Malorientation in half-bivalents at anaphase: analysis of autosomal laggards in untreated, cold-treated, and cold-recovering crane fly spermatocytes

Exposing crane fly larvae to 6 degrees C or returning them to 22 degrees C after exposure to 6, 2, or 0.2 degrees C can induce any number of autosomes in their primary spermatocytes to lag near the spindle equator at anaphase. Autosomal laggards in cold-recovering cells are contained in bivalents until anaphase (Janicke, M. A., and J. R. LaFountain, 1982, Chromosoma, 85:619-631). We report here documentation that lagging autosomes in cold-treated and cold- recovering cells are maloriented. During meiosis I, half-bivalents usually associate with only one pole via kinetochore fibers, with sister chromatids being oriented to the same pole. In contrast, laggards had kinetochore microtubules (kMTs) extending from them toward both poles: one sister was oriented to one pole and the other had some or all of its kMTs extending toward the opposite pole. Bipolar malorientation of autosomal laggards also was observed in one untreated cell. The number of kMTs per half-bivalent was similar in lagging and non-lagging autosomes, and those kMTs were contained in long birefringent kinetochore fibers. The overall spindle structure in cold- recovering cells was similar to that observed in untreated anaphase cells. Giemsa-stained centromeric dots of sister chromatids were contiguous in non-laggards and separated in laggards at anaphase. We conclude that bipolar malorientations can exist at anaphase in chromosomes that remain paired until anaphase, that cold recovery increases the frequency of that anomaly, and that such malorientations may be one cause of anaphase lag.     

Malorientation in half-bivalents at anaphase in crane fly spermatocytes following Colcemid treatment

Addition of Colcemid to the medium in which larvae of the crane fly Nephrotoma suturalis are cultivated induces a number of anomalous patterns of chromosome segregation. One of these is the anaphase lagging of autosomal half-bivalents. To investigate the cause of anaphase lagging, the orientation of sister kinetochores in Colcemidtreated spermatocytes having lagging half-bivalents was analyzed in serial sections. In contrast to nonlaggard halfbivalents that had pure syntelic orientation (sister kinetochores having all of their kinetochores microtubules (KMTs) extending to the same pole), six of the seven autosomal laggards that were selected for analysis had kinetochores with either amphitelic orientation (sister kinetochores each with a bundle of KMTs extending to opposite poles) or merotelic orientation (a single kinetochore having KMTs extending toward both poles). An additional laggard had syntelic orientation but two of the microtubules that were in its kinetochore fiber passed through the kinetochore and extended beyond it toward the equator. The bipolar malorientations observed in anaphase half-bivalents are interpreted to be a cause of the anaphase lagging induced by Colcemid treatment. Furthermore, it is hypothesized that such bipolar malorientations also may be stabilized at metaphase and thus explain the unusual tilting of metaphase bivalents commonly observed in Colcemid-treated cells.     

The X chromosome frequently lags behind in female lymphocyte anaphase

Pancentromeric FISH and X-chromosome painting were used to characterize anaphase aberrations in 2,048 cultured lymphocytes from a healthy 62-year-old woman. Of 163 aberrant anaphases, 66.9% contained either chromosomes or their fragments that lagged behind. Characterization of 200 laggards showed that 49% were autosomes, 33. 5% were autosomal fragments, and 17.5% were X chromosomes. The X chromosome represented one-fourth of all lagging chromosomes and was involved much more often than would be expected by chance (1/23). Labeling of the late-replicating inactive X chromosome with 5-bromo-2'-deoxyuridine revealed that both X homologues contributed equally to the laggards. Among 200 micronuclei examined from interphase cells, the proportion of the X chromosome (31%) and autosomal fragments (50%) was higher than among anaphase laggards, whereas autosomes were involved less often (19%). These findings may reflect either selection or the fact that lagging autosomes, which were more proximal to the poles than were lagging X chromosomes, were more frequently included within the main nucleus. Our results suggest that the well-known high micronucleation and loss of the X chromosome in women's lymphocytes is the result of frequent distal lagging behind in anaphase and effective micronucleation of this chromosome. This lagging appears to affect the inactive and active X chromosomes equally.     

Influence of autosome movements and of sex-chromosome movements on sex-chromosome segregation in crane fly spermatocytes

In crane fly spermatocyte meiosis 3 autosome half-bivalents normally move to each spindle pole in anaphase while the 2 amphitelic sex-chromosome univalents remain at the equator. The sex-chromosome univalents move to opposite poles after the autosomes reach the poles. — We used micromanipulation to detach half-bivalents in anaphase. When re-attached half-bivalents were syntelically oriented to the original pole, sex-chromosome segregation was usually not altered. When re-attached half-bivalents were amphitelically oriented, sex-chromosome segregation was usually altered: usually the amphitelic autosome segregated against one sex-chromosome while the other sex-chromosome remained at the equator. When re-attached half-bivalents were syntelically oriented to the opposite pole, sex-chromosome segregation was often altered: often one sex-chromosome moved normally to the spindle pole with 2 autosomal half-bivalents, while the other sex-chromosome did not move to the spindle pole with 4 autosomal half-bivalents, but remained at the equator. — The direction of motion of a sex-chromosome could be altered even after sex-chromosome segregation had begun, by suitable micromanipulation of the other sex-chromosome. — Amphitelic chromosomes that were not on the equator at the start of anaphase segregated predominantly to the closer spindle pole. Detached half-bivalents showed no preference for the closer pole when they re-attached with syntelic orientation. — We discuss some possible hypotheses for non-independent movements, and some implications of the results.     

Malorientation and abnormal segregation of chromosomes during recovery from colcemid and nocodazole

Reversal of meiotic arrest in crane-fly spermatocytes by U. V. irradiation of Colcemid-arrested cells or by rinsing Nocodazole-arrested cells in fresh buffer results in the induction of chromosome malorientation. Malorientations observed among Colcemid-recovering and Nocodazole-recovering spermatocytes at frequencies higher than normally observed in untreated cells included associations of sister kinetochores of half-bivalents with both spindle poles (amphitely), in contrast with associations of sisters with only one pole (syntely) as is usually found during the first meiotic division. In several cases, prior to anaphase onset, maloriented bivalents appeared unusually tilted with respect to the spindle axis, and during anaphase they gave rise to laggard half-bivalents that did not segregate during anaphase along with half-bivalents having proper syntelic orientation. The results parallel previous findings obtained during cold recovery, and the properties of the drugs used here suggest that their action on microtubules, although reversible, induces malorientation during recovery from meiotic arrest.     

Spermatogenesis in the aphid Amphorophora tuberculata (Homoptera,Aphididae)

Spermatogenesis was studied in Amphorophora tuberculata Brown & Blackman, a species of aphid with n=2. Spermatogonia have 2=3 (AA+XO). In early prophase I the autosomal homologues are united terminally to form a tandem bivalent. No evidence could be found of synapsis or of the formation and terminalisation of chiasmata. The terminal connection of the autosomes is retained until late in prophase II. Sister chromatids separate, and autosomal half-bivalents move apart at anaphase I, but the division is incomplete, the X chromosome forming a thin chromatin bridge between the two autosomal half-bivalents. In prophase II the autosomal half-bivalents double back on themselves, so that non-sister chromatids become aligned in parallel. The X chromosome then becomes associated with one of the autosomal half-bivalents. Anaphase II separates the non-sister chromatids, and meiosis is thus post-reductional.     

Direct visualization of microtubule flux during metaphase and anaphase in crane-fly spermatocytes

Microtubule flux in spindles of insect spermatocytes, long-used models for studies on chromosome behavior during meiosis, was revealed after iontophoretic microinjection of rhodamine-conjugated (rh)-tubulin and fluorescent speckle microscopy. In time-lapse movies of crane-fly spermtocytes, fluorescent speckles generated when rh-tubulin incorporated at microtubule plus ends moved poleward through each half-spindle and then were lost from microtubule minus ends at the spindle poles. The average poleward velocity of approximately 0.7 microm/min for speckles within kinetochore microtubules at metaphase increased during anaphase to approximately 0.9 microm/min. Segregating half-bivalents had an average poleward velocity of approximately 0.5 microm/min, about half that of speckles within shortening kinetochore fibers. When injected during anaphase, rhtubulin was incorporated at kinetochores, and kinetochore fiber fluorescence spread poleward as anaphase progressed. The results show that tubulin subunits are added to the plus end of kinetochore microtubules and are removed from their minus ends at the poles, all while attached chromosomes move poleward during anaphase A. The results cannot be explained by a Pac-man model, in which 1) kinetochore-based, minus end-directed motors generate poleward forces for anaphase A and 2) kinetochore microtubules shorten at their plus ends. Rather, in these cells, kinetochore fiber shortening during anaphase A occurs exclusively at the minus ends of kinetochore microtubules.     

An actin-like component in spermatocytes of a crane fly (Nephrotoma suturalis Loew)

Decorated actin-like filaments were seen in spindles after crane fly spermatocytes were glycerinated and then treated with rabbit skeletal muscle heavy meromyosin (HMM). Both ATP and pyrophosphate inhibited the HMM reaction. In prometaphase, metaphase, and mid-anaphase cells, actin-like filaments were seen near regions where chromosomal spindle fibres are seen in living cells, and were oriented in the pole-to-pole direction. In the interzone of anaphase cells, actin-like filaments were not oriented in a preferential direction when they were not associated with the microtubules attached to the sex chromosomes. No filaments were seen in glycerinated spindles not treated with HMM. We discuss reasons why filaments might not be seen without prior HMM treatment, and we discuss the possible role of the actin-like filaments in the spindles. — Spindle microtubules often were not seen in cells treated with HMM. This depended on the stage of division: in prometaphase no microtubules were seen; in metaphase microtubules were seen, in apparently normal numbers; in mid-anaphase, microtubules between the autosomes and the poles were seen in reduced numbers, those associated with the equatorial sex-chromosomes were seen in apparently normal numbers, while those between the separating autosomal half-bivalents were not seen. Microtubules were not seen in glycerinated spindles not treated with HMM, suggesting that HMM in some way affects microtubule stability. The question of microtubule stability is briefly discussed.     

Chromator is required for proper microtubule spindle formation and mitosis in Drosophila

The chromodomain protein, Chromator, has been shown to have multiple functions that include regulation of chromatin structure as well as coordination of muscle remodeling during metamorphosis depending on the developmental context. In this study we show that mitotic neuroblasts from brain squash preparations from larvae heteroallelic for the two Chromator loss-of-function alleles Chro⁷¹ and Chro⁶¹² have severe microtubule spindle and chromosome segregation defects that were associated with a reduction in brain size. The microtubule spindles formed were incomplete, unfocused, and/or without clear spindle poles and at anaphase chromosomes were lagging and scattered. Time-lapse analysis of mitosis in S2 cells depleted of Chromator by RNAi treatment suggested that the lagging and scattered chromosome phenotypes were caused by incomplete alignment of chromosomes at the metaphase plate, possibly due to a defective spindle-assembly checkpoint, as well as of frayed and unstable microtubule spindles during anaphase. Expression of full-length Chromator transgenes under endogenous promoter control restored both microtubule spindle morphology as well as brain size strongly indicating that the observed mutant defects were directly attributable to lack of Chromator function.     

Midzone organization restricts interpolar microtubule plus‐end dynamics during spindle elongation

To study the dynamics of interpolar microtubules (iMTs) in Saccharomyces cerevisiae cells, we photobleached a considerable portion of the middle region of anaphase spindles in cells expressing tubulin‐green fluorescent protein (GFP) and followed fluorescence recovery at the iMT plus‐ends. We found that during anaphase, iMTs show phases of fast growth and shrinkage that are restricted to the iMT plus‐ends. Our data indicate that iMT plus‐end dynamics are regulated during mitosis, as fluorescence recovery was faster in intermediate anaphase (30 s) compared with long (100 s) and pre‐anaphase (80 s) spindles. We also observed that deletion of Cin8, a microtubule‐crosslinking kinesin‐5 motor protein, reduced the recovery rate in anaphase spindles, indicating that Cin8 contributes to the destabilization of iMT plus‐ends. Finally, we show that in cells lacking the midzone organizing protein Ase1, iMTs are highly dynamic and are exchangeable throughout most of their length, indicating that midzone organization is essential for restricting iMT dynamics.     

The induction of orientational instability and bivalent interlocking at meiosis

The administration of 40° C heat-treatments was found to induce bivalent orientational instability and interlocking at male meiosis in the locust Locusta migratoria. Only the longest members of the complement showed orientational instability and these usually possessed single distally sited chiasmata, with near-maximal intercentromeric distances. An effect on the stability of spindle fibre microtubule association, or attachment to the chromosome, is considered to be a possible explanation of the behaviour found. Bipolar orientation was generally achieved prior to anaphase I so that chromosome segregation was usually normal. Diamphitelic bivalents provided the most common exception to this rule. They sometimes lagged at anaphase, with the separation of half-bivalents and the production of structures indistinguishable from lagging univalents. The bivalent interlocking also involved the longest members of the complement. Most combinations of rod/rod, rod/ring and ring/ring types of interlocking were found. Usually only two bivalents were interlocked in any one cell, although occasionally three were found interlocked. All types appeared to involve an effect on the regulation of chromosome pairing, although at least one of the cells found showed interlocking caused by the metaphase orientational instability. In most cells, interlocked bivalents showed stable orientation and this usually involved the unipolar orientation of each bivalent's two centromeres. Such configurations provide concrete support for the importance of physical tension in the maintenance of metaphase orientational stability. They lead to double non-disjunction at anaphase I. Interlocked bivalents showed normal congression to a mid-equatorial position with no tendency for the re-adjustment of arm ratios to equalise centromere distances from the poles. This behaviour is discussed in relation to spindle fibre dynamics and it is concluded that no hypothesis of congression currently available can satisfactorily explain all that we know of the behaviour of univalents, bivalents, multivalents and interlocked bivalents.     

Cytochalasin induces abnormal anaphase in crane-fly spermatocytes and causes altered distribution of actin and centromeric antigens

Inhibition of cytokinesis by cytochalasins without an effect on karyokinesis has been demonstrated in several types of cells. We report here that treating crane-fly spermatocytes with cytochalasins at concentrations (10 M CE, 100 M CD, and 200 CB) in excess of that needed to inhibit cell division induces one or more half-bivalents to lag at anaphase during the first meiotic division. The behavior of the laggards is similar to that of maloriented half-bivalents. Following treatment at these concentrations, probing with rhodamine-phalloidin or bodipy-phallacidin reveals loss of filamentous actin from the poles and its appearance in the spindle, predominantly in regions where centromeres and kinetochores are normally found. When either N350 anti-actin monoclonal antibody or rhodamine DNase I was used to probe for actin in cytochalasin-treated cells, a similar redistribution of actin was observed. CD and CE treatments alter the pattern of fluorescence at centromere/kinetochore regions after staining with scleroderma CREST serum: CREST-positive structures become broader, with spikes extending from them toward the pole; in addition, some strands of CREST fluorescence appear that are apparently extraneous, and not associated with chromosomes. Probes for actin yield staining patterns in centromere/kinetochore regions that match closely the cytochalasin-altered pattern of CREST staining. Our finding of actin in the vicinity of kinetochores under conditions that result in abnormal chromosome behavior raises numerous questions about the possible role(s) of actin in meiosis, particularly in chromosome orientation.Abbreviations CREST calcinosis, Raynaud's phenomenon, esophageal dysmotility, sclerodactyly, telangiectasia by W.C. Earnshaw     

Microtubules of the kinetochore fiber turn over in metaphase but not in anaphase

In previous work we injected mitotic cells with fluorescent tubulin and photobleached them to mark domains on the spindle microtubules. We concluded that chromosomes move poleward along kinetochore fiber microtubules that remain stationary with respect to the pole while depolymerizing at the kinetochore. In those experiments, bleached zones in anaphase spindles showed some recovery of fluorescence with time. We wished to determine the nature of this recovery. Was it due to turnover of kinetochore fiber microtubules or of nonkinetochore microtubules or both? We also wished to investigate the question of turnover of kinetochore microtubules in metaphase. We microinjected cells with x- rhodamine tubulin (x-rh tubulin) and photobleached spindles in anaphase and metaphase. At various times after photobleaching, cells were detergent lysed in a cold buffer containing 80 microM calcium, conditions that led to the disassembly of almost all nonkinetochore microtubules. Quantitative analysis with a charge coupled device image sensor revealed that the bleached zones in anaphase cells showed no fluorescence recovery, suggesting that these kinetochore fiber microtubules do not turn over. Thus, the partial fluorescence recovery seen in our earlier anaphase experiments was likely due to turnover of nonkinetochore microtubules. In contrast fluorescence in metaphase cells recovered to approximately 70% the control level within 7 min suggesting that many, but perhaps not all, kinetochore fiber microtubules of metaphase cells do turn over. Analysis of the movements of metaphase bleached zones suggested that a slow poleward translocation of kinetochore microtubules occurred. However, within the variation of the data (0.12 +/- 0.24 micron/min), it could not be determined whether the apparent movement was real or artifactual.     

Temperature dependency of induction of sexual agglutinability by α pheromone in the yeast Saccharomyces cerevisiae

When pheromone-pretreated cells of an inducible a strain of Saccharomyces cerevisiae carrying the inducible gene saa1 were incubated in a growth medium at 28°C, induction of sexual agglutinability began after a 10 min lag period. If the cells were incubated at 38°C during the lag period, no induction occurred even after incubation at 28°C. Contrary to this, if the cells were incubated at 28°C during the lag period, almost complete induction occurred, even after transfer to 38°C. Temperature shift experiments revealed that 5 min incubation at 28°C was necessary for the initiation of the temperature-sensitive period and further 5 min incubation for the completion of the period. The temperature-sensitive period was sensitive to phenylmethylsulfonyl fluoride.Non-common abbreviations PBS 10^-2 M phosphate buffer solution, pH 5.5 - PMSF phenylmethylsulfonyl fluoride     

Mechanism of action of cholera toxin: Studies on the lag period

Summary The lag period for activation of adenylate cyclase by choleragen was shorter in mouse neuroblastoma N18 cells than in rat glial C6 cells. N18 cells have 500-fold more toxin receptors than C6 cells. Treatment of C6 cells with ganglioside G_M1 increased the number of toxin receptors and decreased the lag phase. Choleragen concentration also effected the lag phase, which increased as the toxin concentration and the amount of toxin bound decreased. The concentration, however, required for half-maximal activation of adenylate cyclase depended on the exposure time; at 1.5, 24, and 48 hr, the values were 200, 1.1., and 0.35pm, respectively. Under the latter conditions, each cell was exposed to 84 molecules of toxin.The length of the lag period was temperature-dependent. When exposed to choleragen at 37, 24, and 20 °C, C6 cells began to accumulate cyclic AMP after 50, 90, and 180 min, respectively. In G_M1-treated cells, the corresponding times were 35, 60, and 120 min. Cells treated with toxin at 15 °C for up to 22 hr did not accumulate cAMP, whereas above this temperature they did. Antiserum to choleragen, when added prior to choleragen, completely blocked the activation of adenylate cyclase. When added after the toxin, the antitoxin lost its inhibitory capability in a time and temperature-dependent manner. Cells, however, could be preincubated with toxin at 15 °C, and the antitoxin was completely effective when added before the cells were warmed up. Finally, cells exposed to choleragen for >10 min at 37 °C accumulated cyclic AMP when shifted to 15 °C. Under optimum conditions at 37°C, the minimum lag period for adenylate cyclase activation in these cells was 10 min. These findings suggest that the lag period for cholerage action represents a temperature-dependent transmembrane event, during which the toxin (or its active component) gains access to adenylate cyclase.Abbreviations used: ganglioside nomenclature according to Svennerholm [32] (see Table 1 for structures) cAMP adenosine 35-monophosphate - MIX 3-isobutyl-1-methylxanthine - HEPES N-2-hydroxyethylpiperazine-N-2-ethane sulfonic acid - PBS phosphate-buffered saline (pH 7.4)     

Improvement of phytase thermostability by using sorghum liquor wastes supplemented with starch

When a phytase solution, soluble starch, and sorghum liquor wastes were mixed at the ratio of 1:1:10 (v/w/w), the residual phytase activities after 30 min of treatment at 70 and 80 °C were respectively, about 90% and 18% of that at 37 °C. After 10 min treatment, the residual activity was 67% at 80 °C and 10% at 90 °C.     

The effect of hypothermic treatment on the repair or expression of X-ray damage measured in split dose experiments on L5178Y-S cells

Summary The nature of the post-irradiation lesions and processes leading to cellular reproductive death or survival were investigated in mouse lymphoblastic leukemia L5178Y-S (LY-S) cells. Post-(x-)irradiation incubation at 25° C protects LY-S cells against the fixation of biologically expressed damage which takes place at 37° C. An optimal condition for the repair of damage, assayed in split-dose experiments as split-dose recovery (SDR), is 1 h at 37° C followed by 4 h holding at 25° C prior to the second half of a split dose, or 5 h holding at 25° C without a 37° C incubation during the interval between doses. Longer incubations at 37° C resulted in progressively decreased survivals. Postirradiation inhibition of DNA synthesis at 37° C was observed only during the first 30 min; thereafter,³H-dThdR incorporation washigher than in unirradiated controls. Theexcess synthesis effect was removed by shifting irradiated cells to 25° C holding. The inhibition observed at 25° C was reversed by shifting to 37° C. Thus the degree of postirradiation DNA synthesis is inversely related to SDR. DNA filter elution shows complete strand break repair by 20 min at 37° C, and by 3 h at 25° C; DNA double-strand break (DSB) repair plateaus at 80% (37° C) and 60% (25° C) after 90 min. An inverse correlation was found between total strand break repair rate, as assayed by filter elution methods, and cell survival. This work was supported by a grant from The Mathers Charitable Foundation.A preliminary report of this work was presented at the 35th Annual Meeting of the Radiation Research Society, Atlanta, GA 1987, USA     

The HECT E3 ligase Smurf2 is required for Mad2-dependent spindle assembly checkpoint

Activation of the anaphase-promoting complex/cyclosome (APC/C) by Cdc20 is critical for the metaphase–anaphase transition. APC/C-Cdc20 is required for polyubiquitination and degradation of securin and cyclin B at anaphase onset. The spindle assembly checkpoint delays APC/C-Cdc20 activation until all kinetochores attach to mitotic spindles. In this study, we demonstrate that a HECT (homologous to the E6-AP carboxyl terminus) ubiquitin ligase, Smurf2, is required for the spindle checkpoint. Smurf2 localizes to the centrosome, mitotic midbody, and centromeres. Smurf2 depletion or the expression of a catalytically inactive Smurf2 results in misaligned and lagging chromosomes, premature anaphase onset, and defective cytokinesis. Smurf2 inactivation prevents nocodazole-treated cells from accumulating cyclin B and securin and prometaphase arrest. The silencing of Cdc20 in Smurf2-depleted cells restores mitotic accumulation of cyclin B and securin. Smurf2 depletion results in enhanced polyubiquitination and degradation of Mad2, a critical checkpoint effector. Mad2 is mislocalized in Smurf2-depleted cells, suggesting that Smurf2 regulates the localization and stability of Mad2. These data indicate that Smurf2 is a novel mitotic regulator.     

Effects of repeated short-term cold exposures on cold induced thermogenesis of women

The effects of repeated exposures to resting cold air (10°C) on the shivering and thermogenic responses of women to standard cold stress were investigated. Ten women, aged 18 to 34 years, were divided into two groups of five women each. One group, the acclimated (A) was exposed ten times within 2 weeks, the first and the last exposures being the pre-and post-tests respectively. The second group, the control (C) was exposed twice within 18 days. Measurements of rectal and skin temperatures, oxygen uptake, time to onset of shivering (TOS), and perceived cold were performed during all exposures. Shivering responses were evaluated by electromyography and visually. A significant (P<0.05), increase was seen in TOS (from 26.2 min to 55.6 min), and a significant decrease was seen in thermoregulatory heat production (from 14.78 kcal/h to –2.64 kcal/h) in group A; these changes were evident after about five exposures. It is concluded that the women became cold acclimated as a result of the repeated short-term resting cold air exposures.Research supported by Capes/Brazil, and by the Universidade Federal de Minas Gerais/Brazil     

On the mechanism of prometaphase congression: Chromosome velocity as a function of position on the spindle

Rates of movement of univalents at prometaphase and of half-bivalents at anaphase in living cricket and grasshopper spermatocytes were determined as a function of the distance from the pole toward which the movement was directed. In the artificially produced univalents of cricket cells, correlation coefficients for rate versus distance form the pole were widely disparate from movement to movement and there was no consistent relationship between velocity and distance from the pole. However, in the naturally occurring univalents of grasshopper cells, there was a significant positive correlation between velocity and distance from the pole. In both cricket and grasshopper cells, there was no consistent correlation between rate of movement and distance from the pole for half-bivalents at anaphase. The prometaphase data from grasshopper cells support the simple hypothesis of Östergren (1950) that congression results from the application to chromosomes of forces which increase with increasing distance from the pole. Furthermore, these data are consistent with models of force production which suppose that the relationship between force (reflected as velocity) and distance from the pole is a linear one.