Published April 1, 1965
TELOPHASE
SEGREGATION
OF C H R O M O S O M E S
AND
AMITOSIS
J. M O L I ~ - B A J E R , D.Se.
From the Laboratory of Plant Physiology, Jagellonian University, Cracow, Poland. The author's
present address is Department of Biology, University of Oregon, Eugene, Oregon
ABSTRACT
I. I N T R O D U C T I O N
Mitosis consists of series of processes precisely coordinated in a characteristic manner during the
course of normal division. The definition of mitotic
stages was originally based on the course of events
in normal mitosis. During experimentally modified
mitoses, the time relation of individual processes
can be greatly altered, For example, a stage resembling telophase may show certain features
characteristic of anaphase or even metaphase. For
this reason, the usual terminology regarding stages
and even structures, such as the spindle and the
phragmoplast, may not be directly applicable to
the modified types of mitosis.
As is generally realized, the function of the
phragmoplast is the formation of the new cell wall.
It is also widely accepted that the phragmoplast
prevents a backward movement and fusion of the
groups of daughter chromosomes in telophase
nuclei. However, judging from the literature,
phragmoplast action in regard to chromosome
movements has not been analysed in detail. Certain aspects of the problem were discussed by
Ostergren (33), Bajer (3), Mol~-Bajer (27), and
Mol~-Bajer and C)stergren (29).
The purpose of the present paper is to analyse
the segregation of the chromosomal material during the later stages of division in normal and experimentally modified mitosis. The abnormal divisions occurring in the experimental material belong to two widely different types. One type is
represented by certa n types of amitosis; the other
is a modified mitosis in which the whole chromosomes of metaphase type (i.e., each consisting of
two chromatids) are distributed into two separate
groups of anaphase-like character. The latter type
of division is here termed "distributive c-mitosis,"
79
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Cases of "distributive c-mitosis" (the term does not mean that colchicine has been used) in
plant endosperm are described, in which the chromosomes of metaphase type (two-chromatid chromosomes) are distributed at random because of phragmoplast activity in a process
similar to non-disjunction. There is some evidence that chromosmal fibres can be formed
within the phragmoplast under special circumstances; during "distributive c-mitosis"
some kinetochores show active movements due to cooperation with chromosomal fibres
formed in the phragmoplast; while other chromosomes, as indicated by their arrangements
and shape, are moved without any activity of kinetochores. Some components of the phragmoplast have the fastest movements occurring in mitosis. Some cases are described in which
the phragmoplast divides telophase and interphase nuclei into two or more groups and
moves the pieces a considerable distance apart. In a similar way, the phragmoplast may divide newly formed restitution nuclei. This phenomenon leads to a reduction of chromosome
numbers, and the course of the process itself is reminiscent of amitosis.
Published April 1, 1965
using the expression of Levan (25). This is an
original term of Nybom and Knutsson (30) modified by Levan, (25). In plant material, the phragmoplast plays an essential role in both these phenomena, although in distributive c-mitosis it is
modified and may not necessarily form a cell wall.
The reasons for considering this structure to be a
kind of phragmoplast, even if it does not form a
cell wall, are discussed below.
In this connection, some other observations pertaining to the function of the phragmoplast are
also presented; namely, a study on movements
occurring in strands within the phragmoplast.
II.
MATERIAL
AND METHODS
III.
SEGREGATION
SOMES
DUE
OF CHROMOTO D I S T R I B U T I V E
C-MITOSIS
During studies on experimentally modified mitosis,
a type of division was discovered in which chromosomes of metaphase structure are separated into
two or more groups, similar to an anaphase distribution. Other workers have reported similar observations from investigations on fixed material
(8, 30). Nybom and Knutsson (30) speak of "distributed c-mitosis" and Gavaudan (16) of "figures
de pseudodicentrie." Huskins (17-21) has described an effect called "somatic reduction,"
which comprises a group of various kinds of disturbances. Some of the modified mitoses included
under this concept may belong to the distributive
type of c-mitosis. Others are modifications of
normal mitosis.
In cine-film studies on endosperm the course of
such distributive c-mitosis was followed in the living cells treated with methanol. In these cells, the
chromosomes seem to segregate more or less at
random. Naturally, some tendencies to preferential
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THE JOURNAL OF CELL BIOLOGY • VOLUME ~5, 1965
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Endosperm of Haemanthus kathelinae Bak. studied in
vitro was the material for this study. Untreated cells,
as well as those irradiated with E-particles or treated
with 5 per cent methanol in the agar medium, were
used. The time-lapse cinemicrographic technique
(phase contrast) was followed by a frame-by-frame
analysis when necessary. The methods have been
previously described in detail (2, 27, 28). Segregation
due to distributive c-mitosis (paragraph III) was
observed in 4 cells, amitosis (paragraph IV) in more
than 15 cells (both treated with methanol or irradiated, and untreated), and movements in the
phragmoplast (paragraph V) in more than 100
treated and untreated cells.
segregation might exist; to demonstrate such a
possibility, however, would require special studies
and this aspect cannot be considered here.
These modified divisions also show certain
features of normal anaphase, especially sudden
lapse of association of daughter kinetochores at the
beginning of movement. The corresponding
daughter chromosomes then either fail to move
apart from one another, as in c-anaphase under
influence of colchicine, or else separate by active
movements similar to those found in normal anaphase. When they do not move apart, they move
together in the same direction. Since the events
taking place in all 4 cells observed were essentially
similar, they need be described in detail in only
one cell (Fig. 1 a to i). In this case, the observations began after 9 hours of continuous treatment
with 5 per cent methanol in agar medium. It was
found (34) that prolonged treatment with methanol often prevents the formation of the spindle, and
in some cells a kind of c-mitosis develops. The cell
in question showed modified c-mitosis--using the
expression in a wide sense as is done by (~stergren
(32). The clear zone around the prophase nucleus
develops slightly, and then decreases in size. No
metaphase plate is formed. Chromosome contraction proceeds rather normally; but the chromosomes do not show the complete straightening of
their arms which is typical for full c-mitosis, and a
typical c-anaphase did not occur. After 6 hours,
"cytoplasmic boiling" occurred; this is a process
typical for the telophase (cf. reference 7) both in
normal mitosis of untreated cells and in c-mitosis.
Certain time relations of these and other phenomena are shown on graphs in Figs. 5 and 6. The
"boiling" is probably comparable to the bubbling
at telophase of animal cells (12). In the cell in
question, after "cytoplasmic boiling" occurred, the
phragmoplast or phragmoplast-like structure is
formed and the chromosomes start to move in
opposite directions.
The behaviour of whole metaphase-type chromosomes in this cell is similar to that of acentric
fragments and small granules moving within the
phragmoplast of normal mitosis (3-7). During
normal mitosis, "acentric bodies" move away
from the equator of the phragmoplast in the two
opposite "polar directions," that is, towards the
daughter nuclei. The similar behaviour of whole
chromosomes in distributive c-mitosis suggests that
the kinetochores are completely or partly inactive.
Consequently, the conclusion could be drawn that
Published April 1, 1965
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I~GUR~ 1 Distributive c-mitosis. Distribution of chromosomes at random in two polar directions without corresponding separation of most daughter chromosomes caused by the action of the developing
phragmoplast. Clear zone at prophase is very weakly developed and does not grow in size. Chromosomal
cycle proceeds normally. The kinetochores of some chromosomes, marked by arrows (in d), behave
actively. Times after a: b, ~ hours, 9 minutes; c, 6 hours, 11 minutes; d, 6 hours, 16 minutes; e, 6
hours, ~ minutes; f, 6 hours, ~9 minutes; g, 6 hours, 37 minutes; h, 7 hours, 5 minutes; i, 8 hours, 7
minutes--telophase changes. 10-/z scale in a. Cell No. 140/60. )< 500.
J. MOL~-BxJEn
Telophase Segregation of Chromosomes
81
Published April 1, 1965
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l?mtm~, ~ Movement of chromosomes to opposite poles during distributive c-mitosis. The distance of the
kinetoehores from an arbitrarily chosen equatorial plane is plotted against the time. A and D, chromosomes
in which the ehromatids fall apart, but do not separate in anaphase and continue to move close together
in the same direction. B, B', two daughter chromosomes executing simultaneously two kinds of movements: a passive one due to the action of the phragmoplast and an active one due to the action of kinetochores. The direction of their separation movements is parallel to the long axis of the phragmoplast.
E, E', two daughter chromosomes which are eliminated in a polar direction due to the general action of
the phragmoplast together with their own chromosomal fibres. At the same time, daughter chromosomes
separate from each other in a direction perpendicular to the long axis of the phragmoplast.
the chromosomes are not attached, or only weakly
attached, to the spindle-like structure, i.e., the
phragmoplast which plays here a role similar to
that of the spindle during normal mitosis. Fig. 2
gives the time-distance graphs of the chromosome
movements parallel to the phragmoplast axis. The
speed of movement in this cell is much slower than
that of normal anaphase: the separation into two
groups of chromosomes takes here about 60 minutes, compared to 30 to 45 in normal anaphase
(cf. reference 2). When the movements during
distributive c-mitosis start, the units that move
are complete chromosomes of metaphase type
(two chromatids associated in, or close to, the
kinetochore region). During the progress of this
movement, pairs of anaphase-type chromosomes
are formed by lapse of association of sister chromatids at the kinetochores. Most of the chromosomes
continue their passive movements, sister chromatids lying side by side, in a type of non-disjunction
82
T h E JOURNAL OF CELL BIOLOGY • VOLUME
~5,
process. However, in the particular cell described
here, three pairs (two marked by arrows in Fig. 1
d) perform more active separation movements.
In these pairs, the kinetochores separate actively
from one another and the daughter chromosomes
move under the combined influence of a localised
kinetochore activity and the general mechanism
of distributive c-mitosis. Fig. 3 shows paths of these
chromosomes. The kinetochore of each sister
chromatid moves independently and its guiding
role in the movement is visible. One of the chromosomes moves in the opposite direction to the
poleward elimination action of the phragmoplast
(Fig. 2 B0. Curves of these separation movements
are shown in Fig. 4. The course of the movement
is similar to that of normal anaphase, but the
speed isslower: 0.16 to 0.2 #/minute--i.e., 2 to 7
times slower than in normal anaphase. The distance covered by the daughter chromosomes is
short: 5 to 15 # (Figs. 3 to 4), while in normal
1965
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Published April 1, 1965
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anaphase each daughter kinetochore often moves
25 to 30 ~. The direction of movements may be
either parallel or perpendicular to the long axis
of the phragmoplast (Fig. 3 B, parallel, E, perpendicular).
The problem arises as to what stage of mitosis
the chromosomes are in when these events take
place, i.e. whether the movements occur in anaphase or in telophase. In this cell, there is no metaphase plate and the distinction of stages on this
basis is not possible. Measurements of chromosome
length, however, permit a recognition of the stage,
and the curves have very characteristic shape. In
normal mitosis, the chromosomes shorten between
prophase and telophase in a characteristic way (4).
Fig. 5 illustrates the changes in the normal untreated cell. The chromosomes shorten rather
rapidly dur'ng metaphase, and again more rapidly
during anaphase. Anaphase takes place at the
beginning and during the second rapid phase of
chromosome contraction.
In colchicine-treated ceils, in which mitosis is
usually prolonged, the course of chromosome contraction is similar: a steep contraction curve in
c-metaphase and a steep curve again in c-anaphase
(4).
Fig. 6 illustrates the change in length of 4 chromosomes in the analysed cell with distributive
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FTGU•E 4 Active separation movements of daughter
kinetochores simultaneous with the general and unspecific elimination of the chromosomes by the phragmoplast. The distance between pairs of sister kinetochores is (for two such pairs) plotted against time.
The graph shows only active kinetochore movements
(i.e., movements due to the activity of fibres) within
the phragmoplast, and the movements are comparable
to the movements of the kinetochores during normal
anaphase. Movements due to the general elimination
activity of the phragmoplast are not shown on the
graph. E, movement of daughter chromosomes in a
direction perpendicular to the long axis of the phragmoplast. B, movement of daughter chromosomes in a
direction parallel to the long axis of the phragmoplast
(cf. Figs ~ and 3).
J. MOL~-BAJER Telophase Segregation of Chromosomes
83
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I~OURE 3 The paths of the chromosomes which are
distributed to both poles during telophase because of
action of the phragmoplast. A, D, not separating
chromosomes, B, a chromosome in which the daughter
chromosomes execute a separation movement caused
by the action of chromosomal fibres. This separation is
parallel to the long axis of the phraginoplast. E, F,
separating chromosomes as in the ease of B; their
separation movements, however, are perpendicular to
the long axis of the phragmoplast.
mitosis T h e curves of chromosome contraction
closely resemble those of normal mitosis, but both
the passive movements characteristic of distributive cmitosis and the active kinetochore movements occur later than would be expected from
the shape of the curves in normal and colchicinetreated cells (4). In cells with distributive c-mitosis, the contraction process is very prolonged, and
in the present cell at time 7 hours, a plateau comparable to that of the metaphase of normal mitosis
is seen. At about time 9 hours, a second rapid
contraction occurs comparable to that of anaphase
in normal cells, and simultaneously cytoplasmic
boiling takes place. In cells with distributive cmitosis, the active kinetochore separation takes
place after the second rapid chromosome contraction has stopped; i.e., in a stage comparable to
telophase of normal mitosis. Thus, the following
conclusions can be drawn:
1. During the stage in the chromosome cycle
comparable to the anaphase of normal cells, the
chromosomes do not separate and do not move.
2. The chromosomes start to move at the stage
in the chromosome cycle comparable to the telophase of untreated cells.
Published April 1, 1965
Thus, in this cell prometaphase or metaphase
(as no metaphase plate is formed, it is impossible
to distinguish these stages) is directly transformed
into telophase. During telophase, "anaphase"
(i.e., separation of sister chromatids) takes place.
In this case, the two chromosome groups separated
for a short distance only remained connected by
bridges formed by chromosomes that did not move
apart. The reason these chromosomes did not
separate appears to be insufficient growth of the
phragmoplast (towards the right side in Fig. 1).
Although no cell plate was formed by the main
phragmoplast in the cell described, such plates
were actually formed by certain phragmoplasts
lying outside the chromosomal group; see the
farthest right region of Fig. 1 h.
In other cases bi- or muhinucleate cells often
arise through the failure of cell plate formation
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FmultE 5 Changes of chromosome length during normal mitosis. Nuclear membrane disappears sonle
time before the time 0'. Chromosomes decrease rapidly in length in prometaphase (0 to 45 minutes), and
then slowly during a long (unusually long in this cell) metaphase. They start to contract again more
rapidly just before the beginning of anaphase Ao. BCP, beginning of cell plate formation.
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I~GURE 6 Changes of chromosome length in the analysed cell with distributive c-mitosis. Length of four
chromosomes plotted against time. A0, beginning of distributive c-anaphase taking place because of the
formation and action of the phragmoplast. The time scales in this figure and Figs. 1 and 3 do not
correspond to each other.
84
THE
JOURNAL OF CELL BIOLOGY
• VOLUME ~5, 1965
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50
Published April 1, 1965
A
B
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b-katrn~ 7 Partition of a restitution nucleus and formation of a number of daughter nuclei due to action
of the phragmoplast. This process resembles amitosis. Times after A: B, 1 hour, 56 minutes; C, 2 hours, 49
minutes; D, 4 hours, 22 minutes; E, 7 hours, 21 minutes; F, 9 hours, 49 minutes--segregation of chromosomal material by the forming phragmoplast; G, 11 hours, 41 minutes--formation of cell plate; H, 24
hours later; interphase nuclei and cell wall are present.
J. MOL/~-BAa~rt Telophase Segregation of Chromosomes
85
Published April 1, 1965
after the end of division. The course of distributive
c-mitosis in other cells is similar, although the time
relations are slightly different.
IV. AMITOSIS
The process of phragmoplast formation in both
normal untreated and treated endosperm is not
necessarily restricted to the telophase stage. The
phragmoplast may start to appear in late anaphase, or even when the nuclei are at interphase.
Is is not known whether a phragmoplast might
form during interphase in other plant tissues as
well. The phragmoplast not only forms the cell
plate, but shows another kind of activity in which
telophase or interphase nuclei may be pinched
into two or more pieces which are then pushed
apart for a considerable distance. This process
closely resembles amitosis, described previously in
the main for various animal cells in which no
phragmoplast or phragmoplast-like structure is
formed (38). In endosperm, if such a process takes
place in telophase more than two daughter nuclei
are formed after bipolar anaphase.
A good example of the splitting of a nucleus in
late telophase by phragmoplast activity is shown
in Fig. 7. This cell was at least a third progeny of
a E-irradiated cell. It contained a micronucleus
having a few chromosomes only, and it was impossible to determine whether these had kinetochores. After some time, the chromatids separated
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FIGURE 8 Cutting of nuclei by the pllragmoplast after a normal anaphase. A, movement of the distal
parts of the two primary daughter nuclei due to action of the phragmoplast. These nuclei were formed
from a restitution nucleus as a result of fragmentation due to the action of phragmoplast. B, movement
of distal parts of telophase nuclei of the "second order" mainly because of the action of an additional
phragmoplast (the phragmoplast cuts through the nucleus). The arrow indicates the moment of breakage
of the bridge connecting these two telophase nuclei of the "second order."
86
T H E JOURNAL OF CELL BIOLOGY • VOLUME ~5, 1965
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in a process resembling c-mitosis, and finally a
restitution nucleus was formed. The shape of the
nucleus was rather irregular. At a certain moment,
the nucleus was pushed rapidly towards one side
of the cell, pinched into two, and the resulting
nuclei pushed apart by the developing phragmoplast. Finally, a regular cell plate and cell wall
were formed between these nuclei. The separation
of the daughter nuclei was very slow (0.1 to 0.2
~/minute) and lasted for a long time (130 minutes).
This process is shown in Fig. 8 A. Thus, the duration of the movement of the nuclei was about 2 to 4times longer than the movement of chromosomes
in anaphase of normal cells. In normal cells, the
comparable poleward elimination of small particles in the phragmoplast was faster (0.5 to 0.8
/.+/minute) and usually did not last so long.
The phragmoplast is often responsible for the
fragmentation of newly formed daughter nuclei.
Usually, this is caused by an additional phragmoplast which forms some time after the development of the main phragmoplast. This phenomenon
is observed not only after the influence of
methanol, but also in perfectly normal untreated
syncytial and single-celled endosperm. In all
cases, the additional phragmoplast is formed more
or less perpendicular to the cell plate; it cuts
through the daughter nucleus (Fig. 9) or nuclei
(Fig. 10), pushes aside the nuclei of the second
order, and a new cell wall is formed. In this way,
Published April 1, 1965
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FtGURB 9 Splitting of the upper daughter nucleus in telophase due to phragmoplast activity. An additional phragmoplast begins to form (d) perpendicular to the main one, and the nucleus is gradually cut
into two (e to h). In this way, three nuclei are formed after a bipolar anaphase, and the process resembles
amitosis. Untreated cell. Times after a: b, 16 minutes; c, ~7 minutes; d, 3~ minutes; e, 53 minutes; f, 1
hour, ~ minutes; g, 1 hour, 10 minutes; h, 1 hour, ~0 minutes; i, ~ hours. 10-/z scale in A. Cell No. 8/59.
X 950.
Published April 1, 1965
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FIGURE 10 Splitting of the sister nuclei because of phragmoplast activity. Additional phragmoplasts are
formed in d; they cut through the two daughter nuclei and finally, after bipolar anaphase, 4 nuclei are
formed and the process resembles amitosis. This cell was treated with a saturated solution of acenaphthene
(in agar medium). Times after a: b, 1 hour, 14 minutes; c, 2 hours, 24 minutes; d, 3 hours, 26 minutes;
e, 4 hours, 13 minutes; f, 5 hours, 80 minutes; g, 6 hours, 10 minutes; h, 7 hours, 2 minutes; i, 8 hours,
11 minutes. 10-# scale in a. Cell No. 138/57. X 700.
Published April 1, 1965
J
V.
MOVEMENTS
WITHIN
PHRAGMO-
PLAST
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FIGURE 11 Diagram of movements taking place in
the phragmoplast strands or strings. On one string of
the phragmoplast the same "bulb" is represented by a
continuous line, and then by dotted lines, or vice versa.
Movements in neighbouring strings usually slightly
differ. The "bulb" often changes in size during the
movement, and it may disappear and form de novo.
three (Fig. 9) or four (Fig. 10) nuclei m a y be
formed after a bipolar anaphase.
Fig. 8 B represents a g r a p h of the movements
d u r i n g such a process. It is evident t h a t a long
time (180 minutes) is required to transport two
parts of the nucleus to their final opposite positions. T h e distance between the two nuclei at the
end of this process is of the same order as t h a t between the chromosome groups after n o r m a l anaphase. T h e two newly formed nuclei, which were
connected by a bridge, separated at a constant
speed until the breaking of the bridge, after which
the speed of separation of the d a u g h t e r nuclei increased.
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(
T h e next question concerns the mechanical background of the various movements described here.
Observations of movements within the p h r a g m o plast in n o r m a l a n d experimentally modified divisions m a y throw some light on these processes.
I n some cases, b o t h in normal a n d treated cells,
it was possible to observe movements within the
phragmoplast d u r i n g its formation. Such movements are seen in phragmoplasts which manifest a
coarse structure or sometimes contain vacuoles.
Phragmoplasts with this coarse structure are
usually seen in syncytial endosperm, often between
non-sister nuclei. T h e coarse structure a n d
vacuoles often a p p e a r after the cell plate begins to
form. I n such cases, the phragmoplasts seem to be
composed of a series of longitudinally a r r a n g e d
strings or strands on which "swellings" or " b u l b s "
a p p e a r a n d disappear. These " b u l b s " move in a
very characteristic way. T h e movements usually
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FiouR~ 12 Movements in cytoplasmic strings. The
distance of the moving "bulb" or "swelling" from an
arbitrary chosen plane is plotted against time. Black
circles, movement of another bulb on the same string.
J. MOL]~-BAJER Telophase Segregation of Chromosomes
89
Published April 1, 1965
p
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FtGURE 13 Movements in the structure of the phragmoplast. The distance of several moving "bulbs"
from the cell plate plotted against time.
start before the cell plate appears and continue for
a certain period after its formation (usually not
longer than 2 hours, but sometimes even 3 hours).
It is likely that these movements are related to the
occurrence of the oriented structure and, consequently, of the birefringence in the phragmoplast
(22, 23). The movements give the impression of
pulsations, peristaltic motions, or longitudinal
waves. Figs. 11 and 13 illustrate this type of movement. The shifting o f one bulb is often followed
by the formation of another, which again is shifted
after a short time. Often, however, the bulbs
quickly disappear and are formed in another place.
It has to be stressed that it is difficult to give a detailed record of the course of such movements
(more detailed analysis has been made by Bajer,
6). The same type of movements is seen in thin
cytoplasmic strings, but the velocities and distances are much higher in this case (Fig. 12). Such
long cytoplasmic threads often connect different
ceils of the endosperm in vitro. In such cytoplasmic
strings, several swellings may move at the same
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T H E JOURNAL OF CELL BIOLOGY • VOLUME
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time. In the cytoplasm and in the first stages of
phragmoplast development, no regularity of the
movement was found. Phases of movement in the
neighbouring strings differ. In the phragmoplast,
however, movements towards the nearest daughter
nucleus are mostly observed. No consistent differences in the rate and direction of the movements
during various periods of phragmoplast development have been found. The velocity of the tiny
swellings is the highest so far observed during
mitosis: 6.5 to 8.5 /~/minute; while that of the
chromosome parts during neocentric movements
(the fastest chromosome movements (8)) only exceptionally reaches 5 #/minute. The movements of
these bulbs are probably an expression of the
same mechanism that governs the poleward elimination in the phragmoplast during mitosis.
VI.
DISCUSSION
The most characteristic activity of the phragmoplast is the formation of the cell plate, and in endosperm phragmoplasts form between sister and non-
1965
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Minutes
Published April 1, 1965
1 It is sometimes possible to find out with polarized
light to what extent the phragmoplast or spindle is
disturbed. On the other hand, birefringence varies
greatly in normal cells. Even in cells in which birefringence is very weak or not detectable, the course
of mitosis is usually regular (Bajer, 1961, unpublished). Consequently, differences in birefiingence do
not necessarily imply disturbances in the spindle
structure. Therefore, conclusions concerning the organisation of the spindle and phragmoplast drawn on
the basis of chromosome behaviour are, in some instances, more reliable than those depending on birefringence.
section I I I is one of a few situated in a larger cytoplasmic lobe). The elimination movements in one
of the other phragmoplasts start exactly at the
same time as the movements during distributive
c-mitosis in the described cell. Also, slight transverse movements which appear at the beginning
of phragmoplast formation (9) are seen in the cell
in question and in others which show distributive
c-mitosis.
3. Even in more normal divisions--i.e., those
in which lagging chromosomes form strong bridges
between the separated daughter chromosome
groups--cell plate formation does not always follow phragmoplast activity. In the distributive cmitosis described here similar bridges were formed
between the daughter nuclei.
The phragmoplast and the mitotic spindle show
many similarities (31, 36). In their submicroscopic
structure they resemble each other, and studies
with the interference microscope and polarised
light (1, 22, 23, 35) show analogies. The behaviour
of acentric fragments is similar in the prometaphase spindle and in the phragmoplast; in both
cases, acentric fragments are transported towards
the poles. Thus, it is not surprising that, if the
chromosome cycle is disturbed, kinetochores may
show active movements within the phragmoplast;
i.e., chromosomal fibres may be formed from the
phragmoplast as well as from the prometaphase
spindle. This conclusion is based on the analysis
of the curves of chromosome movements (Fig. 4)
and on the paths of kinetochores (Fig. 3). Studies
of the mitotic disturbances due to the influence of
chloral hydrate also support this conclusion (27).
In cells showing "distributive c-mitosis," the
phragmoplast shifts chromosomal fibres as the
whole units, without disturbing the cooperation
between chromosomal fibres and kinetochores.
There is some evidence that similar phenomena
occur during anaphase of normal endosperm
mitosis (5). It could, therefore, be suggested that
the mitotic spindle and the phragmoplast are two
modifications of the same basic structure. The observations suggest one difference between the
prometaphase spindle and the phragmoplast: in
the prometaphase spindle, the chromosomal fibres
are parallel to the long axis of the spindle, while in
the phragmoplast there is no such coordination and
fibres can both form parallel and perpendicular to
the long axis of the phragmoplast. It should be
noted, however, that the difference in these cases
is not merely one between spindle and phragmoplast, but that it is, at the same time, a difference
J. MoL~-BAaER Telophase Segregation of Chromosomes
91
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sister nuclei in telophase (24) or during interphase
(27). It appears, however, that the phragmoplast
may perform other important functions. All acentric bodies lying in the phragmoplast are eliminated towards the daughter nuclei; the phragmoplast may also segregate the chromosomal material of various stages of mitosis in different ways.
In cases reported here, the chromosomes are
transported passively (distributive c-mitosis); some
of them, however, show active kinetochore movements within the phragmoplast. The segregation
movements of whole metaphase chromosomes in a
"distributive c-mitosis" are likely to be caused by
the same mechanism that also moves acentric
fragments and all granules, and which also causes
fragmentation of nuclei in amitosis (section IV).
Thus, the function of the phragmoplast may resemble that of the mitotic spindle. The causes of
segregation due to phragmoplast activity (described here as "distributive c-mitosis") are not
known in detail, but probably the following factors
are involved:
1. Partial disorganisation of the spindle or a disturbance of its formation.
2. Disturbances in the behaviour or function of
the kinetochores.
3. Conditions favourable for the formation o
the phragmoplast. 1
The question arises as to whether it is reasonable
to consider as a phragmoplast a structure in a
modified mitosis which does not organise a cell
plate. The answer is affirmative, because:
1. Movements during distributive c-mitosis
ascribed to phragmoplast activity do not begin
until the time that the phragmoplasts begin to develop in normal division, using the chromosome
contraction cycle as a time scale.
2. Approximately at the same time that movements during distributive c-mitosis start, several
phragmoplasts forming cell plates develop in the
surrounding cytoplasm (the nucleus discussed in
Published April 1, 1965
92
THE JOURNAL OF CELL BIOLOGY • VOLUME
~5,
groups during telophase "amitosis" leads to two
sister nuclei of the second order whose chromosomes are present in approximately the haploid
number, although the division is never precise.
This may be a mechanism leading to a reduction of
the chromosome number in somatic cells. Some of
the processes described by Huskins (17-21) as
"somatic reduction" and by others as "segregational" or "reductional" groupings (11, 14) may
be, therefore, explained, in part, as the result of
phragmoplast activity.
The literature on amitosis is controversial (13,
38) and the descriptions differ. There is not even a
satisfactory definition of amitosis (13). "Amitosis"
means only that the nucleus divides by a process
other than mitosis. Amitosis is often not accompanied by division of the cell, especially in animal
tissues. In the cases of "amitosis" described here in
endosperm, a cell wall is usually formed. There
seems to be little doubt that amitosis may sometimes be explained simply as resulting from
phragmoplast activity during the late stages of
mitosis or interphase (15, 26). In endosperm, the
phragmoplast may cut and push apart daughter or
restitution nuclei, and sometimes a phragmoplast
is formed in endosperm even during interphase.
Such processes are lengthy and two "sister nuclei"
of the second order are, as a rule, connected for a
certain time by a bridge of varying thickness to
give a picture closely resembling the amitotic
figures so often described in the older literature.
Unfortunately, it is not possible in endosperm to
observe the next division of the same cell, and it is
not known whether chromosomes may be fragmented during this process of amitosis. Sometimes,
however, in normal endosperm tissue, cells containing numerous fragments are found, and it is
likely that their origin is explained by phragmoplast activity. It is obvious, however, that not all
amitotic figures may have this simple explanation;
in animal cells in which a phragmoplast does not
exist this process must find another interpretation.
There is also no doubt that certain types of fragmentation of nuclei occur in plant cells in which
no sign of phragmoplast formation is found (e.g.,
under the influence of colchicine--Bajer and
Mol~-Bajer, unpublished).
For critical reading of the manuscript and for valuable and constructive suggestions, the author is indebted to: Prof. R. Boothroyd of the Department of
Genetics, McGill University, Montreal, Canada; Dr.
M. O'Sullivan of the Institute of Genetics, Univer-
1965
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between normal and treated cells. The unusual
directions of some separation movements of kinetochores within the phragmoplast might be due to a
structural disorganisation provoked by the treatment.
The phragmoplast is a structure found in higher
plants only. Though the process of cytokinesis in
plant and animal cells is completely different, there
are several phenomena in the telophase of animal
cells which indicate transport activity similar to
that of the phragmoplast. The elimination of
Feulgen-positive granules in Cyclops, described by
Stich (37), is one of these phenomena. These
granules appeared to be parts of chromosomes
(10). Some films on the division of animal cells
(e.g., those taken by A. Bajer) also give an impression of transport activity. There is no doubt,
however, that the late stages of mitosis in animal
cells deserve more study, and the comparison of
these late stages with phragmoplast development
may elucidate phenomena that are, at present,
little understood.
There seems to be no doubt that the phragmoplast formation can be induced at different stages
of the chromosome cycle and that this phenomenon
explains the unusual behaviour of the chromosomes described here. The problem arises as to
what stimulates the formation of the phragmoplast
under different conditions and in different stages
of mitosis. The factors are probably diverse. A
study of the formation of the phragmoplasts in untreated syncytial endosperm leads to the conclusion that a stimulation in one nucleus can derive
from proximal regions of the same syncytium (24).
There is also some evidence that substances secreted from telophase chromosomes may stimulate
certain mitotic events in animal cells, e.g. bubbling
(12); some substances are secreted from the chromosomes in endosperm from the first stages of
mitosis until the beginning of anaphase (1, 35).
Unfortunately, the nature of these substances is
not known. It is possible that a stimulation for
some mitotic events may derive from another cell
or from the nucleus. In regard to endosperm, this
was already suggested by Jungers (24). Such
stimulations may be disturbed by different factors,
and certain stimuli may reach the nucleus either
prematurely or after a delay, thus causing events
such as telophase segregation of chromosomes.
The segregation of chromosomes due to
phragmoplast activity elucidates some otherwise
not easily interpretable events. For instance, the
random segregation of chromosomes into two
Published April 1, 1965
sity of Lund, Lund, Sweden; and Prof. G. Ostergren
of the Institute of Genetics, University of Uppsala,
Uppsala, Sweden. For many interesting suggestions
and detailed comments, the author is greatly indebted
to Prof. D. Mazia and Mr. P. Luykx of the Department of Zoology, University of California, Berkeley,
California.
Recieved for publ&ation, March .3, x964.
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