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Sister chromatid exchanges in human cells and Chinese hamster cells
Copyright @ 1480 by Academic Press, Inc. All rights of reproduction in any form reserved 0014-4827/8Q/O50179-05%02.00/0
Experimental Cell Research 127 (1980) 179-183
SlSTER
CHROMATID
EXCHANGES
AND CHINESE
HAMSTER
IN HUMAN
CELLS
CELLS
Evidence that the Rate of Sister Chromntid Function qf‘f’loidy
Exc~hanges is 11
M. S. LIN ‘Department of‘Medicu/ Generics, Universifv of South Alabnmcc, Mobile, AL 36617. ctnd Children’s Hospiral of 1.0sAngeles, Los Angeles, CA 90027, USA
SUMMARY The frequency of sister chromatid exchanges (SCE) in the chromosomes of the diploidy and polyploidy of Chinese hamster cells and human cells has been studied using BUdR-DAN (bromodeoxyuridine, 4’-6-diamidino-2-phenylindol) fluorescence. The rate of SC& per cell under constant control conditions is in proportion to the ploidy levels. In addition, the frequency of SCEs observed in a given human chromosome (nos. 1) is also-directly proportional to the number of such chromosomes presented in the cells. The mean of SCEs in human chromosome numbers I is very similar (0.46-0.48) for diploid, triploid, and tetraploid cells. The results suggest that the rate of SCEs is a function of cellular ploidy levels.
The differentiation of sister chromatids and the detection of sister chromatid exchanges (SCE) have been studied intensively in recent years in a large number of organisms. In such investigations, metaphases in cells are grown in the presence of S-bromodeoxyuridine (BUdR) for two cycles of DNA synthesis and stained with fluorochrome dyes or Giemsa [l-5]. The rate of SCEs/cell under identical experimental conditions has been found to be similar in various mammalian species, irrespective of their diploid chromosome numbers [6]. Carrano & Wolff [7] reported that in the Indian muntjac there is a good correlation between the frequency of SCEs in each chromosome and its relative DNA content. Pathak et al. [6] suggest that the
rate of SCE might be correlated to genome size. To test this hypothesis, Kato [8] investigated the rate of SCEs in various mammalian species as compared with their genome size. He found that the number of SCEs is proportional to the genome size in some species, but exceptions to this rule have also been found in some other species of mammals. These results obtained from a number of ‘unrelated’ species may be misleading. In an attempt to clarify the situation, the present investigation of the rate of SCEs was conducted on a diploidy and polyploidy of Chinese hamster and human cells, in order to determine whether the ’ Address for offprint requests. Exp CellRes
!27(1980)
180
M. S. Lin
Table 1. Chromosomal constitution and frequency of sister chromatid exchanges (SCEs) in heteroploidy of Chinese hamster cells after two cycles of BUdR (2~10~~ M) incorporation No. of Cell line
cells examined
Average chromosome no.
Range Total of of chromosomes SCEs
Ave. SCE/ cell
Range of SCE
SD.
E36 2E36 3E36
30 30 30
21.0 38.7 64.5
18-22 35-42 58-70
6.17 11.83 20.33
2-11 8-18 1l-34
2.13 2.60 5.48
203 355 610
x2=4.54
rate of SCEs is a function of cellular DNA content, i.e., whether a doubling of genetically identical DNA would correspondingly double the rate of SCEs in identical cell types from the same species. The work to be described was designed to test this hypethesis. MATERIALS
AND METHODS
Chinese hamster and human cells were used for this studv. The Chinese hamster cell lines used were E36. 2E36, and 3E36. The E36 cells are heterodiploid. The 2E36 cells are heterotetranloid (derived from E36 cells _ after colcemid induction) and 3E36 cells are heterohexoploid, derived from the 2E36 line. The human cells used in the experiments were diploid tibroblasts (1015-2N;46,XY), triploid fibroblasts (GM-1332; 69,XXY) and tetraploid fibroblasts (1015-4N;92, XXYY). To obtain tetraploid cells, diploid fibroblasts were grown in Eagle’s minimum essential medium (MEM)’ containing 0.05 pg/ml colcemid t for 16 h. The cells were washed and then grown in the same medium for an additional 2 days. Those cells that contained diuloid and tetranloid cells were used for experiments. The E36 cells were kindly provided by Dr R. L. Davidson. The cells (GM 1332)were obtained from the Institute for Medical‘Researcd, Camden, NJ. Human fibroblasts during 6-10 passages were used in the experiments. Chinese hamster cells were grown in MEM supplemented with 10% heat-inactivated fetal calf serum (HFCS). Human fibroblasts were also grown in MEM supplemented with 20 % HFCS. In order to differentiate sister chromatids, exnonentially growing cells were incubated in the dark and allowed to grow in medium containing 2X lo+ BUdR for two cycles of DNA replication before fixation for chromosome preparations; 26-30 h for Chinese ham; ster and 50-52 h for human cells. In one series of experiments, the concentration of 4~ 10e5M BUdR was Exp Cell Res 127 (1980)
in
also used to compare the effect of BUdR concentration on ploidy levels. Colcemid (at a final concentration of 0.1 pg/ml) was added to the cultures, and Z-4 h later; the cells were harvested by trypsinization. Chromosome spreads were prepared by a conventional air-dry method. The metaphase chromosomes were stained according to the DAPI method as has been described previously [5]. Chromosome fluorescence was observed using a Leitz Orthoplan microscope equipped with incident illumination using a HBO lOOW/2mercury light source, a BG 12 filter, and TK 455 dichroic mirror for excitation. and KS30 and K510 ftlters for emission. Wellspread ‘and differentiated metaphases were photographed on Kodak Tri-X film. About 25-30 metaphases per experiment were analyzed.
RESULTS The chromosome constitution and the frequency of SCEs in hetero-diploidy and -polyploidy of Chinese hamster cell lines after two cycles of BUdR (2~ low5 M and 4x 10q5 M) incorporation are shown in tables 1 and 2. The averages of SCEslcell in hetero-diploidy, -tetraploidy and -hexaploidy after the cells grown for 2 cycles of 2X10P5 M BUdR incorporation were 6.77, 11.83, and 20.33, respectively (table 1). Thus, the rate of SCEs/cell rose with increasing ploidy levels, similarly, at either of the two concentrations of BUdR employed. The rate of SCEs only showed a slight increase (about 1 SCElcell) when the concen-. tration of BUdR increased from 2~ 10e5M to 4~ 10M5M. The increase in SCE fre-
Sister chromatid exchanges and ploidy
1
Table 2. Chromosomal constitution and frequency of sister chromatid exchanges (SCE) in heteroploidy of Chinese hamster cells after two cycles of BUdR (4X10e5 poration Cell line
No. of cells examined
Average chromosome no.
Range of chromosomes
Total of SCEs
Ave. SCEI cell
Range of SCEs
S.D.
E36 2E36
29 29
20.9 38.3
18-22 35-42
212 373
7.31 12.86
3-12 7-20
2.01 2.72
X”=2.09
quency with the concentration of BUdR indicates that at least some of the exchanges are induced by the drug. This is consistent with the previous observations on the effect of BUdR concentrations on the frequency of SCEs [2,9-l 11. The results of a x2 analysis of the observed and expected number of SCEs indicate that the rate of SCEs/cell is in proportion to the ploidy levels in both concentrations of BUdR (P>O.O5). The experiments were performed on Chinese hamster cells that were heteroploid. In order to make sure that the average SCE value is a function of the ploidy levels, human diploid , triploid , and tetraploid fibroblasts were examined. The frequency
of SCEs in each type of cells was recorded. In addition, the frequency of SCEs in human chromosome I was also recorded since this chromosome can be easily recognized by its morphology. The chromosome constitution and the frequency of SCEs in these human cells are shown in table 3. As for SCEs in a given chromosome, the incidence also increased in proportion to the number of such chromosomes in the cell. However, the average number of SCEs observed in chromosome number 1 (0.46-0.48) is very similar for diploid, triploid, and tetraploid cells. These results suggest that the rate of SCEs is proportional to the unit
Table 3. Chromosomal constitution andfrequency ofsister chromatid exchanges (SCE) in euploidy of human cells after 2 cycles of BUdR (2 x lo--” M) incorporation Chromosomal level (chromosome no. I) Cellular level Cell strain
No. of cells examined
Chromosome Genome no.
Total of SCEs
Ave. Range SCEsl of SCEs S.D. cell
No. of chromosomcs
Total of SCEs
Ave. SCE/ chromosome
10152N GM1322 1015-4N
25 25 25
2N 3N 4N
228 332 444
9.12 13.28 17.76
50 75 100
23 35 48
0.46 0.47 0.4x
46 69 92
4-15 6-25 9-30
2.66 4.22 4.s4
n=2 $-=0.14<~z(P=0,05)=5.99. The expected values for SCEs were calculated assuming that the rate of SClJs in cells is in proportion to the ploidy levels. Exp Cdl Res 127 (1980)
182
M. S. Lin
priate to compare the rates of SCEs obtained between different mammalian groups, since different species may vary in proportions of heterochromatin and euchromatin of chromosomal DNA as well as have different characteristics of chromatin. It is known that there is a non-random distribution of SCEs along the chromosomes. A preferential localization of the SCEs was found in the heterochromatin of Micro&s 2 4 6 0 agrestis [ 121 and in the c-band region of Plotdy levels human and mouse chromosomes [13,14]. In Fig. 1. The relationship between the rate of SCEs and contrast, in the heterochromatin of the the ploidy levels. The regression equation for Chinese Chinese hamster, Microtus montanus, rat hamster (x) is average SCEs=-0.52+3.38 ploidy levels. The correlation coefficient is 0.990. The regres- kangaroo, and Indian muntjac [1.5-161, sion equation for human (0) is average SCEs=0.35+ SCEs occur at a significantly lower rate 4.35 ploidy levels. Correlation coefficient is 0.999. than in euchromatin. Further, in Indian muntjac, rat kangaroo, and human, a high frequency of SCEs was observed in the of DNA content in diploidy, triploidy, and junctions between euchromatin and heterotetraploidy. Therefore, the rate of SCEs is chromatin [lo, 15-16, 171.We do not know an additive effect with the increase of the why the exchange frequency is higher in the identical genetic genomes. At a cellular heterochromatin of some species, while in level, the rate of SCEs/cell also increased others it is higher in the euchromatin. The with the ploidy levels. The data were sub- study of the rate of SCEs and the cellular jected to x2 analysis, and no significant ploidy levels from the same species may value was obtained (P>O.O5) (table 3). It is provide a meaningful comparison between clear that the rate of SCEs rose in propor- the rate of SCEs and cellular DNA content. tion to the ploidy levels. The relationship between the rate of SCEs/cell and the ploidy levels is shown in fig. 1. The curves indicate that the average value of SCEs is roughly linearly related to DISCUSSION the ploidy levels. The correlation coeffiThe results presented in this report indicate cient (r) for the three Chinese hamster cell that the rate of SCEs increased with the ad- lines is 0.990. Although the r-value is high, dition of cellular DNA content. Also, under the results of a t-test analysis showed no constant control conditions, the rate of ex- significant difference (P>O.O5). This may changes/cell appears to be a function of cel- have been due to the fact that Chinese hamlular ploidy levels, as demonstrated by a ster cells are heteroploid. However, in BUdR-DAPI fluorescence technique. human diploid , triploid , and tetraploid cells, There are some conflicting results about the chromosome constitutions are conthe correlation between the rate of SCEs sistent. The tetraploid cells used were and the DNA content in different mam- derived from the same diploid cells of this malian species [Xl. However, it is not appro- study. Therefore, the genetic constitutions Exp Cell Res 127 (1980)
Sister chromatid
are identical except for the doubling of genetically identical DNA. The cells were under the same control conditions and the rate of SCEs/cell is directly proportional to the ploidy levels. The correlation coefficient (Y) is 0.999. A t-test value indicated a highly significant value (PcO.01). Further study on the rate of SCEs in the given human chromosome numbers 1 in diploidy, triploidy, and tetrapIoidy supports this view (table 3). The present findings provide convincing evidence that the rate of SCEs is a function of cellular ploidy levels and support the hypothesis proposed by Pathak et al. [6] that the rate of SCEs are correlated to genome size. The author is grateful to Dr W. Wertelecki for advice and for reviewing the manuscript. This work was supported in part by the Department of Health, Education and Welfare, Maternal and Child Service Project 422 and the Alabama State-Wide Genetics Program.
Printed
in Sweden
exchanges and ploidy
I83
REFERENCES 1. 2. 3. 4.
Latt, S A, Proc natl acad sci US 70 (1973) 3395. Kato, H, Nature 251 (1974) 70. Perry, P & Wolff, S, Nature 251 (1974) 156. Korenberg, .I R & Freedlender, E F, Chromosoma 48 (1974) 335. 5. Lin, M S & Alfi, 0 S, Chromosoma 57 (1976) 219. 6. Pathak, S, Ward, 0 G & Hsu, T C, Experientia 33 (1977) 875. 7. Carrano, A V &Wolff, S, Chromosoma 53 (1975) 361. 8. Kato, H, Int rev cytol47 (1977) 55. 9. Wolff, S & Perry, P, Chromosoma 48 (1974) 341. 10. Latt, S A, Science 185 (1974) 74. 11. Tice, R, Chaillet, .I & Schneider, E L, Exp cell res 102 (1976) 426. 12. Natarajan, A T & Klasterska, I, Hereditas 79 (1975) 150. 13. Tice, R, Chaillet, J & Schneider, E L. Nature 256 (1975) 642. 14. Holmquist, G P & Comings, D E, Cbromosoma 52 (1975) 245. 15. Hsu, T C & Pathak, S, Chromosoma 58 (1976) 269. 16. Carrano, A V & Johnson, G R, Chromosoma 64 (1977) 97. Keccived September 10, 1979 Revised version received November 7, 1979 Accepted November 8, 1979
Exp Cd Rrs 127 11980)
Experimental Cell Research 127 (1980) 179-183
SlSTER
CHROMATID
EXCHANGES
AND CHINESE
HAMSTER
IN HUMAN
CELLS
CELLS
Evidence that the Rate of Sister Chromntid Function qf‘f’loidy
Exc~hanges is 11
M. S. LIN ‘Department of‘Medicu/ Generics, Universifv of South Alabnmcc, Mobile, AL 36617. ctnd Children’s Hospiral of 1.0sAngeles, Los Angeles, CA 90027, USA
SUMMARY The frequency of sister chromatid exchanges (SCE) in the chromosomes of the diploidy and polyploidy of Chinese hamster cells and human cells has been studied using BUdR-DAN (bromodeoxyuridine, 4’-6-diamidino-2-phenylindol) fluorescence. The rate of SC& per cell under constant control conditions is in proportion to the ploidy levels. In addition, the frequency of SCEs observed in a given human chromosome (nos. 1) is also-directly proportional to the number of such chromosomes presented in the cells. The mean of SCEs in human chromosome numbers I is very similar (0.46-0.48) for diploid, triploid, and tetraploid cells. The results suggest that the rate of SCEs is a function of cellular ploidy levels.
The differentiation of sister chromatids and the detection of sister chromatid exchanges (SCE) have been studied intensively in recent years in a large number of organisms. In such investigations, metaphases in cells are grown in the presence of S-bromodeoxyuridine (BUdR) for two cycles of DNA synthesis and stained with fluorochrome dyes or Giemsa [l-5]. The rate of SCEs/cell under identical experimental conditions has been found to be similar in various mammalian species, irrespective of their diploid chromosome numbers [6]. Carrano & Wolff [7] reported that in the Indian muntjac there is a good correlation between the frequency of SCEs in each chromosome and its relative DNA content. Pathak et al. [6] suggest that the
rate of SCE might be correlated to genome size. To test this hypothesis, Kato [8] investigated the rate of SCEs in various mammalian species as compared with their genome size. He found that the number of SCEs is proportional to the genome size in some species, but exceptions to this rule have also been found in some other species of mammals. These results obtained from a number of ‘unrelated’ species may be misleading. In an attempt to clarify the situation, the present investigation of the rate of SCEs was conducted on a diploidy and polyploidy of Chinese hamster and human cells, in order to determine whether the ’ Address for offprint requests. Exp CellRes
!27(1980)
180
M. S. Lin
Table 1. Chromosomal constitution and frequency of sister chromatid exchanges (SCEs) in heteroploidy of Chinese hamster cells after two cycles of BUdR (2~10~~ M) incorporation No. of Cell line
cells examined
Average chromosome no.
Range Total of of chromosomes SCEs
Ave. SCE/ cell
Range of SCE
SD.
E36 2E36 3E36
30 30 30
21.0 38.7 64.5
18-22 35-42 58-70
6.17 11.83 20.33
2-11 8-18 1l-34
2.13 2.60 5.48
203 355 610
x2=4.54
rate of SCEs is a function of cellular DNA content, i.e., whether a doubling of genetically identical DNA would correspondingly double the rate of SCEs in identical cell types from the same species. The work to be described was designed to test this hypethesis. MATERIALS
AND METHODS
Chinese hamster and human cells were used for this studv. The Chinese hamster cell lines used were E36. 2E36, and 3E36. The E36 cells are heterodiploid. The 2E36 cells are heterotetranloid (derived from E36 cells _ after colcemid induction) and 3E36 cells are heterohexoploid, derived from the 2E36 line. The human cells used in the experiments were diploid tibroblasts (1015-2N;46,XY), triploid fibroblasts (GM-1332; 69,XXY) and tetraploid fibroblasts (1015-4N;92, XXYY). To obtain tetraploid cells, diploid fibroblasts were grown in Eagle’s minimum essential medium (MEM)’ containing 0.05 pg/ml colcemid t for 16 h. The cells were washed and then grown in the same medium for an additional 2 days. Those cells that contained diuloid and tetranloid cells were used for experiments. The E36 cells were kindly provided by Dr R. L. Davidson. The cells (GM 1332)were obtained from the Institute for Medical‘Researcd, Camden, NJ. Human fibroblasts during 6-10 passages were used in the experiments. Chinese hamster cells were grown in MEM supplemented with 10% heat-inactivated fetal calf serum (HFCS). Human fibroblasts were also grown in MEM supplemented with 20 % HFCS. In order to differentiate sister chromatids, exnonentially growing cells were incubated in the dark and allowed to grow in medium containing 2X lo+ BUdR for two cycles of DNA replication before fixation for chromosome preparations; 26-30 h for Chinese ham; ster and 50-52 h for human cells. In one series of experiments, the concentration of 4~ 10e5M BUdR was Exp Cell Res 127 (1980)
in
also used to compare the effect of BUdR concentration on ploidy levels. Colcemid (at a final concentration of 0.1 pg/ml) was added to the cultures, and Z-4 h later; the cells were harvested by trypsinization. Chromosome spreads were prepared by a conventional air-dry method. The metaphase chromosomes were stained according to the DAPI method as has been described previously [5]. Chromosome fluorescence was observed using a Leitz Orthoplan microscope equipped with incident illumination using a HBO lOOW/2mercury light source, a BG 12 filter, and TK 455 dichroic mirror for excitation. and KS30 and K510 ftlters for emission. Wellspread ‘and differentiated metaphases were photographed on Kodak Tri-X film. About 25-30 metaphases per experiment were analyzed.
RESULTS The chromosome constitution and the frequency of SCEs in hetero-diploidy and -polyploidy of Chinese hamster cell lines after two cycles of BUdR (2~ low5 M and 4x 10q5 M) incorporation are shown in tables 1 and 2. The averages of SCEslcell in hetero-diploidy, -tetraploidy and -hexaploidy after the cells grown for 2 cycles of 2X10P5 M BUdR incorporation were 6.77, 11.83, and 20.33, respectively (table 1). Thus, the rate of SCEs/cell rose with increasing ploidy levels, similarly, at either of the two concentrations of BUdR employed. The rate of SCEs only showed a slight increase (about 1 SCElcell) when the concen-. tration of BUdR increased from 2~ 10e5M to 4~ 10M5M. The increase in SCE fre-
Sister chromatid exchanges and ploidy
1
Table 2. Chromosomal constitution and frequency of sister chromatid exchanges (SCE) in heteroploidy of Chinese hamster cells after two cycles of BUdR (4X10e5 poration Cell line
No. of cells examined
Average chromosome no.
Range of chromosomes
Total of SCEs
Ave. SCEI cell
Range of SCEs
S.D.
E36 2E36
29 29
20.9 38.3
18-22 35-42
212 373
7.31 12.86
3-12 7-20
2.01 2.72
X”=2.09
quency with the concentration of BUdR indicates that at least some of the exchanges are induced by the drug. This is consistent with the previous observations on the effect of BUdR concentrations on the frequency of SCEs [2,9-l 11. The results of a x2 analysis of the observed and expected number of SCEs indicate that the rate of SCEs/cell is in proportion to the ploidy levels in both concentrations of BUdR (P>O.O5). The experiments were performed on Chinese hamster cells that were heteroploid. In order to make sure that the average SCE value is a function of the ploidy levels, human diploid , triploid , and tetraploid fibroblasts were examined. The frequency
of SCEs in each type of cells was recorded. In addition, the frequency of SCEs in human chromosome I was also recorded since this chromosome can be easily recognized by its morphology. The chromosome constitution and the frequency of SCEs in these human cells are shown in table 3. As for SCEs in a given chromosome, the incidence also increased in proportion to the number of such chromosomes in the cell. However, the average number of SCEs observed in chromosome number 1 (0.46-0.48) is very similar for diploid, triploid, and tetraploid cells. These results suggest that the rate of SCEs is proportional to the unit
Table 3. Chromosomal constitution andfrequency ofsister chromatid exchanges (SCE) in euploidy of human cells after 2 cycles of BUdR (2 x lo--” M) incorporation Chromosomal level (chromosome no. I) Cellular level Cell strain
No. of cells examined
Chromosome Genome no.
Total of SCEs
Ave. Range SCEsl of SCEs S.D. cell
No. of chromosomcs
Total of SCEs
Ave. SCE/ chromosome
10152N GM1322 1015-4N
25 25 25
2N 3N 4N
228 332 444
9.12 13.28 17.76
50 75 100
23 35 48
0.46 0.47 0.4x
46 69 92
4-15 6-25 9-30
2.66 4.22 4.s4
n=2 $-=0.14<~z(P=0,05)=5.99. The expected values for SCEs were calculated assuming that the rate of SClJs in cells is in proportion to the ploidy levels. Exp Cdl Res 127 (1980)
182
M. S. Lin
priate to compare the rates of SCEs obtained between different mammalian groups, since different species may vary in proportions of heterochromatin and euchromatin of chromosomal DNA as well as have different characteristics of chromatin. It is known that there is a non-random distribution of SCEs along the chromosomes. A preferential localization of the SCEs was found in the heterochromatin of Micro&s 2 4 6 0 agrestis [ 121 and in the c-band region of Plotdy levels human and mouse chromosomes [13,14]. In Fig. 1. The relationship between the rate of SCEs and contrast, in the heterochromatin of the the ploidy levels. The regression equation for Chinese Chinese hamster, Microtus montanus, rat hamster (x) is average SCEs=-0.52+3.38 ploidy levels. The correlation coefficient is 0.990. The regres- kangaroo, and Indian muntjac [1.5-161, sion equation for human (0) is average SCEs=0.35+ SCEs occur at a significantly lower rate 4.35 ploidy levels. Correlation coefficient is 0.999. than in euchromatin. Further, in Indian muntjac, rat kangaroo, and human, a high frequency of SCEs was observed in the of DNA content in diploidy, triploidy, and junctions between euchromatin and heterotetraploidy. Therefore, the rate of SCEs is chromatin [lo, 15-16, 171.We do not know an additive effect with the increase of the why the exchange frequency is higher in the identical genetic genomes. At a cellular heterochromatin of some species, while in level, the rate of SCEs/cell also increased others it is higher in the euchromatin. The with the ploidy levels. The data were sub- study of the rate of SCEs and the cellular jected to x2 analysis, and no significant ploidy levels from the same species may value was obtained (P>O.O5) (table 3). It is provide a meaningful comparison between clear that the rate of SCEs rose in propor- the rate of SCEs and cellular DNA content. tion to the ploidy levels. The relationship between the rate of SCEs/cell and the ploidy levels is shown in fig. 1. The curves indicate that the average value of SCEs is roughly linearly related to DISCUSSION the ploidy levels. The correlation coeffiThe results presented in this report indicate cient (r) for the three Chinese hamster cell that the rate of SCEs increased with the ad- lines is 0.990. Although the r-value is high, dition of cellular DNA content. Also, under the results of a t-test analysis showed no constant control conditions, the rate of ex- significant difference (P>O.O5). This may changes/cell appears to be a function of cel- have been due to the fact that Chinese hamlular ploidy levels, as demonstrated by a ster cells are heteroploid. However, in BUdR-DAPI fluorescence technique. human diploid , triploid , and tetraploid cells, There are some conflicting results about the chromosome constitutions are conthe correlation between the rate of SCEs sistent. The tetraploid cells used were and the DNA content in different mam- derived from the same diploid cells of this malian species [Xl. However, it is not appro- study. Therefore, the genetic constitutions Exp Cell Res 127 (1980)
Sister chromatid
are identical except for the doubling of genetically identical DNA. The cells were under the same control conditions and the rate of SCEs/cell is directly proportional to the ploidy levels. The correlation coefficient (Y) is 0.999. A t-test value indicated a highly significant value (PcO.01). Further study on the rate of SCEs in the given human chromosome numbers 1 in diploidy, triploidy, and tetrapIoidy supports this view (table 3). The present findings provide convincing evidence that the rate of SCEs is a function of cellular ploidy levels and support the hypothesis proposed by Pathak et al. [6] that the rate of SCEs are correlated to genome size. The author is grateful to Dr W. Wertelecki for advice and for reviewing the manuscript. This work was supported in part by the Department of Health, Education and Welfare, Maternal and Child Service Project 422 and the Alabama State-Wide Genetics Program.
Printed
in Sweden
exchanges and ploidy
I83
REFERENCES 1. 2. 3. 4.
Latt, S A, Proc natl acad sci US 70 (1973) 3395. Kato, H, Nature 251 (1974) 70. Perry, P & Wolff, S, Nature 251 (1974) 156. Korenberg, .I R & Freedlender, E F, Chromosoma 48 (1974) 335. 5. Lin, M S & Alfi, 0 S, Chromosoma 57 (1976) 219. 6. Pathak, S, Ward, 0 G & Hsu, T C, Experientia 33 (1977) 875. 7. Carrano, A V &Wolff, S, Chromosoma 53 (1975) 361. 8. Kato, H, Int rev cytol47 (1977) 55. 9. Wolff, S & Perry, P, Chromosoma 48 (1974) 341. 10. Latt, S A, Science 185 (1974) 74. 11. Tice, R, Chaillet, .I & Schneider, E L, Exp cell res 102 (1976) 426. 12. Natarajan, A T & Klasterska, I, Hereditas 79 (1975) 150. 13. Tice, R, Chaillet, J & Schneider, E L. Nature 256 (1975) 642. 14. Holmquist, G P & Comings, D E, Cbromosoma 52 (1975) 245. 15. Hsu, T C & Pathak, S, Chromosoma 58 (1976) 269. 16. Carrano, A V & Johnson, G R, Chromosoma 64 (1977) 97. Keccived September 10, 1979 Revised version received November 7, 1979 Accepted November 8, 1979
Exp Cd Rrs 127 11980)