Meiosis-I in Mesostoma ehrenbergii spermatocytes includes distance segregation and inter-polar movements of univalents, and vigorous oscillations of bivalents
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- Ferraro-Gideon, J., Hoang, C. & Forer, A. Protoplasma (2014) 251: 127. doi:10.1007/s00709-013-0532-9
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In this article, we describe meiosis-I in spermatocytes of the free-living freshwater flatworm Mesostoma ehrenbergii. The original observations of Oakley (1983, 1985) and Fuge (Eur J Cell Biol 44:294–298, 1987, Cell Motil Cytoskeleton 13:212–220, 1989, Protoplasma 160:39–48, 1991), the first to describe these cells, challenge our understanding of cell division, and we have expanded on these descriptions with the aim of laying the framework for further experimental work. These cells contain three bivalents and four univalent chromosomes (two pairs). Bivalent kinetochores oscillate vigorously and regularly throughout prometaphase, for up to several hours, until anaphase. Anaphase onset usually begins in the middle of the kinetochore oscillation cycle. Precocious cleavage furrows form at the start of prometaphase, ingress and then remain arrested until the end of anaphase. The four univalents do not pair, yet by anaphase there is one of each kind at each pole, an example of “distance segregation” (Hughes-Schrader in Chromosoma 27:109–129, 1969). Until proper segregation is achieved, univalents move between spindle poles up to seven times in an individual cell; they move with velocities averaging 9 μm/min, which is faster than the oscillatory motions of the bivalent kinetochores (5–6 μm/min), and much faster than the anaphase movements of the segregating half-bivalents (1 μm/min). Bipolar bivalents periodically reorient, most often resulting in the partner kinetochores exchanging poles. We suggest that the large numbers of inter-polar movements of univalents, and the reorientations of bivalents that lead to partners exchanging poles, might be because there is non-random segregation of chromosomes, as in some other cell types.
KeywordsChromosome oscillationsNon-random segregationDistance segregationPrecocious cleavage furrow
We have studied meiosis-I in spermatocytes of the freshwater flatworm Mesostoma ehrenbergii (a relative of Planaria) because several unique aspects of this division raise important questions that challenge our understanding of cell division. One of these aspects is distance segregation, the proper segregation of chromosomes that are not conjoined. The usual picture of meiosis is straightforward: chromosome partners (homologues) are attached at chiasmata, the chromosomes are bipolarly oriented, attached to the two spindle poles, and sister kinetochores (on each partner) are syntelically oriented such that once the partners disjoin the spindle fibres attached to each pole cause the partners to move to opposite poles. In many cell types, however, segregating chromosomes are not attached to each other (e.g. Camenzind and Nicklas 1968; Hughes-Schrader 1969; Forer and Koch 1973; Oakley 1983; Oakley 1985) yet their segregation nonetheless is accurate. In crane-fly spermatocytes, for example, the X and Y univalent chromosomes are not attached, they move separately on the spindle during prometaphase, and when they reach the equator at metaphase both univalent chromosomes have spindle fibre attachments to both poles (amphitelic orientation). As the two sex chromosomes move to opposite poles in anaphase (after the completion of autosomal anaphase) the spindle fibres to both poles persist: as each one moves poleward during anaphase, one of its attached spindle fibres shortens and the other elongates (e.g. Forer, Ferraro-Gideon and Berns 2013). The major puzzle of distance segregation in these cells is how each sex chromosome segregates to its respective spindle pole despite them not having been paired. It is unclear what mechanism is utilized by the sex chromosomes to ensure that movements are to opposite spindle poles, nor is it clear how the two sex chromosomes influence each other’s movements throughout anaphase (e.g. Forer and Koch 1973 and Forer et al. 2013). This example, one of many, raises the issue of how movements of not-attached chromosomes are coordinated to achieve distance segregation and what “communication” takes place between chromosomes that are not attached. An equally challenging univalent segregation pattern occurs in Mesostoma spermatocytes, as previously described by Oakley (1983, 1985), whose conclusions we now summarise.
Mesostoma spermatocytes have three autosomal bivalents and four univalents. The univalents appear as two pairs, two of each kind, distinguishable by shape and position of their kinetochores (Husted and Ruebush 1940). At the start of spindle formation, the univalents move to the two spindle poles as the bivalents become bipolarly oriented. The univalents remain at the spindle poles throughout prometaphase and metaphase but sometimes they move between the poles. At anaphase each pole is associated with one of each kind of univalent but early in division there often is “mis-segregation” of univalents: three at one pole and one at the other, or two of the same kind at each pole. The movements between poles thus seem to correct the mis-segregation and allow proper segregation of the univalents. Oakley speculated that not only do the univalents assort one of each kind at each pole, but they do so non-randomly (e.g. female derived univalents at one pole and male derived univalents at the other), the kind of non-random segregation that is well documented in other cells [e.g. Gryllotalpa hexadactyla (mole crickets) (Camenzind and Nicklas 1968); Sciara (Gerbi 1986) and mealy-bugs (Nur 1982; Schrader 1921)]. Oakley based this suggestion on two observations: (1) that univalents seem to move between poles more often than necessary in order to achieve one of each kind at each pole, and (2) that in some cells univalents of the same kind changed poles after proper segregation had been achieved. In this article we describe inter-polar movements of univalents throughout prometaphase as they move from one spindle pole to the other, prior to the onset of anaphase. We describe the frequency and velocities of univalent movements, and whether they affect bivalent kinetochore oscillations.
A second remarkable feature of Mesostoma spermatocytes is the presence of a precocious cleavage furrow. The cleavage furrow begins ingression shortly after the bivalents become bipolarly oriented in prometaphase and the furrowing then becomes arrested shortly thereafter. The furrow can gradually become slightly more constricted as the cell gets closer to anaphase but the furrow continues its ingression and cleaves the cell only after anaphase (Forer and Pickett-Heaps 2010). This seems remarkable in itself since most textbooks and articles assume that the stimulus for cleavage furrow ingression is released after anaphase onset. Almost more remarkable is that the position of the furrow along the length of the cell changes when a univalent moves from one pole to the other (Forer and Pickett-Heaps 2010), when a bivalent re-orients, or when a component of the spindle is altered by UV microbeam irradiation (unpublished data). We do not deal with this phenomenon herein, except in passing.
A third noteworthy feature of Mesostoma spermatocytes is the vigorous and persistent oscillations of bivalent kinetochores, as described by Fuge (1987, 1989). Oscillations that occur in prometaphase and/or metaphase cells in other cell types generally are of the entire chromosomes and are irregular, for short periods prior to anaphase (up to 10–25 min), for short distances (at most 1–3 μm), perhaps tapering off as chromosomes approach the metaphase plate, and at moderate speeds of the order of 1–3 μm/min (Skibbens et al. 1993; Ke et al. 2009; Jaqaman et al. 2010), though sometimes oscillations are more regular (Wan et al. 2012; Civelekoglu-Scholey et al. 2013). Speeds and distances can be larger under unusual experimental conditions such as in monopolar spindles (Bajer 1982; Ault et al. 1991; Skibbens et al. 1993; Cassimeris et al. 1994) or in chromosomes with only one kinetochore (e.g. Skibbens et al. 1995; Khodjakov and Rieder 1996; Khodjakov et al. 1997), and can be larger for oscillations of entire nuclei (e.g. Aist and Bayles 1988) or spindle poles (Riche et al. 2013). Compared with this usual description of entire chromosomes that oscillate, oscillations in Mesostoma spermatocytes as described by Fuge (1987, 1989), are regular, cover longer distances, take place throughout most of prometaphase and are best described as kinetochore oscillations since kinetochores on the same chromosome do not always act the same. According to Fuge, each kinetochore oscillates regularly to and from its spindle pole during the maximum observation period (up to an hour) with one kinetochore oscillation cycle to and from the pole taking about 100 s, encompassing distances of 5–7 μm or more with velocities from 8–10 μm/min. The oscillations last throughout prometaphase and there is no real metaphase plate: individual bivalents centre at the metaphase plate only when the two kinetochores move toward their respective poles at the same time. This sometimes happens but often does not, so when one kinetochore moves toward its pole and the other moves away from its pole, the bivalent shifts off the equator. The oscillation distances are so large that the chromosomes often are greatly stretched: during oscillations interkinetochore distances of the bivalents vary from 27 to 40 μm (Fuge 1987, 1989). Fuge (1989) noted that kinetochores sometimes remain more-or-less motionless for varying periods, but that the oscillations generally are regular. Since he did not follow the cells into anaphase, there is no information about when or whether kinetochore oscillations stop prior to anaphase onset. Fuge (1989) also suggested (based on chromosome behaviour in a few cells) that there is some coordination between movements to the same pole: of the kinetochores of the three bivalents in the cell, he suggested, two pairs move in phase and the third moves in opposite phase. In this article we present extensive data on oscillatory movements of kinetochores in Mesostoma spermatocytes, including description of chromosome movements during anaphase.
Materials and methods
Living cell preparations
We reared laboratory stocks of M. ehrenbergii and obtained spermatocytes from Mesostoma testes according to the protocol described by Hoang et al. 2013. Briefly, in our Mesostoma stocks we kept approximately five to 15 animals in plastic jars filled with dechlorinated water and fed them daily with laboratory-reared live brine shrimp (Brine Shrimp Direct, Ogden, Utah, USA). The jars were stored at 25 °C in incubators with a 16 h light:8 h dark cycle. We removed the testes from 3- to 4-week-old animals by inserting through the body wall of the animal a glass needle, pulled from 10 μl micropipettes (Fisher), and sucking up the testes using Tygon tubing (Fisher) attached to the needle. We expelled the testes from the needle onto a glass coverslip, added an equal-sized drop of Mesostoma Ringer’s solution (61 mM NaCl, 2.3 mM KCl, 0.5 mM CaCl2 and 2.3 mM phosphate buffer, pH 6.9) that contained 0.2 mg/mL Fibrinogen (Calbiochem), as previously described (Forer and Pickett-Heaps 2005), spread the fibrogen/testes mixture on the coverslip, and added a drop of thrombin (Sigma) to create a fibrin clot. The coverslip was then inverted onto a perfusion chamber (Forer and Pickett-Heaps 2005) was sealed with wax, and perfused with 2–3 mL of Mesostoma Ringer’s solution.
Phase-contrast images of living cells were obtained using a Nikon Diaphot microscope with ×100 (NA1.3) phase-contrast objective, were recorded on DVDs, then time-lapsed using Virtual Dub (http://www.virtualdub.org), a freeware program, and analysed as previously described (e.g. Sheykhani et al. 2013). Briefly, we used an in-house program (Wong and Forer 2003) in which we marked the position of a fixed point (the spindle pole) and then marked each kinetochore. The measured distances between pole and kinetochores were then imported into a commercially available program (SlideWrite) for analysis.
Student's t tests were conducted to determine statistical significance of velocities, amplitudes and periods of oscillations. When the values in question were calculated as percentages (e.g. Fig. 5), the percentages were first converted to proportions, and the proportions were transformed by taking their square root and then the arcsin (sin−1) of the square roots; Student's t tests were performed on the transformed values.
Overview of Mesostoma spermatocytes
Kinetochores of the three bivalents oscillate to and from the spindle poles throughout prometaphase (Fuge 1987; Fuge 1989; Fuge 1991). The general impression from viewing time lapsed movies of these oscillations is that there is continuous and rapid movement of all kinetochores toward and away from the spindle poles throughout prometaphase and until the very start of anaphase, which occurs as long as 2 h after the start of prometaphase. The oscillatory movements are so fast that one can easily see them in living cells when viewed on a TV screen. Univalents, on the other hand, do not oscillate. They remain stationary at the spindle poles (Oakley 1985), and only move from one pole to the other, presumably to achieve the proper distribution of two univalents at each pole, one of each kind, before the onset of anaphase (Oakley 1985). Univalent movements between spindle poles can easily be seen in time lapsed movies and are more rapid than the oscillatory movements of the bivalents, as we describe in detail below.
Most Mesostoma spermatocytes have a dumbbell-shaped appearance resulting from a precocious, "pre-anaphase", cleavage furrow that begins ingression once the bivalents have established bipolar orientation; ingression of the furrow then ceases until the onset of anaphase (Forer and Pickett-Heaps 2010).
We now describe these phenomena in more detail, namely kinetochore oscillations, anaphase chromosome movement, bivalent reorientations and univalent movements between spindle poles in Mesostoma spermatocytes.
Detailed descriptions of Mesostoma spermatocytes
Summary of the number of kinetochores that oscillate throughout prometaphase compared to the number of kinetochores that remain stationary and do not move throughout prometaphase
Total number of KTs (n = number of cells)
Number of moving KTs
Number of KTs NOT moving
Number of cells with kinetochores not moving
Two KTs to the same pole
KTs to different poles
152 (n = 40)
42 (n = 28)
16 (57 %)
8 (29 %)
3 (11 %)
1 (3 %)
Average velocities, amplitudes and periods
Summary of the velocity, amplitude, and period of kinetochore movement to the pole and away from the pole in Mesostoma primary spermatocytes and secondary spermatocyte
Number of kinetochore measurements (n = number of cells)
Length of time series analyzed
Range of velocities (μm/min)
Average velocities (μm/min)
Average amplitude (μm)
Average period (s)
Mesostoma primary spermatocytes
Away from polea
2,491.5 min (41.5 h)
5.23 ± 1.82
4.0 ± 1.38 (1.0–9.0)
92.5 ± 20 (50–180)
To the polea
6.24 ± 2.23
1,760 (n = 88)
5.08 ± 1.75
Mesostoma secondary spermatocyte
Away from pole
20 (n = 1)
To the pole
20 (n = 1)
Using the same 176 kinetochores, we determined the amplitudes of kinetochore movement by measuring the distance from the trough to the crest of each sawtooth wave, and we determined the periods by measuring the time interval between troughs of multiple waves. The average distance the kinetochores moved away from the pole was 4.0 μm (Table 2, Fig. 3b). The average time it took a kinetochore to move away from the pole and then back to the pole was 92.5 s (Table 2, Fig. 3c). At low time-resolution, kinetochores seem to reverse their directions immediately as the tips of the sawtooth waves appear pointed (Fig. 2a), but at a higher time-resolution the tips of the sawtooth waves appear rounded (Fig. 2b). The lag time (i.e. the ‘rounded’ period between the two linear parts of the curve) for kinetochores at the pole was 11.4 ± 11.4 (SD) seconds (n = 495) and for those away from the pole was 10.6 ± 10 s (n = 509). These are not statistically different.
Difference in the velocity of kinetochore movement away from the pole, velocity of kinetochore movement to the pole, amplitude of kinetochore movement to and away from the pole and period of kinetochore oscillations for partner kinetochores, for kinetochores moving to the same pole and for non-partner kinetochores moving to opposite poles in the same cell
Type of kinetochore
Total number of cells
Difference in velocity away from pole
Difference in velocity to the pole
Greater than 20 %
Greater than 20 %
22 (61 %)
5 (13 %)
9 (25 %)
12 (33 %)
10 (27 %)
14 (36 %)
KTs to the same pole
17 (50 %)
8 (24 %)
9 (26 %)
14 (42 %)
9 (27 %)
11 (32 %)
Non-partner KTs moving to opposite poles
17 (54 %)
9 (28 %)
6 (19 %)
12 (38 %)
7 (22 %)
13 (41 %)
Difference in amplitude
Difference in period
Greater than 20 %
Greater than 20 %
14 (39 %)
11 (30.5 %)
11 (30.5 %)
22 (61 %)
6 (17 %)
8 (22 %)
KTs to the same pole
12 (35 %)
10 (29 %)
12 (35 %)
14 (41 %)
7 (21 %)
13 (38 %)
Non-partner KTs moving to opposite poles
12 (36 %)
7 (22 %)
13 (41 %)
18 (56 %)
7 (22 %)
7 (22 %)
Summary of the number of phase shifts that occur throughout prometaphase at the pole versus away from the pole and a summary of the length of time single bivalents oscillate in-phase versus out-of-phase
In-phase vs out-of-phase
Length of time series analyzed (min)
When KTs were at the pole (# of phase shifts)
When KTs were away from the pole (# of phase shifts)
Length of time series analyzed (min)
Length of time IN phase (min)
Length of time OUT of phase (min)
1 in ∼38 min
1 in ∼138 min
Granules in the spindle
Phase dense fibres between kinetochores and poles
Summary of the average velocities of kinetochore movement for bivalent kinetochore oscillations, anaphase chromosome movement, univalent movement and bivalent reorientations in control Mesostoma spermatocytes
Mesostoma spermatocyte control cells
Number of cells
Number of kinetochores analyzed
Range of velocities (μm/min)
Average velocities (μm/min)
5.1 ± 1.75
1.2 ± 0.9
9.4 ± 4.3
8.2 ± 2.7
We analyzed distance versus time graphs to see if we could better understand when anaphase onset takes place based on possible irregularities in the sawtooth waves prior to anaphase. In the five sawtooth waves immediately preceding the final sawtooth wave prior to onset of anaphase there was no change in the amplitude or period and there was little to no change (less than 15 % difference) in the velocity of kinetochore movement to the pole; however, there was a significant decrease (approximately 30–50 %) in the velocity of kinetochore movement away from the pole, with the most drastic decrease (50 %) seen in the two sawtooth waves immediately preceding anaphase onset. In 16 of the 20 cells we studied, anaphase occurred as half-bivalent kinetochores started to move away from pole: the half-bivalents disjoined and moved towards their respective spindle poles before reaching their furthest away-from-the-pole position (Fig. 11b). In most (10/16) of these cells, bivalents disjoined after half-bivalent kinetochores moved approximately 2 μm away from the pole; bivalents also disjoined after half-bivalent kinetochores moved only 1 μm away from the pole (4/20), less than 1 μm away from the pole (1/20) or greater than 2 μm away from the pole (1/20). In the other four of the 20 cells we studied, anaphase occurred as half-bivalent kinetochores moved to the pole, the bivalents actually disjoining once the kinetochores reached their respective spindle poles (Fig. 11a). This information does not help us predict when anaphase will occur when we are watching a live cell, but it does show that bivalents usually separate and enter into anaphase when half-bivalent kinetochores are moving away from the pole.
We now describe reorientations of bivalents, and movements of univalents in primary Mesostoma spermatocytes.
We have observed 49 bivalent reorientations in time lapsed movies of 25 Mesostoma spermatocytes corresponding to 2,492 min (41.5 h) of filming; one example is seen in Supplementary Movie 1. We saw two kinds of bivalent reorientations: (1) bivalents that were mono-oriented when we first starting filming that subsequently became bipolarly oriented; (2) bipolarly oriented bivalents, with normal oscillations, that became mono-oriented after a half-bivalent kinetochore detached and re-attached to the opposite pole (i.e. the same pole as its partner). After the bipolar univalents became monopolar, bipolarity was re-established when one of the two monopolarly oriented kinetochores detached and re-oriented to the opposite pole; the bipolarity was re-established either when the previously detached kinetochore returned to its original pole or when the two kinetochores exchanged places. Reorientations occurred after stable bipolar orientation of the bivalents in question, not just after initial, perhaps incomplete, attachments, because we have filmed up to 75 min of normal oscillation behaviour before some detachments and reorientations.
In 8/49 cells with reorientations, mono-oriented bivalents that were mono-oriented when we started filming became bipolarly oriented during our filming sequence. In seven of the eight cells, no further reorientations took place once the mono-oriented bivalent became bipolarly oriented.
In 4/41 kinetochore detachments and movement to the opposite pole, the bivalent remained mono-oriented for as long as we followed the cell (up to 30 min), with both kinetochores either oscillating toward the same pole or remaining stationary.
From distance versus time graphs, we determined that some reorienting kinetochores continued to oscillate (albeit with dampened amplitude) as they moved away from the pole, with linear movement between oscillations (e.g. Fig. 13b, d). Other reorienting kinetochores moved to the opposite pole with constant velocity (Fig. 13c, d). When a kinetochore initially detached and moved to the opposite pole, the oscillations of its partner kinetochore had reduced amplitudes and velocities (with unchanged periods), e.g. Fig. 13c, d, and then it stopped oscillating completely after the detached kinetochore reached the pole.
Reorienting bivalent kinetochores moved between spindle poles with velocities that on average were 3 μm/min faster than bivalent kinetochore oscillations, 8.2 ± 2.7 (SD) μm/min (range 4.9 to 14.1 μm/min, n = 16 kinetochores), compared to 5 μm/min (Table 5). In the cells in which the bivalents were monopolarly oriented when we first started filming, when the four monopolar bivalents became bipolarly oriented the kinetochore reorientation velocities were the same as movements of detached bipolarly oriented bivalents, 8.3 ± 2.0 μm/min (range 6.3 to 10.3 μm/min).
We have described cell division in meiosis-I spermatocytes of M. ehrenbergii with the aim of laying the framework for experimental work on this system, with its several unique attributes. We described the regular oscillations of each bivalent that occur throughout prometaphase and until anaphase, and that there is no metaphase configuration recognisable as such; that the oscillations to the pole are faster than those away from the pole; that oscillating kinetochores periodically shift phase; that the free arms of the bivalents usually move with the kinetochores, but sometimes do not; and that while the three bivalents appear to oscillate together, the oscillation parameters of each kinetochore and of the kinetochore in its partner half-bivalent in the same bivalent differ by more than 20 % in 20–40 % of the cells, so that the kinetochore oscillations in a bivalent (and between bivalents) seem independent. Anaphase most often occurs in the middle of an oscillation cycle, and anaphase movements are much slower than oscillation velocities (∼1 μm/min rather than 6 μm/min). We described how bipolar bivalents periodically reorient, most often resulting in the partner kinetochores exchanging poles. We described the pole-to-pole movements of univalent chromosomes, and how these generally do not affect the oscillations of the bivalents.
Mesostoma spermatocyte control cells
Our data North American M ehrenbergii
Average velocity (range including to the pole and away from pole)
8 to 10 μm/min
5 to 6 μm/min
5 to 7 μm
Amount of time in-phase: amount of time out-of-phase
Anaphase chromosome movements started in the middle of oscillation cycles, most commonly when kinetochores moved away from the pole, interrupting the oscillatory movement away from pole. Anaphase chromosome velocities to the pole are much slower than oscillations, 1 μm/min compared to 6 μm/min; at first glance this might seem puzzling since microtubule depolymerisation acts as the rate-limiting step for movement velocity (Forer et al. 2003, Forer et al. 2008, Pickett-Heaps and Forer 2009) and since both prometaphase and anaphase require depolymerisation of kinetochore microtubules. We think that the explanation for this is something related to kinetochore microtubules changes at or immediately prior to anaphase. It might be, for example, that during prometaphase oscillations kinetochore microtubules depolymerise at the kinetochore and the kinetochores "chew" their way to the pole as described by the PacMan model (Rieder and Salmon 1994) but that during anaphase kinetochore microtubules depolymerise at the pole, as in the classic traction fibre theory or the "flux model" (Cameron et al. 2006). In general, one could explain the slower speeds during anaphase by the microtubule depolymerising enzymes at pole and kinetochore (Sharp and Ross 2012) changing their depolymerisation rates at anaphase; if the spindle matrix (Forer et al. 2008; Pickett-Heaps and Forer 2009; Johansen et al. 2011) propels microtubules and chromosomes poleward, the rate of movement would change depending on the rates of microtubule depolymerisation at the two ends of the microtubules.
Univalents periodically moved between spindle poles, sometimes up to seven times in the same cell. Many cells have a 2:2 distribution of univalents at the poles from early prometaphase, but some have 3:1 or even 4:0 distributions. If pole-to-pole movements of univalents were required only to achieve distance segregation, then the cell would need at most one or two movements to have one X univalent and one Y univalent at each pole. In 6/28 cells, however, there were three or more excursions (Fig. 14). Oakley attributed the more-than-needed number of excursions to the need to not only balance one X univalent and one Y univalent at each pole (distance segregation) but because there is non-random segregation of the univalents, requiring X1Y1 at one pole and X2Y2 at the other. Our data confirm that there are more-than-needed univalent movements in Mesostoma spermatocytes, which is consistent with Oakley’s interpretation that there may be non-random segregation of the univalents. Perhaps relevant to this are the bivalent reorientations that occur in these cells.
Bivalent reorientations occur with reasonable frequency in the cells we studied: 49 reorientation in 2,490 min of filming, or one reorientation every 50 min on average. This seems to us to be quite high, since we know of no reports on any other not-treated cells that describe any reorientation of bipolarly oriented bivalents, though in acentrosomal mouse oocytes there seems to be considerable chromosome instability prior to bipolar orientation of bivalents (Katajima et al. 2011). The high frequency is not because this is correction of faulty initial attachment: we expect the attachments to be stable because they occurred after lengthy periods of normal oscillations and hence seemingly normal attachments, for periods of up to 75 min. That there are so many bivalent reorientations, and that most reorientations (13/17) resulted in the sister kinetochores switching poles, may be related to the speculation by Oakley (1983, 1985) that univalent movements between the poles is required both for distance segregation and to achieve non-random segregation (for example that male-derived and female-derived univalents must be at different poles). It may be that bipolar attachment of bivalents is necessary but not sufficient, that if directed, non-random segregation is required as well, then the reorientation of bivalents may be the mechanism used so that kinetochores of some bivalents can switch the poles to which they are oriented.
Bivalents oscillate regularly and continuously until anaphase in Mesostoma spermatocytes. In other cells described in the literature bivalents oscillate irregularly and only briefly throughout prometaphase before stabilizing at the metaphase plate (Pickett-Heaps et al. 1979; Pickett-Heap and Tippit 1980; Skibbens et al. 1993; Civelekoglu-Scholey et al. 2006; Jaqaman et al. 2010), though in some cells regular oscillations for some chromosomes continue through metaphase (Wan et al. 2012). While kinetochores in Mesostoma spermatocytes oscillate until anaphase, several lines of evidence show that oscillations are not required for anaphase onset. Mesostoma spermatocytes sometimes enter into anaphase when one or more kinetochores are not moving, as described above; they also enter anaphase when kinetochore oscillations are stopped with an optical trap (Ferraro-Gideon et al. 2013). Therefore, the lengthy prometaphase oscillations observed in Mesostoma spermatocytes are not actually required for anaphase onset.
During prometaphase oscillations in Mesostoma spermatocytes, kinetochore movements to the pole are statistically faster than movements away from the pole. This is not necessarily the case in other cells. In more usual cells, the velocity of kinetochore movement to and away from the pole are the same, though different mechanisms have been thought to be required to produce these movements (Ault et al. 1991; Skibbens et al. 1993; Skibbens et al. 1995; Khodjakov and Rieder 1996; Campas and Sens 2006; Ke et al. 2009). In one model, to the pole and away from pole movement are both thought to be generated by tension in the kinetochore (Skibbens et al. 1993; Skibbens et al. 1995); in a different model away from pole movement is thought to be generated by polar ejection forces acting on the chromosome arms (Ault et al. 1991; Khodjakov and Rieder 1996; Liu et al. 2007; Campas and Sens 2006; Ke et al. 2009). Our data indicate that tension alone is not responsible for producing kinetochore oscillations, as discussed above, but a combination of these models may explain why kinetochore movement to the pole is significantly faster than kinetochore movement away from the pole in Mesostoma spermatocytes.
In summary, we have described cell division in meiosis-I spermatocytes of M. ehrenbergii and we have provided detailed description of the regular bivalent oscillations that occur throughout prometaphase; the periodic reorientations of bipolar bivalents and the pole-to-pole movements of univalent chromosomes. We hope our descriptions help lay a foundation for further experimental work on these unique oscillations, on trying to understand the distance segregation of univalents, on the possible non-random segregation of univalents and half-bivalents, on the normally occurring reorientation of bipolarly oriented bivalents and on mechanisms that control their precocious cleavage furrows.
This work was supported by grants from the Canadian Natural Sciences and Engineering Council to A.F.
Conflict of interest
None of the authors have any conflict of interest.