vertebrates.Edited by P. Simpson     

Meiosis in male PL/J mice: a genetic model for gametic aneuploidy

Sperm from mice of the PL/J strain have a high frequency of sperm-head morphology abnormalities. Fluorescence in situ hybridization (FISH) methods revealed that PL/J sperm are also characterized by a high frequency of aneuploidy. The traits of abnormal sperm head morphology and aneuploidy are associated with numerous meiotic abnormalities. Spermatocytes of PL/J mice exhibit chromosome asynapsis during meiotic prophase as well as reduced crossing over, revealed by analysis of both MLH1 foci in pachytene spermatocytes and chiasmata seen at the first meiotic metaphase. During the first meiotic division, roughly one-third of the PL/J spermatocytes exhibit aberrant spindle morphology, with abnormalities including monopolar spindles, split spindle poles, and incomplete spindle formation and centrosomal abnormalities. F1 progeny of a cross between PL/J and C57BL/6J did not exhibit a high frequency of either sperm aneuploidy or sperm head morphology aberrations, as would be expected if the PL/J traits were dominant. Among progeny of a backcross of F1 mice to PL/J, none of 16 males assessed exhibited elevated frequencies of sperm head morphology abnormalities. Four of the individuals exhibited elevated sperm aneuploidy, but not at the levels of the PL/J parents. Thus, it is likely that the aberrant PL/J traits are due to several genes and/or modifiers affecting the generation of both sperm aneuploidy and abnormal sperm head morphology.     

Patch leaving strategies and superparasitism: an asymmetric generalized war of attrition

When several competitors deplete a patch, it can be advantageous for each of them to stay provided that others leave, whereas, on the other hand, staying longer decreases the expected payoff for everyone. This situation can be considered as a generalized war of attrition. Previous studies have shown that optimal patch leaving strategies become stochastic and the expected leaving time is much larger than predicted by the marginal value theorem when competitors interfere. The possibility of superparasitism, as occurs for example in parasitoids, induces such interference. In addition, it gives several complications. First, the payoff of females that have left the patch is affected by the ovipositions of the remaining individuals. Second, differences in the arrival time of females cause payoff-relevant asymmetries, since females that arrived early on have parasitized more hosts in a patch at the moment superparasitism starts than those that arrived later. We show that this can be modelled as an asymmetric generalized war of attrition, and derive global characteristics of the ESS for simultaneous decisions on when to start superparasitism and when to leave a patch.     

Ca2+ signal blockers can inhibit M/A transition in mammalian cells by interfering with the spindle checkpoint

A key step in mitosis is the sister-chromatid separation at the metaphase-anaphase (M/A) transition. Several earlier studies had suggested that Ca(2+) signal is involved in regulating this process in somatic cells. The detailed mechanisms, however, are not yet well understood. In this study, we used the GFP-gene fusion method and a living-cell imaging technique to examine the effects of suppressing cytosolic Ca(2+) level on the mitotic process in HeLa and PtK2 cells. We observed that application of the Ca(2+) chelator BAPTA/AM can block or severely delay the M/A transition. This blockage was caused by a failure in activating the anaphase-promoting complex (APC), since both cyclin B and securin could not be degraded under this situation. Furthermore, using YFP-labeled tubulin, we found that the mitotic spindle structure in most of the BAPTA-treated cells gradually deformed with time. Other Ca(2+) signal blockers, such as heparin, also produced a similar effect. These results suggest that one pathway for the blockage of M/A transition by suppressing cytosolic Ca(2+) level is due to its interference with the mitotic spindle checkpoint.