J. Cell Sci. 37, 69-84 (i979)
Printed in Great Brtitain © Company of Biologists Limited 1979
69
FINE STRUCTURE OF MEIOTIC PROPHASE
CHROMOSOMES AND MODIFIED
SYNAPTONEMAL COMPLEXES IN DIPLOID
AND TRIPLOID RHOEO SPATHACEA
YUE J. LIN
Department of Genetics, Ohio State University, Columbus Ohio 43210,
U.S.A. Present address: Department of Biological Sciences, St John's
University ,Jamaica, New York 11439, U.S.A.
SUMMARY
The synaptonemal complex (SC) in the diploid Rhoeo consists of 2 amorphous lateral elements,
each about 46-0 nm thick, and one amorphous central element about 300 nra thick. The central
region is about 1150 nm wide. SC in the triploid have essentially the same dimensions as those
of the diploid; both lateral (460 nm) and central (30-0 nm) elements are amorphous, and the
central region is about 1175 nm wide. The coil, observed in both diploid and triploid, is a
modified short segment of SC with several twists at the end of a synapsed bivalent that is attached to the nuclear membrane. Serial sections in a diploid cell reveal that a coil extends inwards about 3-5 /4m from the nuclear membrane and makes a complete turn at a distance of
every 0-5 (tm. There is a correlation between the modified ends of SC and terminal chiasmata
in Rhoeo. The coils might have a positive role in the process of crossing over, or alternatively
might be involved in ring formation by holding chromosome ends together while chiasmata are
not involved. SC are present in chromocentres of both diploid and triploid. Chromocentres in
diploid and triploid are indistinguishable, and appear to be formed from the aggregation of
pericentromeric heterochromatin as a result of translocations which occurred close to the centromeres. 3-dimensional hypothetical pachytene configuration of the diploid is presented.
INTRODUCTION
Diploid Rhoeo spathacea (Swartz) Stern {zn = 12) is characterized by chromosomal
structural hybridity to such an extent that no 2 chromosomes in the diploid plant are
genetically identical. At first meiotic division, the 12 chromosomes are usually arranged
into a ring. One, two or three chains are often observed (Lin & Paddock, 19736). The
ring has been interpreted to be a result of extensive reciprocal translocation, so that
the 2 arms of each chromosome synapse respectively with an arm of each of 2 other
chromosomes. Rhoeo has not been satisfactory for light-microscope studies of synapsis
because of the unfavourable and elongated state of chromosomes at zygonema and
pachynema (Figs. 5-8). Electron microscopy allows the study of synapsis at an ultrastructural level.
Synaptonemal complexes (SC hereafter) have been shown to be associated with
bivalents at meiotic prophase in many species of eukaryotes and have become regarded
to be involved in synapsis and perhaps subsequent crossing over and chiasma formation although the exact function is yet not clear (see Moses, 1968; Westergaard &
Wettstein, 1972; Gillies, 1975, for review).
70
Y.J.Lin
SC among different species have a remarkable morphological consistency despite
great phylogenetic diversity. An SC usually consists of a set of 3 parallel strands, lying
in a single plane that curves and twists along the axis of the synapsed chromosomes in
which it lies. Two dense lateral elements are held in register by the central region
which contains the central element. The central element is generally less dense and is
joined to the lateral elements by thin transverse filaments.
Modified lateral elements of SC have been reported. In an allotriploid lily, Moens
(1968) observed unusual tube-like structures over a distance of a few micrometres in
one or both of the lateral elements in nearly every section of a pachytene nucleus. He
speculated that this deformed structure might be associated with the chromosome
heterozygosity because this species is an allotriploid derived from hybridization of two
or more closely related species. Further studies of SC in an autotetraploid lily (Moens,
1970) revealed no such abnormality. Later a very similar deformity was reported in
pollen mother cells of both diploid and triploid Phaedranassa viridiflora (LaCour &
Wells, 19736). LaCour & Wells (1973 a) also studied the SC in a lily hybrid, which
had almost complete bivalent formation at metaphase I, and observed rather bizarre
irregularities in lateral elements and in whole SC.
In his studies of the SC in diploid Rhoeo, Moens (1972) found coiled SC attaching
to the inner membrane of the double nuclear membrane. Therefore, diploid Rhoeo
presents another example of a modified SC. The coiled SC, hereafter simply the coil,
extends approximately 4 /tm from the nuclear membrane and as many as 10 such
attachments (of 12 possible attachments) were observed in a small region of the nuclear membrane near the nucleolus (Moens, 1972). Moens concluded that perhaps all
the ends of SC in diploid Rhoeo are coiled and attached to the nuclear membrane.
This type of coil has not been reported except in a grasshopper, Chloealtis conspersa
(Moens, 1974). Later McQuade & Wells (1975) confirmed Moens' finding of coils in
diploid Rhoeo.
Studies on SC have been concentrated mostly in diploid organisms. This study
was aimed at investigating the SC and its possible structural modification in triploid
Rhoeo, and comparing the SC in the triploid with that in the diploid.
MATERIALS AND METHODS
The process of microsporogenesis and maturation is often synchronous within an anther but
is less so among the anthers in aflowerof Rhoeo. For accurate identification of the meiotic stages
in pollen mother cells (PMC hereafter), an anther in proper size was first cut into 2 halves which
werefixedimmediately: one half was placed infixativefor squash preparation, and the other was
fixed in glutaraldehyde for electron microscopy. If the PMC in squash preparation were found to
be in the proper stages, the other half was further prepared for electron-microscope studies.
Further stage identification was done with thick sections (o-8 fim) prior to ultrathin sectioning.
The method of making slides for light microscopy has been described (Lin & Paddock,
1973a)For electron microscopy, excised anthers were fixed immediately for 1 h in 2 % glutaraldehyde buffered in o-2M Sorensen's phosphate buffer at pH 7-2, rinsed in buffer at pH 7-2 for
30 min (2 changes), and postfixed for 1 h in phosphate-buffered 1 % osmium tetroxide. Anthers
were then dehydrated in a graded ethanol series at 4 °C, and embedded in hard Spurr's low
viscosity resin (Spurr, 1969) at room temperature. Thick sections (o-8 /im) were obtained with
Synaptonemal complexes in Rhoeo
71
an ultramicrotome, then stained with polychrome stain (Sato & Shamato, 1973), and examined
with a light microscope for additional stage identification. Only those blocks containing PMC in
proper stages were further sectioned into ultrathin sections. The thin sections were mounted on
grids, stained with a saturated aqueous solution of uranyl acetate (3 min) followed by lead
citrate (3 min) and examined with an RCA-EMU 3G electron microscope.
RESULTS
Synapsed chromosomes in diploid
Chromosomes at leptonema are thin and unsynapsed, and the two sister chromatids
are not 9een as two distinct strands under the light microscope (Fig. 1). At the ultraStructural level, chromosomes are diffuse and the axial element is elusive in the early
stage of leptonema (Fig. 2). At a later stage of leptonema, the medial longitudinal
axial elements can be detected occasionally in the space between the sister chromatids
(Figs. 3, 4). The axial element has a lower electron density than the chromatin
surrounding it. In cross-sections, the axial element can be identified only with
ambiguity or cannot be identified at all. Heterochromatic chromocentres (Figs. 2, 3)
were observed in leptonema, zygonema and pachynema. The chromocentral chromatins can be readily distinguished from euchromatin in electron micrographs (Figs.
2, 3). The chromocentres are often large and composed of heterochromatins in 2 variable states. Chromosomal attachments to the chromocentre were often seen (Fig. 2).
At early zygonema, large portions of the chromosomes are not synapsed. The axial
element in the unsynapsed portion of a univalent at zygonema (Fig. 4) is similar in
electron density and appearance to that at leptonema (Fig. 3).
In squash preparations, some synapsed chromosome segments can be seen in lessclumped areas of zygotene cells (Figs. 5-7). Part of zygotene chromosome configuration of the cell in Fig. 7A is interpreted in Fig. 7B-F and a differential segment is
identified. A pachytene cell is seen in Fig. 8.
A synapsed bivalent invariably possesses an SC running its central axis (Figs. 10-12).
Fig. 4 is an electron micrograph of part of a zygotene cell with SC and axial elements.
As synapsis proceeds at zygonema, homologous chromosomes (univalents) with their
axial elements come into contact and are held in register at a distance of about 115-0
nm by a central region. The axial element at leptonema is now called the lateral element of the SC in the synapsed bivalent. Each lateral element measures about 46-0 nm
in thickness. In the middle of the central region is a central element about 30-0 nm
thick.
Both lateral and central elements have the same electron density and are all less
extensively stained than the chromatin. These elements do not have evident substructures so they are referred to as amorphous (Westergaard & Wettstein, 1972). The
region between the lateral and the central elements appears to be light and loosely
textured. The transverse filaments crossing this region are poorly defined (Figs. 10-12).
In cross-sections, the tripartite SC appears as a near-circle in outline and lies in the
centre like a core in the chromatin of synapsed bivalents. The lateral elements are
semicircular in outline and the central element appears to be a rather flat structure
(Fig. I2B).
Synaptonemal complexes in Rhoeo
73
The twists and turns of SC, revealed by serial section reconstruction or whole mount
preparations, are generally very gentle (Gillies, 1972; Counce & Meyer, 1973). In the
diploid, coiling was observed at the ends of synapsed bivalents (Figs. 9, 13). The coils
were always near the nuclear membrane on which the synapsed chromosome ends
were attached. The coil in Fig. 13 extends inwards about 3-5 /tm from the nuclear
membrane and makes one complete turn at a distance of every 0-5 /tm. The dimensions
of the lateral and the central elements in the coil are the same as those where the complexes are not coiled.
SC were also observed in chromocentres (Fig. 14). The SC in chromocentres and
the role of the coil are considered in the Discussion.
Measurements of SC in the diploid are compared with the measurements made by
McQuade & Wells (1975) in Table 1: the methods of fixation appear to alter the
dimensions of SC. The authors' measurements of SC in Rhoeo are very close to the
average dimensions reported from 1968 to 1971 in various organisms as shown in
table I of Westergaard & Wettstein (1972). McQuade & Wells' measurements of the
lateral and central elements in the diploid are apparently much smaller than the average.
Synapsed chromosomes in triploid
Leptotene chromosomes in triploid (Fig. 15) have an appearance similar to those
of diploid (Fig. 1). At zygonema, the homologous chromosomes synapse (Fig. 16). A
pachytene cell is in Fig. 18. Two-by-two synapsis leaving the third unsynapsed has
been observed in a squash preparation (Lin & Paddock, 1978). With the electron
microscope, SC were also found to be present in synapsed bivalents in triploid and
they were not distinguishable from those in diploid. In Fig. 17 a portion of a zygotene
cell with SC distributed over the section is shown. Figs. 19-21 are electron micrographs
of SC in various cells. The 2 lateral elements appear as 2 ribbons each about 46-0 nm
thick and separated by a region in the centre of which lies a strip of central element
about 30-0 nm thick. Fine filaments traversing the central region between the lateral
elements are more obvious in Fig. 21. The distance between the lateral elements is
about 117-5 n m (Table 1). A cross-sectional view of an SC is in Fig. 20B.
Chromocentres were also observed at prophase I in triploid (Figs. 22, 23). Diffuse
chromosomes and chromocentres of a leptotene cell can be seen in Fig. 22; in some
sections, some chromocentres did have a suggestion of SC (Fig. 23).
Figs. 1-3. Diploid leptonema.
Fig. 1. Diploid leptotene cell from a squash preparation, x 1850.
Fig. 2. Section through a diploid early leptotene cell. Chromatin (ch) in electron
micrograph appears quite diffuse and the axial element is hardly seen. Arrow marks
the chromosome attachment with chromocentre (cc). x 20000.
Fig. 3. Diploid leptotene chromosomes with axial elements (ae) in the space between
the sister chromatids prior to synapsis. Note the 2 variable states of heterochromatin
in chromocentre (cc). x 15000.
Fig. 4. Synapsed chromosomes with an SC and unsynapsed chromosomes with axial
elements (ae) in a diploid zygotene cell, x 15000.
74
Y.J.Lin
5B
Synaptonemal complexes in Rhoeo
75
Coiling at the ends of synapsed bivalents was also observed in triploid (Fig. 24). In
Fig. 24, the end of the chromosome is associated with a nucleolus and the nuclear
membrane. It appears then that a nucleolar organizer region is located at the tip of
this chromosome.
No SC were found inside nucleoli in this study although central region materials
have been found inside nucleoli of a fungus, Neottiella (Westergaard & Wettstein,
1970). Triple chromosome synapsis as reported in triploid chickens (Comings &
Okada, 1971) was not observed in triploid Rhoeo.
No abnormal tubular lateral elements such as those described in triploid lily (Moens,
1968) and triploid Phaedranassa (LaCour & Wells, 19736) were observed either in
triploid or in diploid Rhoeo in this study. Therefore the deformed lateral elements in
these 2 instances may not be due to the triploidy.
DISCUSSION
Coils and chiasmata in Rhoeo
The association of chromosome ends with the nuclear membrane at prophase I has
been found consistently in many animals (insects: Moens, 1969; Church, 1976;
Wettstein & Sotelo, 1967; mammals: Baker & Franchi, 1967; birds: Ford & Woolam,
1964; snail: Gall, 1961; and rat: Esponda& Gime"nez-Martin, 1972). Occasionally, the
association was observed in higher plants (Figs. 9, 13, 24 in Rhoeo; Gillies, 1973, in
maize). In the ascomycete fungi Neottiella (Westergaard & Wettstein, 1970) and
Neurospora crassa (Gillies, 1972) the ends of chromosomes are also connected with
the nuclear membrane. Both ends of the same chromosomes attaching to the nuclear
membrane were observed in some organisms (Ford & Woollam, 1964; Wettstein &
Sotelo, 1967; Moens, 1969V
The attachment points may be polarized to a small region near the centrioles
making a bouquet configuration in Locusta migratoria and synapsis was found to be
initiated near the attachments and proceeded away from the nuclear membrane in
this organism (Moens, 1969).
Fig. 5. A, diploid zygotenc cell, B, an interpretation of A. Chromosome 11' is synapsing
with 2 chromosomes J 2 and 1' 2'. x 1850.
Fig. 6. Synapsed segments (arrows) of a bivalent are shown in this diploid zygotene
cell. Synapsis appears to be initiated at the same time in various segments of homologous chromosomes, x 1850.
Fig. 7. A, squash preparation of a diploid zygotene cell. Arrows indicate synapsed
chromosome ends, B-F, an interpretation of A. The added dotted lines are not seen
in the photo-micrograph. Each chromosome is synapsed with 2 or more different
chromosomes, e.g. chromosome 13 is synapsed with 3 chromosomes: 1' 2'', 1 2 and 3 3'.
Chromosome 1' 3' is synapsed with 2 chromosomes: 1' 2' and 3 3'. The chromocentre is not associated with the 2 translocation points. The segment between 2
translocation points is called a differential segment (ds). x 1850.
Fig. 8. Diploid pachytene cell, x 1850.
Fig. 9. Section through a diploid zygotene cell with 3 coils at the ends of 3 synapsed
bivalents. x 12000.
Y.J.Lin
D
Figs. 10-12. Synaptonemal complexes in diploid. ce, central element; le, lateral element, x 40000. Fig. i2B. Cross-sectional view of 2 SC in diploid. Both LE and CE
are less dense than the chromatin. x 40000.
Fig. 13. A-D, 4 consecutive sections of zygotene bivalent in diploid at its point of
attachment to the nuclear membrane. The coil structure is evident, E, reconstruction
of lateral elements. F, reconstruction of central element; the lateral elements are shown
by dotted lines, x 20000.
Synaptonemal complexes in Rhoeo
77
Fig. 14. Chromocentre with SC (arrow) in diploid. x 20000.
Fig. 15. A triploid leptotene cell, x 1850.
Fig. I6A, B. A triploid zygotene cell and diagrammatic representation. The arrow indicates a point where the synaptic partners change which may be a translocation point,
x 1850.
Fig. 17. Section through a triploid zygotene cell. Many chromosomes contain SC.
x12
000.
Fig. 18. Triploid pachytene cell in a squash preparation, x 1850.
Fig. 19. Synaptonemal complexes in triploid. ce, central element; le, lateral element,
x 40000.
fi
CEL
37
78
Y. J. Lin
The polarization of chromosome ends at prophase I is probably important to the
process of synapsis. It may provide a firm association of chromosome ends with each
other in the beginning of synapsis and promote the chance of the synaptic partners
meeting.
Table 1. Dimensions (in nm) of synaptonemal complexes related to
methods of fixation
Fixation
Diploid Rhoeo
McQuade & Wells (1975)0
Formalin; OsO4
GAP"; OsO4
Glutaraldehyde only
mly
Glutaraldehyde; EDTA
Glutaraldehyde; ethanolic PTA°
Lin
Glutaraldehyde; OsO4
Triploid Rhoeo
Lin
Glutaraldehyde; OsO4
Average monocot.''
Average of reported dimensions"
ions"
Central
region
Lateral
element
Central
element
76-4
867
178
150
224
I9-3
14-2
93-8
97-6
24-4
I2'2
166
I3-O
iiS-o
46-0
3 O-O
"7-5
io6-2
46-0
40-0
3 O-O
105-0
45O
as -o
892
a. Their Table I.
b. GAP = glutaraldehyde-acrolein-paraformaldehyde technique.
3O-O
c. PTA = phosphotungstic acid.
d. See table I of Westergaard & Wettstein (1972).
e. This is the average of all reported dimensions in various organisms from 1968 to 1971 as
reviewed by Westergaard & Wettstein (1972) in their table I.
The attachment of chromosome ends with nuclear membrane has been noted only
rarely in higher plants (Westergaard & Wettstein, 1972). In my study, both diploid
and triploid Rhoeo were found to have not only the attachments but also coils (Figs.
9, 13, 24). From a large number of serial sections, as many as 10 such attachments were
observed in a cell by Moens (1972) and 11 by McQuade & Wells (1975). The 3 nuclei
examined with serial sections by Moens had all the attachments polarized on a small
area of the nuclear membrane. Three coils together in a small region of nucleus can
be seen clearly in a single thin section (my Fig. 9). Similarly to Moens' (1969) observation, McQuade & Wells' (1975) data indicate that the initiation of synapsis in diploid
Rhoeo probably begins near the nuclear membrane. Synapsis in triploid Rhoeo perhaps
proceeds in a similar fashion.
The existence of coils has been reported only in Rhoeo and a grasshopper, Chloealtis
conspersa (Moens, 1974). The coils in C. conspersa are located in the terminal portion of
some of the bivalents.
There is a striking correlation between such modified ends of SC and terminal
chiasmata. In diploid Rhoeo, the 12 meiotic chromosomes at diakinesis are joined by
terminal chiasmata and form a ring. In C. conspersa, there are 3 pairs of large chromosomes with terminal chiasmata which always form rings at meiosis (Moens, 1974).
Another species of grasshopper, Chorthippus longicornis, has a karyotype similar to
Synaptonemal complexes in Rhoeo
^
&.
.
>
MX
ce
24
Figs. 20, 21. SC in triploid. c«, central element; le, lateral element. Fig. 20B is a crosssectional view. Transverse filaments (//) are more evident in Fig. 21. x 40000.
Fig. 22. Electron micrograph of a triploid leptotene cell with 2 chromocentres (cc).
Chromosomes (ch) are diffuse and the axial element is hardly discernible, x 10000.
Fig. 23. A chromocentre in triploid with a suggestion of SC. x 20000.
Fig. 24. A coil in triploid. The nucleolus («) is attached to this chromosome which is
associated with the nuclear membrane (rnn). x 25000.
80
Y. J. Lin
C. conspersa but there are no terminal chiasmata, and coils have not been found in this
species (Moens, 1974). Such correlations suggest that the modified SC are probably
involved in terminal chiasma formation.
X
X
~
y
y
x'
y
y'
Fig. 25. A. Reciprocal translocation between 2 pairs of non-homologous chromosomes
may result in a chromocentre at pachynema. The translocation points are near the
centromeres. B. An additional ring-enlarging translocation may result in a larger chromocentre.
The indication from light-microscope studies that synapsis in diploid Rhoeo is limited to short terminal regions of chromosomes (Lin & Paddock, 19736) is in conflict
with the observation that synapsis (SC formation) occurs all over in the interior of the
nucleus (McQuade & Wells, 1975). This can be explained, however, by the fact that
SC does not always lead to chiasma formation. The role of the modified SC, coils,
perhaps is in enhancing the chance of crossing-over at terminal regions. Light-microscope studies showed the frequency of terminal chiasmata tends to be high in diploid
Rhoeo meiotic cells (Lin & Paddock, 19736). Only a very small percentage of cells
had less than 9 terminal chiasmata. On the other hand, prophase I cells seem to have
a high number of coils in each cell because the 2 zygotene and 1 pachytene nuclei
studied had 8, 10 and 11 attachments (coils), respectively (McQuade & Wells, 1975).
Thus, the number of terminal chiasmata probably corresponds to the number of coils.
The chiasma failure might be due simply to the failure in coil formation at the attachment during synapsis. An alternative hypothesis is that the chromosome ends are held
together by nothing more than the coils themselves, i.e. chiasmata are not involved
in the ring formation at all.
Synaptonemal complexes in Rhoeo
81
Chromocentre and synopsis
Heterochromatic chromocentres are often found at prophase I in Rhoeo, and exist in
2 states of condensation (Figs. 3, 22). Chromosome-chromocentre attachment can be
observed in Fig. 2. The chromocentres in diploid and triploid cells are indistinguishable
(compare Fig. 3 with 22 and 23). In squash preparations of prophase I cells, the chromocentres and nucleolus usually do not appear as distinct entities among the chromosomal materials and are thus difficult to detect (Figs. 6, 7,16). However, the nucleolus is obvious in Fig. 8. Natarajan & Natarajan (1972) were able to show clumping of
1
2
2
1
3
3
6 J
6'
1'
6 '
5
6
6
5
5
5'
r
2
2
2'
3'
1
3
«••••-
2'
3
4
-o
o.
4'
-.... 3'
4
0
4'
2
2
1
3
3
44-
v
1'
2'
2'
X
3'
3'
I
— 5'
— 5'
— ;'
-~ 2'
3'
4'
3'
4'
5
5
5'
5'
1
2
2
1
3
J
4
4
**
2'
3'
2'
4'
3'
4'
D
\
Fig. 26A-G. Five successive hypothetical translocations, 2 occurring in centromeric
regions, result in a branched 2-dimensional configuration involving 12 chromosomes
which, having no chiasma failures, become a ring-of-12 at diakinesis. See Fig. 27 for
a 3-dimensional representation.
82
Y. J. Lin
chromocentres at pachynema in diploid. The heterochromatins in diploid are situated
around the centromeres (pericentromeric) in all the 12 chromosomes and are late
replicating (Natarajan & Natarajan, 1972). They are constitutive heterochromatins and
may be considered to contain highly repetitive DNA sequences (Yunis & Yasmineh,
1971). Aggregation of heterochromatin has been shown in many organisms (Yunis &
Yasmineh, 1971). In a recent report (Godin & Stack, 1975), association of telomeric
heterochromatins was shown in rye, Secale cereale. In Rhoeo, the chromocentre formation thus appears to be from the aggregation of at least part of the pericentromeric
heterochromatins. It can be conceived as a result of extensive reciprocal translocation
Fig. 27. Hypothetical pachytene configuration in diploid, based on Fig. 2 6 c Only
lateral elements are shown. All chromosome ends are polarized to a small region in the
nucleus. Coiled SC are located at the ends of synapsed bivalents. Two adjacent translocation points neither of which is involved in a chromocentre are at left, cc, chromocentre.
in the following way. It is generally accepted that the structural hybridity in Rhoeo has
resulted from a series of translocations. As shown in Fig. 25 A if the break points were
at or near the centromeres, then heterochromatins in the 4 chromosomes could be
brought together at pachynema and a large mass of heterochromatic chromocentre
formed. A ring-enlarging translocation, if at centromeric vicinities, will give rise to an
even larger chromocentre by clumping together of centromeric heterochromatins in
6 chromosomes (Fig. 25 B).
The indefinite shapes of chromocentres may reflect that the various reciprocal translocations involved in building the ring did not occur equally distant from their respective centromeres.
Short stretches of synaptonemal complexes were occasionally found in chromocentres in both diploid and triploid. Those in Figs. 14 and 23 and in McQuade & Wells'
(1975) figs. 11 and 12 are all in the less-condensed area which has a similar appearance
to that of chromosome. It is possible but doubtful that SC in Rhoeo do not occur in the
highly condensed region of the chromocentres.
Synaptonemal complexes in Rhoeo
83
Electron-microscope studies of meiosis in Fritillaria lanceolata (LaCour & Wells,
1970) also showed the presence of SC in the chromocentres.
The view that heterochromatin as such may aid in initial alignment prior to synapsis
was criticized by Maguire (1972) who considers that a direct functional role of heterochromatin in synapsis still lacks sound evidence. In Rhoeo, chromocentres are probably
a result of synapsis causing association of pericentromeric heterochromatins.
Based on considerations in the foregoing discussion and some observations in the
study, theoretical configurations of diploid pachytene cells are presented in Figs. 26
and 27. Only lateral elements of SC are presented. As shown in Fig. 26 G, 5 successive
translocations among non-homologous chromosomes occurring 2 in centromeric
regions and 3 in non-centromeric regions result in a branched 2-dimensional pachytene
configuration containing 12 chromosomes that can become a ring at diakinesis. A
corresponding 3-dimensional pachytene configuration is in Fig. 27. A chromocentre
forms wherever more than 2 centromeric regions aggregate. Where translocation has
not occurred at a centromeric region, such as in Fig. 26B, D and E, no chromocentre is
expected to be present at that point. The speculation that break points are not necessarily localized at centromeric regions is based on the interpretation in Fig. 7B-F
in which neither of 2 adjacent translocation points is associated with the chromocentres
(see also Fig. 27).
This paper represents part of a dissertation submitted for a Ph.D. degree at The Ohio State
University, Columbus, Ohio, U.S.A. I am thankful to Dr Elton F. Paddock for his guidance,
patience and encouragement during the course of this study.
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