264 Cytologia 28
Cytogenetical Studies of Oryza sativa L.
and its Related Species
5. Differential condensation and chromosome pairing
in the hybrid O. sativa•~O. australiensis1
H. W. Li, C. C. Chen, H. K. Wu and Katherine C. L. Lug
Institute of Botany, Academia Sinica, Nankang, Taipei, Taiwan, Free China
Received February 7, 1963
O. australiensis was designated recently as having genome E (Li et al.
1963) after it was crossed successfully by various rice geneticists with O.
sativa, O. officinalis, O. minuta, O. paraguaiensis and O. alta (Gopalakri
shnan 1959, cited by Richharia 1960, Nezu et al. 1960, Morinaga and Ku
riyama 1960, Morinaga et al. 1960, Li et al. 1961, and Hu unpubl.). In
the 3n hybrid of O. sativa•~O. australiensis, Li et al. (1963) found that
the 12 pairs of large chromosomes were derived from the unreduced male
gamete of O. australiensis. Accordingly, in the diploid hybrid of the same
cross, there were twelve distinctly large chromosomes of O. australiensis and
twelve much smaller chromosomes of O. sativa. The chromosomes of O.
australiensis migrated frequently ahead those of O. sativa at MI-AI and the
chromosomes of the latter tended to lag mostly on the equator. In 150 micro
sporocytes studied there were only 2.40 bivalents as an average per cell with a
range of 0-7. Of these bivalents, 0.23 was found to be autosyndetic pairing
of australiensis chromosomes (AA), 0.55 autosyndetic pairing of sativa chro
mosomes (SS) and 1.66 allosyndetic pairing (AS) (Table 2). Of the 35 austra
liensis bivalents (AA) found in 150 PMC's 18 were open type and 17 closed
ones. Of the 83 sativa bivalents found, 82 were open type and only 1 closed
one. Of the 248 AS bivalents observed, most of them were of the open type.
After our manuscript was sent to press. We came across a paper by
Shastry and Ranga Rao 1961 (received in Nov. 1962) dealing with the micro
sporogenesis of the hybrid O. sativa•~O. australiensis, in which they made
these observations and conclusions:
1. Several PMC's at pachytene stage were analysed and the chromo
somes were found to be univalent nature.
2. In the PMC's analysed at diplotene stage, only twelve configurations
were visible and they were assumed to be of O. australiensis complement .
3. In 44 PMC's analysed at diakinesis, twelve O. australiensis chromo
somes were observed but with varying number of O. sativa univalents.
1 Contribution from Institute of Botany, Academia Sinica , Taiwan, China. This work
was partly supported by a subsidy from the National Council on Science Development.
2 Research Fellow, Assistant Research Fellows and Research Assistant respectively .
1963 Cytogenetical Studies of Oryza sativa L . and its Related Species 265
4. True allosyndetic bivalents were not found at MI . The most frequent associations were non-chiasmatic, end-to-end pseudobivalents .
5. Autosyndetic bivalents were recorded mostly in the complement be
longing to O. sativa.
6. The meiotic cycle exhibited timing imbalance with earlier condensa
tion, and possibly earlier migration of the australiensis bivalents .All these observations and conclusions were somewhat at variance with
our own, so a more detailed study was made from the fixed material on hand.
The results are here reported in this paper .
Material and methods
The F1 diploid hybrid of O. sativa•~O. australiensis was furnished by
Dr. C. H. Hu of Chung Hsing University to whom the authors were greatly
grateful. The female parent was Taichung No. 65, a japonica variety. The
accession number of the male parent, O. australiensis, was W 008 of Dr.
Oka's collection. Propiono-carmine smear method was used exclusively through
out this study.
Observations
The data from which the conclusions were based were founded on examination of hundreds of PMC's at pachynema and diplonema in F1. Counts were made from 50 PMC's at diakinesis and 150 at MI-AI. Studies on
pachynema of both parents were done by one of us who examined hundreds of nice preparations. Only samples were presented here. Detailed report on this will be presented elsewhere.
1. Pachytene stage. The twelve pairs of chromosomes could be identified with ease both for O. sativa and O. australiensis (Figs. 1, 2, 2a). However, there was some difficulty in the identification of the centromere in some chromosomes of O. sativa but not with O. australiensis. The salient feature to be mentioned here was the fact that practically most, if not all, of the chromosomes of O. australiensis were prominently made up partly of heterochromatin. One chromosome was almost entirely composed of heterochromatin. In others, the heterochromatic material was found in the proximal region, distal end or ends. One of these chromosomes seemed to be made up mostly of euchromatin (Fig. 2a). Our observation seemed to agree approximately with those of Shastry and Mohan Rao (1961). On the contrary, the chromosomes of O. sativa were made up mainly, if not all, of euchromatin. In general, the chromosomes of O. australiensis did look much thicker and darker stained in appearance than those of O. sativa at this stage even though there seemed to have no appreciable difference in the length of the chromosomes of these two species (Shastry and Mohan Rao 1961, and H. K. Wu unpubl.).
266 H. W. Li, C. C. Chen, H. K. Wu and Katherine C. L. Lu Cytologia 28
Figs. 1-4a. Pachytene stage of the parents and their F1 hybrid. 1, O. sativa. Notice that
there are no observable heterochromatic regions on these chromosomes. 2, O. australiensis.
Notice the heterochromatic regions on almost all of the chromosomes. 2a, camera lucida
drawing of the individual chromosomes of Fig. 2, showing noticeably heterochromatic re
gions on different chromosomes. One of them seems to be devoid of any heterochromatin.
3, one segment of a bivalent of the F1 hybrid showing the loose nature of synapsis. The
identity of these paired chromosomes is not recognized. 3a, camera lucida drawing of the
same as Fig. 3. 4, another paired segment of the F1 hybrid. 4a, camera lucida drawing
of the same as Fig. 4. Figs. 1, 2, 2a•~1570. Figs. 3, 4•~2500. Figs. 3a, 4a•~2250.
In the F1 hybrid, attempts were repeatedly tried to make preparations so
that chromosomes could be more or less identifiable but failed. The chromo
somes were clumped tightly together. Efforts were made assiduosly and patiently
1963 Cytogenetical Studies of Oryza sativa L. and its Related Species 267
therefore to find out whether or not any chromosome was in paired condition. Finally, the dividend was paid off, and many such chromosomes were found to be recognizably paired (Figs. 3, 3a , 4, 4a), at least in the region which stood out separately from the rest of the clump . These paired chromosomes were typified by having loops and bumps which were characteristic of having structural segmental difference. The relational coiling of these paired chromosomes was recognizable and the ends of the two paired chromosomes were usually widely separated, showing that synapsis took place only between certain segments of the chromosomes concerned. It was unfortunate indeed that the identity of these paired chromosomes was not recognized as to be either autosyndetical or allosyndetical. Other chromosomes were seemingly of univalent nature. Some chromosomes, (partly), presumably belonging to australiensis were seen to be darker stained and were highly impregnated with heterochromatin, and they were definitely thicker. Others were lighter in staining and thinner in appearance. Presumably, these were the sativa chromosomes.
2. Diplotene stage. With propiono-carmine, the chromosomes of O. sativa at mid-diplonema did not take stain up readily sometimes so that they were rather faintly stained (H. K. Wu unpubl.) (Fig. 5). On the other hand, the chromosomes of O. australiensis were deeply stained and had loops and nodes characteristic of the presence of chiasmata (Fig. 6).
The chromosomes of the F1 hybrid were rather sticky, so that they were more or less clumped together. The darkly stained thicker chromosomes of O. australiensis were rather conspicuous at this stage. But the faintly stained thinner chromosomes of O. sativa were also discernible with careful observation (Fig. 7). However, the individual chromosomes of these two species were not easily separately identified, particularly the sativa chromosomes. At this stage, most of the bivalents were end-to-end paired (Figs. 8, 9). Efforts were attempted to find allosyndetically paired chromosomes with more than one chiasma. Only two were finally detected (Figs. 10, 11). One of them could be very easily separated by their degree of staining (Fig. 10). The other was found with 3 distinct chiasmata at late diplonema (Fig. 11), the only distinct one of its kind in our studies so far. It seemed that the chromosomes of O. australiensis contracted more at this stage than its paired O. sativa chromosome (Fig. 10). Mention should be made here that there were heterochromatic connections (thin threads) linking many chromosomes together.
3. Diakinesis. In the F1 hybrid, many PMC's at this stage, both early and late were studied. It was found that the chromosomes of both australiensis and sativa contracted further but at a different rate. As a result, the chromosomes of australiensis were more darkly stained. At this stage, the chromosomes of O. australiensis and O. sativa were approaching one another in size more or less. In some PMC's, however, the chromosomes of sativa were only faintly stained that they were almost unrecognizable. Nevertheless, with careful scrutiny, they could be identified (Fig. 13). Thus, attempts were
268 H. W. Li, C. C. Chen, H. K. Wu and Katherine C. L. Lu Cytologia 28
1963 Cytogenetical Studies of Oryza sativa L . and its Related Species 269
made to study the association of the chromosomes and these results are pre
sented in Table 1.
From Table 1, it can be found that the association of the chromosomes
of australiensis and sativa was more or less similar to that found at MI -AI as presented in Table 2 in a summarized form . The significance of this will be discussed later.
Most of the bivalents found were end-to-end configuration at this stage . But efforts were again made to find whether or not the so-called allosyndetic
pairs (AS) were in existence. Several such bivalents were found to be ringshaped signifying the existence of two chiasmata per bivalent (Figs . 14, 14a, 20, 20a).
4. MI-AI. We do agree with Shastry and Ranga Rao (1961) that owing
to the presence of so many univalents (19 .02 per cell, Table 2) in a PMC, MI and early AI can not be differentiated with ease . From the study of 150 PMC's, we found AA association was about half as much as SS association . But this was very different from the finding of Shastry and Ranga Rao (1961) . They found almost nil in AA association. The same proportion of AA and
SS was found in diakinesis (Table 1) from a study of 50 PMC's . With AS association, there were 1.66 such bivalents per PMC at this stage as against 1.36 found at diakinesis.
In our study, with the AA bivalents, 17 were closed type (Fig. 22) and
18 open type (Fig. 21) in a total of 35 such bivalents found at this stage.
Of 83 SS bivalents found, all but one were open type (Fig. 23). With AS
bivalents, most of them were of open type (Fig. 24), but the proportion of
open and closed ones was not recorded. There were some bivalents of closed
type (Figs. 25, 25a, 26, 26a) at this stage upon reexamination, clearly indicat
- Figs. 5-20a. 5-12a. Diplotene stage of the parents and their F1 hybrid. 5, O. sativa. 6, O.
australiensis. 7, F1 hybrid, showing about 24 elements. The australiensis chromosomes are
darkly stained in the heterochromatic regions, whereas the sativa chromosomes are faintly
stained at this stage. 8, camera lucida drawing of a pair of sativa chromosomes of the F1
hybrid. In contrast, a darkly stained more condensed australiensis univalents is shown on
the top of the drawing. 9, camera lucida drawing of an allosyndetic rod-shaped bivalent. 10,
camera lucida drawing of an allosyndetic ring-shaped bivalent at early diplonema showing
the existence of two chiasmata. 11, camera lucida drawing of an allosyndetic bivalent
showing the existence of three chiasmata. 12, an autosyndetic ring-shaped australiensis
bivalent of the F1 hybrid. 12a, camera lucida drawing of the same as Fig. 12. Notice the
hetero- and euchromatic regions. 13-20a. Diakinesis of the F1 hybrid. 13, late diakinesis
showing 24 elements. Twelve of them are darkly stained whereas the remainder lightly stained.
Notice that the size of these two different sets of chromosomes is more or less the same.
14, middiakinesis showing one allosyndetic ring-shaped bivalent. 14a, camera lucida drawing
of the same as Fig. 14. 15, camera lucida drawing of a sativa trivalent. 16 and 17, camera
lucida drawing of one rod-shaped australiensis bivalent. 18 and 19, camera lucida drawing
of one rod-shaped allosyndetic bivalent. 20, a ring-shaped allosyndetic bivalent. Notice
that the size of these two chromosomes are almost the same. 20a, camera lucida drawing
of the same as Fig. 20. Figs. 5-6. •~1570. Fig. 7. •~1910. Figs. 8-11, 12a, 14a-19. •~2250.
Figs. 12, 20. •~2500. Fig. 20a. •~3000. Figs. 13-14. •~1230.
270 H. W. Li, C. C. Chen, H. K. Wu and Katherine C. L. Lu Cytologia 28
Table 1. Chromosome association at diakinesis in the hydrid O. sativa•~O. australiensis
1 A and S represent chromosomes of O. australiensis and O. sativa respectively.
Table 2. Chromosome association at MI-AI in the hybrid O. sativa•~O. australiensis
1 A and S represent chromosomes of O . australiensis and O. sativa respectively.
ing the existence of two chiasmata (Figs. 25, 25a). It is inferred therefore that the open type bivalents would have one chiasma.
1963 Cytogenetical Studies of Oryza sativa L . and its Related Species 271
Figs. 21-32. MI-AI of the F1 hybrid. 21, a rod-shaped autosyndetic australiensis bivalent
. 22, a ring-shaped autosyndetic australiensis bivalent. 23, a rod-shaped autosyndetic sativa
bivalent. 24, a rod-shaped allosyndetic bivalent. 25, an allosyndetic bivalent on the right
showing the existence of two chiasmata. 25a, camera lucida drawing of the same as Fig.
25. 26, another allosyndetic bivalent with two chiasmata. 26a, camera lucida drawing of
the same as Fig. 26. 27, prometaphase showing chromosomes are all over the cell. 28, the
congression of the chromosomes at the equator. 29 and 30, the differential migration of the
chromosomes of the two complements. 31, the bivalents as well as the australiensis uni
valents are moving to the respective poles. 32, all of the chromosomes are dividing at this
stage. Figs. 21-25, 26. •~1910. Figs. 25a, 26a. •~3375. Figs. 27-32 •~1417.
From our former study, we found that the sativa chromosomes were about 1/4-1/2 the size of australiensis chromosomes. It was found at diakinesis
272 H. W. Li, C. C. Chen, H. K. Wu and Katherine C. L. Lu Cytologia 28
(especially late stage) that the size of the chromosomes of A and S was about equal. Definite comparison could be made from the paired bivalent of AS type (Figs. 20, 20a). The two chromosomes were about the same size. However at metaphase, the paired AS bivalent was very much different in size (Figs. 24, 25, 25a, 26, 26a). It shows that the chromosomes of sativa with mostly euchromatic constitution did contract further at this stage. They contracted slower at early stages but completed its final degree of contraction at metaphase. On the contrary, the chromosomes of australiensis which were made up of heterochromatin and euchromatin contracted faster in the earlier stages and stopped their contraction quite early perhaps as early as late diplonema or diakinesis.
There seemed to have some difference in stainability of the chromosomes of australiensis and sativa complement at this stage (Fig. 26) in some PMC's. But in many other PMC's studied, except the size difference, the chromosomes of both complements stained almost identically.
There seemed to have a definite trend for the australiensis chromosomes to migrate to the poles ahead of those of sativa. Table 3 shows the results of our counts which were more or less similar to the data obtained by Shastry and Ranga Rao (1961).
In the movement of chromosomes at this stage, we must take these into consideration:
1. There are chromosomes of two separate complements and,2. There are mostly univalents and none or few bivalents or rarely
multivalents.
Table 3. Frequency distribution of sativa and australiensis chromosomes at both of the
poles at MI-AI in FL hybrid of O. sativa •~O. australiensis
A test of independence was calculated with x2=171.27 and p<0.01 with 110 degrees of
freedom showing that the hypothesis of independent migration of chromosomes of O. sativa
and O. australiensis to poles was not valid.
1963 Cytogenetical Studies of Oryza sativa L. and its Related Species 273
At prometaphase, the chromosomes were distributed all over the cell (Fig . 27). Presumably, congression was taken place as a rule (Fig . 28). The migration of the australiensis univalents to the poles was followed by the separation of the bivalents leaving most of the sativa univalents as laggards on the equator (Figs. 29, 30, 31). In some PMC's , only rarely the australiensis univalents divided at the poles, with the sativa univalents dividing at the equator after the chromosomes of the bivalents reached their respective poles (Fig. 32).
In case the chromosomes of all the PMC's would follow the model just described then we could safely conclude that the migration of the australiensis chromosomes was ahead of those sativa chromosomes and even ahead of the bivalents which were bound together by their chiasma or chiasmata. In Bromus hybrids, Walters (1958) found that during metaphase the numerous univalents moved to the poles then returned to the equator, and the spindle increased and decreased in length. Anaphase might begin at any time during the return of the univalents to the equator. In the hybrid studied the movement of chromosomes did not behave like those in Bromus hybrids.
Discussion
1. Bivalents or pseudobivalents
Because of the appreciable difference in the size of the chromosomes of
the two complements concerned in the Fl hybrid at MI-AI, autosyndetical
pairing can be rather easily differentiated from allosyndetical one. About two thirds of the bivalents recorded are of allosyndetical pairing. These were considered to be pseudo-bivalents by Shastry and Ranga Rao (1961) with their
hybrid material studied. The use of the terms 'pseudo-bivalent' (Walters
1954), 'quasi-bivalent' (Ostergren and Vigfusson 1953), 'association not due
to chiasmata' (Shastry et al. 1961) etc., signifies that these kind of bivalents
are not true bivalents. By true bivalent formation, according to Shastry and
Ranga Rao (1961), it "is indicative of homologous pairing at prophase fol
lowed by persistent chiasma formation" at later stages. In this hybrid, ac
cording to Shastry and Ranga Rao (1961), "the strongest point in favor of
their reflecting homology between the genomes are that all of those associa
tions are allosyndetic and that time imbalance does not provide the necessary
conditions for the formation of true bivalents. The weakest point of their being indicator of homology are end-to-end associations and that the karyotype
of one of the species (O. australiensis) has extensive heterochromatic segments, which can exhibit stickiness".
With our hybrid material, we found paired segments at pachynema even
though the identity of these was not revealed as to whether they are allo- or
autosyndetical pairing. The rather loosely paired condition would indicate
structure difference of the paired segments. At diplonema the chiasma or chiasmata of the bivalents found (auto- as well as allosyndetical) are com-
Cytologia 28, 1963 18
274 H. W. Li, C. C. Chen, H. K. Wu and Katherine C. L. Lu Cytologia 28
pletely terminated so that usually only the end-to-end bivalents each with one terminal chiasma are found at diakinesis. The persistent occurrence of two
or more chiasmata in an allosyndetical pair at diplonema, diakinesis and meta
phase I would indicate that these bivalents are nothing but true bivalents. Accordingly, the end-to-end bivalents at this stage are also true bivalents having
only one chiasma.
2. Differential condensation and chromosome pairing in the hybridThe behavior of particular chromosomes or parts of chromosomes has
long been known to be different in stainability from that of the rest of the
complement. There is the different behavior at telophase and prophase of
mitosis. Certain parts of most of the chromosomes, usually proximal or
distal, stain more deeply than the rest but not with Feulgen reagent. This differential material is the heterochromatin. There is a uniform precocious
condensation in some organisms by all the chromosomes in their proximal
parts at the pachytene and diplotene stages of meiosis. For instance in Fritillaria, it is associated with early pairing and early diplotene separation
of the condensed part. This can be represented as a timing difference at meiosis, the proximal parts being in advance of the distal parts (Darlington
1937, pp. 208-309). Speaking about the behavior of heterochromatin as con
trasted with that of euchromatin in general, the differential condensation does not show itself before the prophase of meiosis, and is seen only in the more
rapid condensation of the affected chromosomes after zygonema. The pre
cocious chromosomes do not contract further than the rest at metaphase, but
during the ensuing interphase and at second metaphase, they continue to show
exceptional condensation (Darlington 1937, p. 311).
The Y-chromosomes in Drosophila melanogaster is made up of hetero
chromatin, and is larger than the X-chromosomes in somatic cells. The Y
chromosome possesses heterochromatin at the proximal region, but the bulk is made up of euchromatin (Swanson 1957, p. 114). In maize, the B-chromosome
has six chromomeres at the proximal region (almost terminal centromere)
which are euchromatic, but the rest is heterochromatic (McClintock 1933).
In the F1 hybrid, there is marked differential condensation of the two
complements. This was termed by Shastry and Ranga Rao (1961) as timing
imbalance in the condensation of the chromosomes. Most of the chromo
somes of O. australiensis probably with the exception of one, are made up
in parts of heterochromatin. On the other hand, all the chromosomes of O. sativa are seemingly made up of euchromatin. Thus , heterochromaticloaded australiensis chromosomes condense precociously at zygonema and
onward till diakinesis and no further. The euchromatic-loaded sativa chro
mosomes condense later at the start but terminate the process of condensation
till MI. Stages further than AI are not observed, so whether or not further
condensation is taken place, nothing is known. This conclusion is evidenced
by the existence of more or less equal sized ring bivalents at diakinesis (Figs.
1963 Cytogenetical Studies of Oryza sativa L. and its Related Species 275
20, 20a), but the chromosomes of the allosyndetic bivalents are markedly different in size at MI (Figs. 24, 25, 26). Furthermore , the univalents of the two
complements are more or less similar in size at diakinesis but very much different at MI (Figs. 13, 27, 30).
Feulgen reaction is not tried as yet . With the propiono-carmine stain used in our experiment, the chromosomes containing heterochromatin are much darker stained from mid-prophase on till diakinesis. This differential staining persists sometimes in some PMC's at MI (Fig . 26).
From our observation, the chromosomes of these two complements can be separated before diakinesis by difference in staining, but they are to be differentiated only by size alone at MI-AI.
In Rhytidolomia senilis the X- and Y-chromosomes formed a terminal union of euchromatic ends at diakinesis when the chromosomes are highly condensed with the heterochromatic regions not fully condensed at this time (Swanson 1957, p. 208). This is the reverse situation from our case as far as condensation is concerned. The heterochromatin loaded australiensis chromosomes are rather localized. The heterochromatic segment may be at the proximal region, or distal end or ends but not in the whole chromosome. Thus the homologous segments of the chromosomes from two complements can be synapsed together. Consequently chiasma or chiasmata is formed after being loosely paired at pachynema. The presence of heterochromatic region on one
partner of the paired bivalents probably hasten the completion of terminalization at an earlier stage, in this case, diplotene.3. Differential migration of the chromosomes
It is noted both by the Indian workers and us that australiensis univalents move ahead of the sativa univalents. By virtue of having heterochromatic segments in almost all the chromosomes of australiensis, this inherent property would render them to differ markedly from the almost heterochromatin free chromosomes of sativa, so their coiling process would be more rapid at mid-prophase and onward to diakinesis. Perhaps this residual "tempo" left may have been carried over to the first metaphase so that these univalents may complete their congression and separation a little earlier than those euchromatic chromosomes of sativa. Consequently, they migrate to the poles ahead of the sativa chromosomes.
Should this residual "tempo" be absent, then another assumption may be made for its stead. The australiensis univalents simply do not congress at the equator as the bivalents and sativa univalents do, but remain at the nearest pole where they are located to start with. Nothing can be ascertained about the validity of these two assumptions.
Summary
The chromosomes of O. australiensis were made up partly of hetero
chromatin whereas those of O. sativa were almost of euchromatin.
18*
276 H. W. Li, C. C. Chen, H. K. Wu and Katherine C. L. Lu Cytologia 28
In the F1 hybrid of O. sativa•~O. australiensis, there was differential
condensation in these two morphologically different types of chromosomes.
The ones with partly heterochromatin and partly euchromatin condensed early
starting off presumably from pachynema on till diakinesis. Whereas the ones
with only euchromatin seemingly started their condensation later but condensed
more complete at first metaphase. Thus before diakinesis the australiensis
chromosomes were darker in staining and were 2-4 times the size of the sativa
chromosomes at MI-AI.
At either diakinesis or MI-AI, about two bivalents could be found per
PMC. These bivalents could be separated into two types at MI-AI by size,
or by difference in taken up the stain at diakinesis:
1. autosyndetic
2. allosyndetic (multivalents were also found only very rarely)
All these bivalents were proved to be authentically true bivalents. The
evidences were:
1. There were loosely paired chromosomal segment observed repeatedly
in many PMC's at pachynema.
2. At diplonema, allosyndetically paired bivalents were found to have
one chiasma mostly, or two or more chiasmata in some PMC's.
3. At diakinesis, these allosyndetically paired bivalents were found to be
ring-shaped as well as the end-to-end ones.
4. Closed allosyndetic bivalents with two chiasmata were frequently ob
served at MI-AI.
Pairing of the homologous segments in these allosyndetic pairs was as
sumed to be carried out at the euchromatic regions of the two chromosomes
concerned. Presumably, these euchromatic regions of the chromosomes from
two different species might have same rate of condensation at various stages
of meiosis.
Literature cited
Darlington, C. D. 1937. Recent Advances in Cytology. Churchill. London.
Hu, C. H. 1961. Comparative karyological studies of wild and cultivated species of Oryza
(in Japanese, unpubl.).
Li, H. W., Weng, T. S., Chen, C. C. and Wang, W. H. 1961. Cytogenetical studies of Oryza
sativa L. and its related species. 1. Hybrids O. paraguaiensis Wedd. •~O. brachy
antha Rhev. et Roehr., O. paraguaiensis Wedd. •~O. australiensis Domin. and O.
australiensis Domin. •~O. alta Swallen. Bot. Bull. Academia Sinica 2: 79-85.
- Chen, C. C., Weng, T. S. and Wuu, K. D. 1963. Cytogenetical studies of Oryza sativa L.
and its related species. 4. Interspecific crosses involving O. australiensis with O.
sativa and O. minuta. Bot. Bull. Academia Sinica 4: 65-74.
McClintock, B. 1933. The association of the non-homologous parts of chromosomes in the
mid-prophase of Zea mays. Zeitschr. Zellf. u. Mikr. Anat. 19: 191-237.
Morinaga, T. and Kuriyama, H. 1960. Interspecific crosses of rice species and their genome
constitution (in Japanese). Agric. and Hort. 35: 1-15.
- Kuriyama, H. and Ono, S. 1960. On the interspecific hybrid of Oryza minuta and O .
australiensis. Jap. Jour. Genet. 35: 277-278.
1963 Cytogenetical Studies of Oryza sativa L. and its Related Species 277
Nezu, M., Katayama, T. C. and Kihara, H. 1961. Genetic study of the genus Oryza. 1.
Crossability and chromosomal affinity among 17 species. Seiken Ziho 11: 1-11.
Ostergren, G. and Vigfusson, E. 1953. On position correlations of univalents and quasivalents
formed by sticky univalents. Hereditas 39: 33-50.
Richharia, R. H. 1960. Origin of cultivated rices. Ind. Jour. Genet. and Pl. Breeding 20:
1-14.
Shastry, S. V. S., Sharma, S. D. and Ranga Rao., D. R. 1961. Pachytene analysis in Oryza.
III. Meiosis in an intersectional hybrid, O. sativa•~O. officinalis. The Nucleus 4:
67-80.
- and Mohan Rao, P. K. 1961. Pachytene analysis in Oryza. IV. Chromosome morphology
of O. australiensis Domin., O. glaberrima Steud. and O. stap fii Rosch. Proc. Indian
Acad. Sci. 54: 100-112.
- and Ranga Rao, D. R. 1961. Timing imbalance in the meiosis of the F1 hybrid Oryza
sativa•~O. australiensis. Genet. Res. 2: 373-383.
Swanson, C. P. 1957. Cytology and Cytogenetics. Prentice Hall.
Walters, M. S. 1954. A study of pseudobivalents in meiosis of two interspecific hybrids of
Bromus. Amer. Jour. Bot. 41: 160-171.
- 1958. Aberrant chromosome movement and spindle formation in meiosis of Bromus hy
brids: an interpretation of spindle organization. Amer. Jour. Bot. 54: 271-289.
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