J. CellSci. 13, 83^5("973)
83
Printed in Great Britain
THE SYNAPTONEMAL COMPLEX AND THE
ACHIASMATIC CONDITION
S. STACK
Department of Botany and Plant Pathology, Colorado State
Fort Collins, Colorado 80521, U.S.A.
University,
SUMMARY
The onion species AUium amplectans includes both a triploid and a tetraploid variety. By
light microscopy both varieties appear to have normal synapsis during pachytene of meiosis.
However, the triploid does not form chiasmata and exhibits almost total asynapsis following
pachytene. The tetraploid forms at least one chiasma per homologue and retains pairing through
metaphase I. Electron-microscopic examination of pachytene nuclei in these 2 varieties reveals
apparently identical synaptonemal complexes. Three-dimensional reconstructions of chromosome arrangements in triploid pachytene nuclei confirm that synapsis is as complete as could
be expected in an autotriploid. These observations give firm support to the hypothesis that
the presence of apparently structurally normal synaptonemal complexes is not a sufficient
prerequisite to ensure chiasma formation. It is suggested that a faulty or missing endonuclease
which is normally involved in crossing over is responsible for the achiasmatic condition in
triploid A. amplectans.
INTRODUCTION
The synaptonemal complex (SC) is a structure which occurs concomitantly with
the intimate association of homologous chromosomes (synapsis) observed in pachytene
of meiosis (Moses, 1968). The SC has been implicated in genetic crossing over primarily
on the following evidence: (1) The SC binds homologues together with homologous
chromomeres aligned. Since genetic crossing over is thought to require alignment of
homologous genes, the level of association mediated by the SC has been extrapolated
to involve a homologous gene-for-gene pairing of the chromosomes (Moses & Coleman,
1964; Ratnayake, i968;Westergaard&vonWettstein, 1970). (2) Chiasmata are usually
interpreted to be the first cytologically observable results of genetic crossing over.
Since chiasmata are observed as soon as SCs break down at the end of pachytene, it
seems likely that the events leading to chiasma formation (crossing over) occurred
during pachytene when SCs are present. (3) In several dipteran species the males are
normally achiasmatic and lack genetic crossing over. Drosophila females homozygous
for the C3G gene likewise are asynaptic and lack crossing over. Meyer (1964) and
Smith & King (1968) determined that SCs are totally lacking in these achiasmatic
males and females. (4) Finally, experiments designed to show the time of crossing
over and/or chiasma formation have most often been interpreted to indicate that
crossing over occurs at zygotene-pachytene (Church & Wimber, 1971, for review).
Although great effort has been expended to determine the exact relation of the SC
to crossing over, so far only correlations have been established, and indeed, some
6-2
84
S. Stack
negative evidence has emerged. After exhaustive examination of SC structure in many
separate laboratories, no alteration in the structure of an SC has been observed which
definitely indicates its direct involvement in chiasma formation. Prokaryotes, parasexual fungi, and dipterans with somatic crossing over are known to have genetic
crossing over without forming SCs. Furthermore, some organisms thought to have
unusually low rates of crossing over may produce SCs which appear to be more or
less normal. Menzel & Price (1966) reported observations on a haploid tomato which
had an average of one synapsed bivalent per pachytene microsporocyte and practically
no chiasmata at metaphase I. Because SCs were occasionally observed by electron
microscopy of this material, the bivalents were thought likely to be bound together by
SCs. The authors, however, suggested that the few SCs observed could have occurred
in diploid cells in a haploid-diploid chimera. Menzel & Price (1966) further reported
SCs in a diploid hybrid between Lycopersicon esculentum and Solatium lycopersicoides.
In this hybrid most chromosomes were synapsed, but at diplotene one to several bivalents fail to form chiasmata and univalents result. In this case the number of chiasmata
was reduced even though most chromosomes may have been more or less involved
in SCs. Gassner (1967,1969) reported that spermatogenesis in the scorpionflyPanorpa
neptialis and the mantid Bolbe nigra is not only achiasmatic but that bivalents are
bound together by what appears to be normal SCs from pachytene through metaphase I.
When the chromosomes separate at anaphase I, no chiasmata are visible between the
bivalents. Gassner pointed out that the usual diplotene-metaphase I repulsion of
bivalents which would normally reveal chiasmata does not occur because the chromosomes remain synapsed through metaphase I. Thus it is possible that chiasmata may
have been formed but did not become visible due to terminalization and loss. Peacock
(1970) suppressed chiasma formation in males of the grasshopper Goniaea australasiae
by exposing them to elevated temperature. Chiasma frequency could be reduced as
low as one chiasma per cell. Electron microscopy of control (non-heat-treated) and
heat-treated pachytene microsporocyte nuclei showed apparently identical SCs with
no observable differences in SC frequency. One might argue in this case that the heat
affected the function of the SC in chiasma formation. Also, since 3-dimensional
reconstructions of the nuclei were not performed, a definite statement of the completeness of the SC-mediated synapsis could not be made. Parchman & Roth (1971) found
that Trillium microsporocytes explanted just prior to leptotene and cultured may
undergo achiasmatic meiosis. They found some SCs to be present in cultures which
contained univalents, but the exact relationship between SCs and chiasmata could not
be determined. Although none of these reports is conclusive in itself, taken collectively
they strongly suggest that the presence of the SC does not guarantee crossing over
even under normal conditions (Moses, 1968, 1969).
A truly definitive system to support the hypothesis that the presence of the SC is
not in itself sufficient to assure the occurrence of crossing over would require an
organism with complete pachytene synapsis mediated by SCs and having total failure
of chiasma formation (or crossing over) at diplotene-metaphase I. There have been
numerous reports of organisms with achiasmatic meiosis (Beadle, 1933; Sears, 1941;
Soost, 1950), but the problem with most of these organisms is that they are really only
Synaptonemal complex and achiasmatic condition
85
partially achiasmatic. One exception to this generalization was reported by Levan in
1940 in the triploid variety of the onion Allium amplectans (371 = 21). From a lightmicroscopic examination pachytene synapsis was reported to be as complete as possible
in an autotriploid while chiasma formation at diplotene-metaphase I was almost a total
failure. Levan recorded an average of less than one chiasma per 100 microsporocytes.
This indicates a cross-over frequency comparable to those observed in parasexual
fungi (Pontecorvo, 1958) and twin spotting in Drosophila (Stern, 1936). The crossing
over in triploid A. amplectans may be thought of as a background level on which
meiotic crossing over is normally superimposed.
A study of meiosis in triploid A. amplectans avoids some of the ambiguities encountered in previous studies. Unlike haploid tomato, synapsis in triploid A. amplectans
is as complete as possible. Unlike the Lycopersicon-Solanum hybrid and explanted
Trillium microsporocytes, the achiasmatic condition in triploid Allium amplectans is
essentially complete. Unlike Bolbe nigra and Panorpa neptialis, the chromosomes of
triploid Allium amplectans separate totally in diplotene-metaphase I. And unlike heattreated Goniaea australasiae, triploid Allium amplectans is asynaptic under normal
environmental conditions. Furthermore, an autotetraploid variety of A. amplectans
(471 = 28) is known in which synapsis and chiasma formation are normal (Levan,
1940). An electron-microscopic examination and comparison of synapsis in these 2
varieties of A. amplectans would appear to offer an excellent test for the relation of the
SC to chiasma formation.
MATERIALS AND METHODS
Both triploid and tetraploid bulbs of Allium amplectans were provided by the Berkeley
Botanical Garden. These bulbs were grown to flowering, and anthers were examined by the
aceto-orcein squash technique to verify the light-microscopic sequence of meiotic events
reported earlier by Levan (1940). It was found that anthers o-8 mm in length contain microsporocytes in pachytene. Anthers of this length were fixed for electron microscopy by 2 different
methods - either in phosphate-buffered 3 % glutaraldehyde for 30 min followed by phosphatebuffered 2 % osmium tetroxide for 1 h or in Navashin's fluid for 24 h (Darlington & La Cour,
1962). The anthers were then dehydrated through an ethanol-acetone series and embedded in
Epon-Araldite (Mollenhauer, 1964). Pachytene microsporocyte nuclei fixed by both methods
were serially sectioned on an LKB 4801A ultramicrotome and mounted on Formvar-coated
slot grids. Well over 100 sections at an average thickness of 120 nm are required to section through
a single nucleus. Sections taken from anthers fixed by both methods were post-stained in
aqueous 0'5 % uranyl acetate followed by lead citrate (Reynolds, 1963). Sections were examined
and photographed in an AEI EM6b electron microscope. Because of the low magnification
(5000 x ) necessary to photograph a whole nucleus and the complexity of the nuclear image
produced by the glutaraldehyde-osmium tetroxide fixation, this fixation was not used for
making large-scale nuclear reconstructions. In contrast, the Navashin's fixation presented a
comparatively clear picture of chromosome relationships, and one complete nucleus and
portions of 4 other nuclei fixed in this manner were used for large 3-dimensional reconstructions
(compare Figs. 6 and 7). Fine-structural studies and small reconstructions were performed on
material fixed in glutaraldehyde-osmium tetroxide. In one comparison of SCs from the tetraploid and triploid varieties, the distances between the inner margins of the lateral elements of
the SCs were measured in 127 examples, and a t test was performed on these measurements to
determine whether there is any difference between the 2 groups (Simpson, Roe & Lewontin,
i960).
86
S. Stack
OBSERVATIONS
Light-microscopic examination of aceto-orcein squash preparations of anthers at
various stages of meiosis confirmed Levan's (1940) earlier description of meiosis in
both the triploid and tetraploid varieties of A. amplectans. Meiosis in the tetraploid
variety is normal in terms of virtually complete synapsis at pachytene (Fig. 2) with
the subsequent formation and maintenance of chiasmata from diplotene through early
anaphase I (Fig. 4). The triploid variety shows as complete synapsis at pachytene as
one could expect in an autotriploid with the equivalent of 1 univalent per bivalent
remaining unsynapsed at pachytene (Fig. 3). However, as diplotene begins in the
triploid one can discern that no chiasmata exist, and by metaphase I all chromosomes
are univalents completely separate from each other on the metaphase plate (Fig. 5).
When pachytene microsporocytes, from both the tetraploid and triploid varieties,
fixed with glutaraldehyde-osmium tetroxide are examined, one observes SCs which
are indistinguishable by structural criteria (Moses, 1968) (Figs. 8-11). The distances
between the inner margins of the lateral elements of triploid and tetraploid SCs were
compared using the t test, and the difference is not significant even at P = 0-2. In
both groups the lateral elements are connected by transverse microfibrils which are
3-4 nm in diameter. When both groups of SCs are viewed in cross-section, the transverse microfibrils appear to lie in 3 to 4 tiers (Figs. 9, 11). Transverse elements appear
in equal density in both the triploids and tetraploids. The appearance of the chromatin
around SCs varied according to section thickness, the degree of chromatin condensation, and the intensity of staining. With these variables taken into consideration, no
difference in the chromatin of the triploid and tetraploid was discerned.
Three-dimensional reconstruction of triploid pachytene nuclei fixed in Navashin's
fluid completely supports the impression of synapsis one gains from squash preparations. Although there may be one or two partner exchanges per bivalent, an SC binds
the equivalent of 2 homologous chromosomes together throughout their length while
the equivalent of a third is in close association but usually not involved in the SC
(Figs. 1, 11). In the Navashin's fluid-fixed material, chromosomes bound together by
SCs are easily differentiated from univalents because univalents are half the size of
synapsed chromosome pairs (as is also true of glutaraldehyde-osmium tetroxide-fixed
material) (Figs. 7, 11).
Many SCs in the triploid were followed for up to 10 /im by using small 3-dimensional reconstructions and by visual examination of serial sections in the electron
microscope. Almost all SCs showed the typical structure without interference of the
univalent. Occasionally in both light and electron-microscope preparations partner
switches were observed in triploids and tetraploids (Figs. 2, 3). However, partner
switching seems to be less common than reported by Moens (1969, 1970) in triploid
and tetraploid varieties of Lilium tigrinum. Although it is possible that chiasma formation is inhibited in these disrupted regions of SC, it seems unlikely that these disruptions
could inhibit chiasma formation generally since they involve so little of the total length
of SCs and since partner switching does not inhibit chiasma formation in the tetraploid.
Furthermore, Maguire (1965, 1966) has shown in corn that a comparatively small
Synaptonemal complex and achiasmatic condition
87
Fig. 1. Three-dimensional reconstruction of a typical bivalent-univalent association
in a pachytene nucleus of a triploid A. amplectans microsporocyte. The bivalent is
represented by the thick strand, and the univalent is represented by the thin strand.
(Compare with the exposed loop in Fig. 3.) Each of the circles or ellipses which make
up the chromosomal strands represents approximately the chromosomal material of
1 bivalent or univalent in 1 section. Both strands terminate at either end on the nuclear
envelope which is represented by single thick lines. The nuclear envelope line with the
smaller circumference represents the cross-sectional appearance of the nuclear envelope
in the first section. The nuclear envelope line with the larger circumference represents
the cross-sectional appearance of the nuclear envelope in the 88th section, x 5000.
region of synapsis is sufficient to assume a crossover will occur in that region. Autotriploidy per se does not appear to prevent either synapsis or crossing over (Redfield,
1930; Catcheside, 1954; Moens, 1969; Reddi, 1970; Raicu & Chirila, 1971), and
Rhoades (1933) calculated that there is an equal amount of crossing over in diploid
and triploid members of a stock of Drosophila melanogaster. Apparently the triploid
condition in A. amplectans cannot be immediately responsible for its desynapsis and
achiasmatic condition.
Later meiotic stages in tetraploid A. amplectans are normal except for fairly frequent
micronuclei caused by pairing and disjunction problems which are common in autotetraploids. Four more or less normal diploid pollen grains result from the meiotic
divisions. In the triploid, however, the first meiotic anaphase fails completely with a
restitution nucleus forming around the univalents. The second meiotic division is
essentially a normal mitosis which yields 2 triploid daughter cells each of which forms
a pollen grain. The importance of this triploid pollen in fertilization and development
is not known.
88
S. Stack
DISCUSSION
This report of seemingly typical and identical SCs in both chiasmatic tetraploid
A. amplectans and achiasmatic triploid A. amplectans is the most definitive evidence
to date that the presence of apparently structurally normal SCs is not sufficient to
guarantee crossing over and chiasma formation, in so far as genetic crossing over may
be equated with chiasma formation (Brown & Zohary, 1955; Kayano, i960; Taylor,
1965; Peacock, 1970).
However, there is little question that the SC is involved in crossing over since its
absence results in an achiasmatic condition (Meyer, 1964; La Cour & Wells, 1970).
Moses (1968) and others have postulated that if the SC is not a sufficient prerequisite
for crossing over, it is a necessary prerequisite for the high rates of crossing over
observed in meiotic tissue compared to somatic tissue. This granted, the question
remains, what function does the SC perform in crossing over ? Studies on the molecular
composition of SCs suggest some limitations on the manner in which it could be involved in crossing over. When SCs are subjected to digestive enzymes for DNA, RNA,
and protein, it has generally been shown that DNA does not constitute a detectable
part of the SC and that the SC is primarily constituted either of protein and RNA or
protein alone (Swift, 1962; Nebel & Coulon, 1962; Brinkley & Bryan, 1964; Comings
& Okada, 1970; Westergaard & von Wettstein, 1970). Coleman & Moses (1964) used
indium trichloride as a specific stain for DNA in electron microscopy. They concluded
that DNA resides in the fibrillar chromatin surrounding the SC, in the lateral elements,
and possibly in the transverse microfibrils. There has never been any confirmation of
this report of DNA in an SC and the authors have not supported it strongly (Moses,
1969). Stern & Hotta (1967) reported that inhibition of DNA synthesis during zygotene
prevents SC formation. The function of DNA synthesized in zygotene is not clear, and
if it plays a structural role in the SC, it must be a small one. In some cases structurally
intact SCs appear to be discarded at diplotene, which is difficult to reconcile with the
idea that genetic material (DNA) extends into the SC (Roth, 1966). Although Solari
(1970) and Westergaard & von Wettstein (1970) have reported short remnants of SC
in sites that appear to be chiasmata in early diplotene, these remnants of SC are also
sloughed off shortly. If DNA does not extend into the SC, then DNA hybridization
within the SC cannot be responsible for chromomere-to-chromomere synapsis or
gene-for-gene pairing. Comings & Okada (1970) and von Wettstein (1971) have
proposed models of synapsis which do not depend on DNA hybridization.
It is generally concluded that most and possibly all of the DNA in SC-associated
chromosomes resides in the chromatin which lies peripheral but attached to the lateral
elements of the SC (Swift, 1962; Nebel & Coulon, 1962; Coleman & Moses, 1964;
Brinkley & Bryan, 1964; von Wettstein, 1971) (Figs. 8-11), and it is possible that it
is this DNA which must be homologously paired before reciprocal crossing over can
occur. Indeed, the chromatin of SC-associated chromosomes is often observed to be
in contact above and below the plane of the SC (Westergaard & von Wettstein, 1966;
Moens, 1969, 1970) (Figs. 8, 10). In explaining gene-for-gene pairing the most recent
hypotheses rely on DNA hybridization (Hotta & Stern, 1971) or some kind of protein-
Synaptonemal complex and achiasmatic condition
89
protein recognition (Comings & Riggs, 1971). Regardless of how gene-for-gene pairing
is achieved, the actual event of crossing over seems to require an enzyme for introducing DNA breaks (an endonuclease), and an enzyme for repairing those breaks (a
DNA ligase) in such a way that crossing over could result. Howell & Stern (1971)
have reported finding a battery of enzymes in Lilium anthers which could perform
these functions. The enzymes they describe include an endonuclease which produces
single-strand nicks in double-stranded DNA, a polynucleotide ligase which repairs
the breaks in single-stranded DNA, and 2 enzymes, polynucleotide kinase and polynucleotide phosphatase, which are capable of rearranging the phosphate groups at
the single-stranded DNA breaks in such a way that the ligase can bind the strands
together.
The question arises, at what point in the sequence of chromosome pairing-chromosome synapsis-DNA pairing-crossing over has triploid A. amplectans broken the usual
chain of events ? Since chromosome pairing and synapsis are apparently normal and
complete and DNA homology and pairing should be good in an autotriploid, the
crossing over enzymes become suspect by default. If we assume that a series of enzymes
such as have been described by Howell & Stern (1971) are involved in crossing over,
we can select a most likely faulty enzyme. If any of the repair enzymes, i.e. polynucleotide ligase, polynucleotide kinase, or polynucleotide phosphatase, were inoperative or absent, then breaks introduced by the endonuclease would not be repaired,
and eventually DNA and chromosome fragmentation would result. Stern & Hotta
(1967) suppressed zygotene-pachytene DNA synthesis (repair?) in Lilium with
deoxyadenosine, and observed chromosome fragmentation as one result. Chromosome
fragmentation has not been observed in triploid A. amplectans, and thus its DNA
repair enzymes are probably present and functional. Alternatively if the meiotic endonuclease were absent or inactive, there would be no breaks, no need for repair, and no
crossing over. When SCs separate from the chromosomes in diplotene, the chromosomes would merely fall apart without chiasmata to hold them together. This is just
what is observed in triploid A. amplectans, and by this rather tenuous line of reasoning,
I conclude that the meiotic endonuclease is the most likely missing or faulty enzyme
to account for the achiasmatic condition in triploid A. amplectans.
This work was supported in part by grants from the Faculty Improvement and Biomedical
Science Support Committees at Colorado State University.
The author thanks Dr Walter Brown for his suggestion to use Navashin's fluid as a fixative
for electron microscopy and his careful reading of the manuscript.
REFERENCES
BEADLE, G. W. (1933). Further studies of asynaptic maize. Cytologia 4, 269-287.
BRINKLEY, B. R. & BRYAN, J. H. D. (1964). The ultrastructure of meiotic prophase chromosomes as revealed by silver aldehyde staining. J. Cell Biol. 23, 14 A.
BROWN, S. W. & ZOHARY, D. (1955). The relationship of chiasmata and crossing over in Lilium
formosanum. Genetics, Princeton 40, 850-873.
CATCHESIDE, D. G. (1954). The genetics of Brevistylis in Oenothera. Heredity, Lond. 8, 125-137.
CHURCH, K. & WIMBER, D. (1971). Meiosis in Ornithogahim virens (Liliaceae) II. Univalent
production by preprophase cold treatment. Expl Cell Res. 64, 119-124.
90
S. Stack
J. R. & MOSES, M. J. (1964). DNA and the fine structure of synaptic chromosomes
in the domestic rooster (Galltis domesticus).J. Cell Biol. 23, 63-78.
COMINGS, D. E. & OKADA, T. A. (1970). Mechanism of chromosome pairing during meiosis.
Nature, Lond. 227, 451-456.
COMINGS, D. E. & RIGGS, A. D. (1971). Molecular mechanisms of chromosome pairing, folding
and function. Nature, Lond. 233, 48-50.
DARLINGTON, C. D. & LA COUR, L. F. (1962). The Handling of Chromosomes. New York:
Hafner Publishing Company.
GASSNER, G. (1967). Synaptinemal complexes: recentfindings.J. Cell Biol. 35, 166 A.
GASSNER, G. (1969). Synaptinemal complexes in the achiasmatic spermatogenesis of Bolbe
nigra Giglio-Tos (Mantoidea). Chromosoma 26, 22-34.
HOTTA, Y. & STERN, H. (1971). Analysis of DNA synthesis during meiotic prophase in Lilium.
J. molec. Biol. 55, 337-355HOWELL, S. H. & STERN, H. (1971). The appearance of DNA breakage and repair activities in
the synchronous meiotic cycle of Lilium. J. molec. Biol. 55, 357-378.
KAYANO, H. (i960). Chiasma studies in structural hybrids III. Reductional and equational
separation in Disporum sessil. Cytologia 25, 461-467.
LA COUR, L. F. &WELLS, B. (1970). Meiotic prophase in anthers of asynaptic wheat. Chromosoma
29, 4I9-427LEVAN, A. (1940). The cytology of Allium amplectans and the occurrence in nature of its asynapsis.
Hereditas 26, 353-394.
MAGUIRE, M. P. (1965). The relationship of crossover frequency to synaptic extent at pachytene
in maize. Genetics, Princeton 51, 23-40.
MAGUIRE, M. P. (1966). The relationship of crossing over to chromosome synapsis in a short
paracentric inversion. Genetics, Princeton 53, 1071-1077.
MENZEL, M. Y. & PRICE, J. M. (1966). Fine structure of synapsed chromosomes in Fj^ Lycopersicon esculentum-Solanum lycopersicoides and its parents. Am.J. Bot. 53, 1079-1086.
MEYER, G. (1964). A possible conelation between the submicroscopic structure of meiotic
chromosomes and crossing over. In Proc. 3rd Europ. reg. Conf. Electron Microsc, vol. B
(ed. M. Titlbach), pp. 461-462. Prague: Academia.
MOENS, P. B. (1969). The fine structure of meiotic chromosome pairing in the triploid, Lilium
tigrinum. J. Cell Biol. 40, 273-279.
MOENS, P. B. (1970). The fine structure of meiotic chromosome pairing in natural and artificial
Lilium polyploids. J. Cell Sci. 7, 55-64.
MOLLENHAUER, H. H. (1964). Plastic embedding mixtures for use in electron microscopy.
Stain Technol. 39, m-114.
MOSES, M. J. (1968). Synaptinemal complex. A. Rev. Genet. 2, 363-412.
MOSES, M. J. (1969). Structure and function of the synaptonemal complex. Genetics, Princeton,
Suppl. 61, 41-51.
MOSES, M. J. & COLEMAN, J. R. (1964). Structural patterns and the functional organization of
chromosomes. In The Role of Chromosomes in Development (ed. M. Locke), pp. 11-49. New
York: Academic Press.
NEBEL, B. R. & COULON, E. M. (1962). Enzyme effects on pachytene chromosomes of the male
pigeon evaluated with the electron microscope. Chromosoma 13, 292-299.
PARCHMAN, L. G. & ROTH, T. F. (1971). Pachytene synaptonemal complexes and meiotic
achiasmatic chromosomes. Chromosoma 33, 129-145.
PEACOCK, W. J. (1970). Replication, recombination and chiasmata in Goniaea atistralasiae
(Orthoptera: Acrididae). Genetics, Princeton 65, 593-617.
PONTECORVO, G. (1958). Trends in Genetic Analysis, pp. 114-127. New York: Columbia University Press.
RAICU, P. & CHIRILA, R. (1971). Cytogenetic study of triploid maize and the frequency of
aneuploids. Caryologia 24, 13-18.
RATNAYAKE, W. E. (1968). Hypothesis to explain synapsis of meiotic chromosomes. Nature,
Lond. 217, 1070.
REDDI, V. R. (1970). Pachytene pairing and the nature of polyploidy in Sorgum arundinaceum.
Caryologia 23, 295-302.
REDFIELD, H. (1930). Crossing over in the third chromosomes of triploids of Drosophila melanogaster. Genetics, Princeton 15, 205-252.
COLEMAN,
Synaptonemal complex and achiasmatic condition
REYNOLDS, E.
91
S. (I 963). The use of lead citrate at high pH as an electron-opaque stain in electron
microscopy. J. Cell Biol. 17, 208-212.
RHOADES, M. M. (1933). An experimental and theoretical study of chromatid crossing over.
Genetics, Princeton 18, 535-555.
ROTH, T. F. (1966). Changes in the synaptinemal complex during meiotic prophase in mosquito
oocytes. Protoplasma 61, 346-386.
SEARS, E. R. (1941). Nullisomics in Triticum vulgare. Genetics, Princeton 26, 167-168.
SIMPSON, G. G., ROE, A. & LEWONTIN, R. C. (i960). Quantitative Zoology, pp. 176-178. New
York: Harcourt, Brace and World, Inc.
SMITH, R. A. & KING, R. C. (1968). Genetic control of synaptinemal complexes in Drosophila
melanogaster. Genetics, Princeton 6o, 335-351.
SOLAIU, A. J. (1970). The behavior of chromosomal axes during diplotene in mouse spermatocytes. Chromosoma 31, 217-230.
SOOST, R. K. (1950). Comparative cytology and genetics of asynaptic mutants in Lycopersicon
esculentum Mill. Genetics, Princeton 36, 410-434.
STERN, C. (1936). Somatic crossing over and segregation in D. melanogaster. Genetics, Princeton
21, 625-730.
STERN, H. & HOTTA, Y. (1967). Chromosome behavior during development of meiotic tissue.
In The Control of Nuclear Activity (ed. L. Goldstein), pp. 47-78. Englewood Cliffs, New
Jersey: Prentice-Hall, Inc.
SWIFT, H. W. (1962). In The Interpretation of Ultrastructure (ed. R. J. C. Harris), pp. 213-232.
New York: Academic Press.
TAYLOR, J. H. (1965). Distribution of tritium labelled DNA among chromosomes during
meiosis I. Spermatogenesis in the grasshoppei. J. Cell Biol. 25, 57-67.
VON WETTSTEIN, D. (1971). The synaptinemal complex and four-strand crossing over. Proc.
natn. Acad. Sd. U.S.A. 68, 851-855.
WESTERCAARD, M. & VON WETTSTEIN, D. (1970). Studies on the mechanism of crossing over IV.
The molecular organization of the synaptinemal complex in Neotiella (Cooke) saccardo
(Ascomycetes). C. r. Trav. Lab. Carlsberg 37, 239-268.
{Received 23 October 1972)
92
5. Stack
Fig. 2. Aceto-orcein squash preparation of a tetraploid A. amplectans microsporocyte
nucleus in pachytene. Almost all strands may be observed to consist of 2 synapsed
chromosomes, x 1750.
Fig. 3. Aceto-orcein squash preparation of a triploid A. amplectans microsporocyte
nucleus in pachytene. Note the doubly thick thread associated with a thin thread at
the top of the photograph. The thick thread is composed of 2 synapsed homologous
chromosomes and the thin thread is the third unsynapsed homologue. An arrow points
to a partner switch, x 2000.
Fig. 4. Aceto-orcein squash preparation of a tetraploid A. amplectans microsporocyte
nucleus in early anaphase I. Numerous chiasmata bind bivalents and quadrivalents
together, x 1100.
Fig. 5. Aceto-orcein squash preparation of a triploid A. amplectans microsporocyte
nucleus in metaphase I. Note the absence of chiasmata and bivalents among the 21
duplicated but unseparated chromosomes, x 1100.
Synaptonemal complex and achiasmatic condition
93
94
S. Slack
Fig. 6. Triploid A. amplectans primary microsporocyte nucleusfixedin glutaraldehydeosmium tetroxide at pachytene. SCs are common (arrows). Univalent-bivalent pairs
are identified with difficulty (double arrow), x 5000.
Fig. 7. Triploid A. amplectans primary microsporocyte fixed in Navashin's fluid at
pachytene. Double arrows point to easily identified homologous bivalent-univalent
pairs in transverse section, x 5000.
Fig. 8. Slightly oblique frontal section of an SC from tetraploid A. amplectans (see
Moses, 1968, for terminology)- x 50000.
Fig. 9. Transverse section of an SC from tetraploid A. amplectans. Arrows indicate
lateral elements, x 50000.
Fig. 10. Frontal section of an SC from triploid A. amplectans. x 50000.
Fig. 11. Transverse section of an SC (short arrows point to its lateral elements) and
its associated univalent (long arrow). Note that the univalent typically does not interfere
with the structure of the SC. x 50000.
Synaptonemal complex and achiasmatic condition