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A correlation between the localisation of chiasmata and the replication pattern of chromosomal DNA
@ 1966 by Academic Experimental
Press Inc.
Cell Research
44, 161-161
161
(1966)
A CORRELATION BETWEEN THE LOCALISATION CHIASMATA AND THE REPLICATION PATTERN CHROMOSOMAL DNA H. Department
of Agricultural
REES
and
Botany,
University
College
April
28, 1966
Received
G.
M.
OF OF
EVANS of Wales,
Aberystwyth,
U.K.
TIIE following
kinds of evidence suggest that the pattern of distribution of chiasmata at meiosis may be correlated with that of DNA replication during the synthesis, S, phase of the division cycle. (1) The influence of the centromere, its “interference” to DNA replication at late S in Scilla [a], suggests a parallel with the centromere interference to chiasma formation found in many organisms including Scilla. (2) In rye [2] and in Tradescantia [6] late DNA synthesis is found mainly in distal regions of the chromosomes. So, also, are the chiasmata at meiosis. (3) Temperature shocks and exposure to ionising radiations at a particular phase of DNA synthesis affect chiasma formation at the subsequent meiosis [3, 11. The indications are that DNh synthesis is in some way connected with chiasma formation and it may be that the chromosome regions synthesising DNA at this critical, sensitive, phase are those later involved in forming chiasmata. For these reasons an investigation was made, in Scilla campanulata, to determine, as far as possible, the relationship between the pattern of chromosome replication and the localisation of chiasmata at meiosis. The species is particularly suitable for the purpose. In the first place, a detailed account is already available of the DNA replication pattern for all eight pairs of chromosomes [2] and, secondly, the distribution of chiasmata at meiosis can be determined fairly easily in specific chromosomes at metaphase 1 in pollen mother cells. RESULTS
A first metaphase in a pollen mother cell of Scilla campanulata is shown in Fig. 1. For scoring, the different chromosome types were classified and numbered I to VIII according to a system earlier used by Rees [5]. Each chromosome was then arbitrarily divided into segments of roughly equal II-
661809
Experimental
Cell
Research
44
162
H. Rees and
Fig.
l.-First
metaphase
I. Average
TABLE
of meiosis
chiasma
in a pollen
G. M. Evans
mother
frequencies
per
cell of SciUu
cell
in
campanulata.
chromosome
x ca 1450.
segments
of
Scilla campanulata. Data
for each replicate
from
10 pollen mother cells. corresponding segments
Chromosome
Chiasma
I
Replicate Replicate
1 2
I I-IV
Replicate Replicate
1 2
v
Replicate Replicate
1 2
VI
Replicate Replicate
1 2
VII
Replicate Replicate
1 2
VIII
Replicate Replicate
1 2
Mean
Mean
0.20 0.60 0.40
0.40 0.20 0.30 0.13 0.13 0.13
Mean
Mean
Mean
Mean Experimental
Data for chromosome in the two arms.
Cell
Research
44
0.20 0.10 0.15 0.10 0 0.05 0.30 0.20 0.25
frequency 0.10 0.30 0.20 0.23 0.23 0.23 0.05 0.05 0.05 0.30 0.20 0.25 0 0.10 0.05 0.20 0.50 0.45
V have
been pooled
for
per segment 0.60 0.40 0.50 0.33 0.50 0.42 0.55 0.55 0.55 0.60 0.62 0.61 0.90 0.50 0.70 0.60 0.40 0..50
0.40 0.80 0.60 0.70 0.40 0.55 0.40 0.35 0.38 0.40 0.20 0.30 0.40 0.40 0.40
0.40 0.30 0.35 0.57 0.27 0.42
Chiasmata
localization
and DNA
163
replication
lengths (see Table I) and the chiasma frequency for each segment estimated from 10 pollen mother cells in each of two separate slides. Because chromosomes II, III and IV are not always identifiable at first metaphase mean values for the three were estimated from pooled data from each pollen mother cell. For chromosome V, which has a very nearly median centromere, the chiasma frequencies were likewise pooled for corresponding segments in the long and short arms which cannot be distinguished from one another at meiosis. The chiasma frequencies are plotted also in Fig. 2. The results plotted, in view of their consistency, were pooled over replicates.
4-
-0.8
VIII
V 0
3 I/ p”o-
-0.4
0
0’ / 0 I
I (\ I
I
I
I
CSegments Fig. Z.-The distribution of mean chiasma frequencies (broken lines) and of mean grain counts following tritium labelling at late S (solid line) in segments of Scilla campanulata chromosomes. Areas with high grain counts are those which are late in synthesising DNA. Note the correlation between late synthesis and chiasma localisation. Experimental
Cell Research
44
164
H. Rees and G. hf. Evans
Also in Fig. 2 are superimposed the grain counts from autoradiographs prepared during the labelling experiment referred to above [2]. Segments showing high and low grain counts, respectively, are those with high and low amounts of DNA replication at late S of mitosis. CONCLUSIONS
From Fig. 2 it will be observed that there is an extremely close correlation between the distribution curves relating to chiasmata and to DNA synthesis at late S. Chromosome regions with high chiasma frequencies are those which, in late S, show a high degree of DNA synthesis, and vice versa. It is, of course, appreciated that the labelling results pertain to mitosis and not to meiosis and any speculation upon a possible causal relationship between chiasma formation and DNA synthesis must, hence, be qualified. Until we know more of chromosome fine structure and behaviour at meiosis it would in any case be rash to attempt to explain what, precisely, this relationship might be. One thing is certain. L4ny relationship between DNA replication at late S and chiasma localisation at meiosis can neither be direct nor simple because for one thing the chiasmata, it is generally agreed, are actually formed when all, or virtually all, the DNA synthesis is completed. Rather than add to the present welter of speculation about the mechanics of chiasma formation it would, perhaps, be best simply to re-emphasise that these results show a correlation between the patterns of DNA replication and of chiasma localisation and that the correlation may have a causal basis. SUMMARY
The distribution of chiasmata at meiosis within chromosomes of Scilla is correlated with the pattern of DNA replication of late S. Chromosome regions with the highest degree of DNA synthesis at late S, namely those interstitial segments away from the chromosome ends and from the centromeres, have the highest chiasma frequencies. A causal relation is suggested. campanulata
REFERENCES 1. 2. 3. 4. 5. 6.
DARLINGTON, C. D. and H~QUE, A., Chromosomes today 1, 102 (1966). EVANS, G. M. and REES, H., Ezpff Ceff Res. 44, 150 (1966). LAWRENCE, C. W., Heredity 16, 83 (1964). -Nafure 106, 789 (1965). REES, H., Suppl. Heredity 6, 235 (1952). WIMBER, D. E., Exptf Cell Res. 23, 402 (1961).
Experimental
Cell
Research
44
Press Inc.
Cell Research
44, 161-161
161
(1966)
A CORRELATION BETWEEN THE LOCALISATION CHIASMATA AND THE REPLICATION PATTERN CHROMOSOMAL DNA H. Department
of Agricultural
REES
and
Botany,
University
College
April
28, 1966
Received
G.
M.
OF OF
EVANS of Wales,
Aberystwyth,
U.K.
TIIE following
kinds of evidence suggest that the pattern of distribution of chiasmata at meiosis may be correlated with that of DNA replication during the synthesis, S, phase of the division cycle. (1) The influence of the centromere, its “interference” to DNA replication at late S in Scilla [a], suggests a parallel with the centromere interference to chiasma formation found in many organisms including Scilla. (2) In rye [2] and in Tradescantia [6] late DNA synthesis is found mainly in distal regions of the chromosomes. So, also, are the chiasmata at meiosis. (3) Temperature shocks and exposure to ionising radiations at a particular phase of DNA synthesis affect chiasma formation at the subsequent meiosis [3, 11. The indications are that DNh synthesis is in some way connected with chiasma formation and it may be that the chromosome regions synthesising DNA at this critical, sensitive, phase are those later involved in forming chiasmata. For these reasons an investigation was made, in Scilla campanulata, to determine, as far as possible, the relationship between the pattern of chromosome replication and the localisation of chiasmata at meiosis. The species is particularly suitable for the purpose. In the first place, a detailed account is already available of the DNA replication pattern for all eight pairs of chromosomes [2] and, secondly, the distribution of chiasmata at meiosis can be determined fairly easily in specific chromosomes at metaphase 1 in pollen mother cells. RESULTS
A first metaphase in a pollen mother cell of Scilla campanulata is shown in Fig. 1. For scoring, the different chromosome types were classified and numbered I to VIII according to a system earlier used by Rees [5]. Each chromosome was then arbitrarily divided into segments of roughly equal II-
661809
Experimental
Cell
Research
44
162
H. Rees and
Fig.
l.-First
metaphase
I. Average
TABLE
of meiosis
chiasma
in a pollen
G. M. Evans
mother
frequencies
per
cell of SciUu
cell
in
campanulata.
chromosome
x ca 1450.
segments
of
Scilla campanulata. Data
for each replicate
from
10 pollen mother cells. corresponding segments
Chromosome
Chiasma
I
Replicate Replicate
1 2
I I-IV
Replicate Replicate
1 2
v
Replicate Replicate
1 2
VI
Replicate Replicate
1 2
VII
Replicate Replicate
1 2
VIII
Replicate Replicate
1 2
Mean
Mean
0.20 0.60 0.40
0.40 0.20 0.30 0.13 0.13 0.13
Mean
Mean
Mean
Mean Experimental
Data for chromosome in the two arms.
Cell
Research
44
0.20 0.10 0.15 0.10 0 0.05 0.30 0.20 0.25
frequency 0.10 0.30 0.20 0.23 0.23 0.23 0.05 0.05 0.05 0.30 0.20 0.25 0 0.10 0.05 0.20 0.50 0.45
V have
been pooled
for
per segment 0.60 0.40 0.50 0.33 0.50 0.42 0.55 0.55 0.55 0.60 0.62 0.61 0.90 0.50 0.70 0.60 0.40 0..50
0.40 0.80 0.60 0.70 0.40 0.55 0.40 0.35 0.38 0.40 0.20 0.30 0.40 0.40 0.40
0.40 0.30 0.35 0.57 0.27 0.42
Chiasmata
localization
and DNA
163
replication
lengths (see Table I) and the chiasma frequency for each segment estimated from 10 pollen mother cells in each of two separate slides. Because chromosomes II, III and IV are not always identifiable at first metaphase mean values for the three were estimated from pooled data from each pollen mother cell. For chromosome V, which has a very nearly median centromere, the chiasma frequencies were likewise pooled for corresponding segments in the long and short arms which cannot be distinguished from one another at meiosis. The chiasma frequencies are plotted also in Fig. 2. The results plotted, in view of their consistency, were pooled over replicates.
4-
-0.8
VIII
V 0
3 I/ p”o-
-0.4
0
0’ / 0 I
I (\ I
I
I
I
CSegments Fig. Z.-The distribution of mean chiasma frequencies (broken lines) and of mean grain counts following tritium labelling at late S (solid line) in segments of Scilla campanulata chromosomes. Areas with high grain counts are those which are late in synthesising DNA. Note the correlation between late synthesis and chiasma localisation. Experimental
Cell Research
44
164
H. Rees and G. hf. Evans
Also in Fig. 2 are superimposed the grain counts from autoradiographs prepared during the labelling experiment referred to above [2]. Segments showing high and low grain counts, respectively, are those with high and low amounts of DNA replication at late S of mitosis. CONCLUSIONS
From Fig. 2 it will be observed that there is an extremely close correlation between the distribution curves relating to chiasmata and to DNA synthesis at late S. Chromosome regions with high chiasma frequencies are those which, in late S, show a high degree of DNA synthesis, and vice versa. It is, of course, appreciated that the labelling results pertain to mitosis and not to meiosis and any speculation upon a possible causal relationship between chiasma formation and DNA synthesis must, hence, be qualified. Until we know more of chromosome fine structure and behaviour at meiosis it would in any case be rash to attempt to explain what, precisely, this relationship might be. One thing is certain. L4ny relationship between DNA replication at late S and chiasma localisation at meiosis can neither be direct nor simple because for one thing the chiasmata, it is generally agreed, are actually formed when all, or virtually all, the DNA synthesis is completed. Rather than add to the present welter of speculation about the mechanics of chiasma formation it would, perhaps, be best simply to re-emphasise that these results show a correlation between the patterns of DNA replication and of chiasma localisation and that the correlation may have a causal basis. SUMMARY
The distribution of chiasmata at meiosis within chromosomes of Scilla is correlated with the pattern of DNA replication of late S. Chromosome regions with the highest degree of DNA synthesis at late S, namely those interstitial segments away from the chromosome ends and from the centromeres, have the highest chiasma frequencies. A causal relation is suggested. campanulata
REFERENCES 1. 2. 3. 4. 5. 6.
DARLINGTON, C. D. and H~QUE, A., Chromosomes today 1, 102 (1966). EVANS, G. M. and REES, H., Ezpff Ceff Res. 44, 150 (1966). LAWRENCE, C. W., Heredity 16, 83 (1964). -Nafure 106, 789 (1965). REES, H., Suppl. Heredity 6, 235 (1952). WIMBER, D. E., Exptf Cell Res. 23, 402 (1961).
Experimental
Cell
Research
44