INTERCHROMOSOMAL EFFECTS OF AUTOSOMAL TRANSLOCATIONS
ON RECOMBINATION IN DROSOPHZLA MELANOGASTER’
JOHN H. WILLIAMSONZ
Department of Zoology, University of Georgia, Athens, Georgia
Received August 19, 1966
ETEROZYGOUS autosomal translocations have been shown to affect recomHbination
in the X chromosome in various ways (HINTON1965); translocations with long interstitial distances increased recombination in X, those with
short interstitial distances decreased recombination in X, and those with intermediate interstitial distances had little or no effect. Other incidental tests of translocation heterozygotes have yielded negative results which nevertheless were concorrelation (ZIMMERING
and BARBOUR
1952; RAMEL1962;
sistent with HINTON’S
RAMEL.GOLDMAN
and KJELLSTROM1964). In this paper, the interchromosomal
effects of translocations will be further defined.
MATERIALS A N D METHODS
From among the 26 2;3-translocations described and tested by HINTON(1965), five were
selected for more intensive study on the basis of their fertility as homozygotes and their interchromosomal effects as heterozygotes; T1/+ and T6/f increased, T16/+ had no effect on, and
T18/+ and T33/+ decreased recombination of X chromosome markers. The structures of these
translocations are schematically represented in Figure 1.
X chromosome mutants used as markers included yellow ( y and y * ) , scute (sc), white (w),
crossveinless (cu), vermilion ( U ) , wavy ( w y ) , forked (f),carnation (car) and Bar ( B ) . Use of
the y + marker of the scvI duplication in the right arm of the X chromosome (from Znp(1)scv’)
allowed recognition of exchanges in the centromere region (GRELL1962); B was used to detect
nondisjunction of the X chromosomes. Autosomal mutants used included vestigial-Depilate ( vg’)
and brown (bw)in the second chromosome and scarlet ( s t ) in the third chromosome; the multiply
inverted SM5,Cy chromosome was used as a translocation balancer. Detailed descriptions of these
and GRELL(1966).
mutants are given by LINDSLEY
The culture medium was a mixture of water, molasses, corn meal, dried brewer’s yeast, agar
and propionic acid and was inoculated with live propionic acid-resistant yeast. Salivary gland
squashes (NICOLETTI1959) from all experimental stocks were checked to exclude introduction of
rearrangements other than the translocations. Recombination data were obtained from the male
progeny of single females mated and maintained in quarter-pint bottles at 25 i- 1°C for eight
days beginning no later than 12 hours after eclosion. In a n attempt to control the genetic background of the tested females. comparisons were made between the progeny of full sisters except
in the tests involving two translocations. Ratios of recombination frequencies from translocation
heterozygotes and homozygotes to the recombination frequencies from the standard homozygotes
were used to measure the effects of the translocations. Thus ratios greater than one indicated
increases and ratios less than one indicated decreases in recombination; the 95% confidence
limits of these ratios were calculated to test their statistical significance (SUZUKI1962, 1964).
This iniestigation was supported by Public Health Service Research Grant HD01235.
Present address: Biology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
Genetics 54: 1431-1440 December 1!)66.
1432
J. H . WILLIAMSON
T16/+
II
FIGURE
1.-The five translocations studied, represented as heterozygotes. Centromeres are
indicated as circles, heterochromatin as wavy lines, chromosome 2 as thick lines and chromosome
3 as thin lines.
RESULTS
To compare the interchromosomal effects of heterozygous and homozygous
autosomal translocations, pair matings were made to produce y z sc u f.sc”/y2cu
wy car females with autosomal constitutions of
T(2;3)/+ and T(2;3)/
T(2;3). Single females were mated to 3 to 5 B; bw; st males; since the tested
translocations carried no markers, 7 to 10 male progeny of each female were
mated to bw; st females to determine the autosomal constitution of the tested
females. Neither sex ratio (varying from 0.44 to 0.50), marker allele recovery
(varying from 0.43 to 0.56), nor primary exceptions (overall incidence of
0.0002) varied significantly with autosomal constitution.
Total X chromosome recombination as well as regional responses are presented
in Table 1 and in Figure 2. The responses of the translocation heterozygotes were
as expected from previous tests (HINTON
1965), i.e., T1/+ and T6/+ increased,
T16/+ did not affect, and T18/+ and T33/+ decreased X chromosome recombination; also, the greatest responses occurred in distal and proximal, as compared
to medial, regions. In cases where regional responses were significantly different
from the control values, the signs of the ratios were the same as the overall
response. In the case of T16/+ recombination was significantly increased in the
region spanning the centromere (car-scv’) although the net response was not
significant.
Regardless of their effects as heterozygotes, all five translocations when homozygous significantly decreased recombination in the X chromosome, with no
apparent correlation between the magnitude of the decreases and the effects in
+/+,
1433
TRANSLOCATIONS A N D R E C O M B I N A T I O N
T33
I
SC
cv
v
WY
f
car
y'
X chromosome markers
FIGURE
2.-Recombination ratios in each region of the X chromosome of translocation heterozygotes (solid lines) and homozygotes (broken lines).
the heterozygotes. Again distal regions gave the greatest responses, proximal
regions next and intermediate regions least of all. In only one case, the sc-cu
region of T6/T6, did a significant increase occur. Graphic representation of these
results (Figure 2) suggests some degree of similarity between the effects of translocation heterozygotes and homozygotes along the length of the X chromosome.
All of the translocation homozygotes decreased multiple exchange strands and
increased noncrossover strands; single exchange strands were decreased slightly.
Comparisons of inclusive coincidence values ( WEINSTEIN
1958) suggested no
pattern of alteration of interference associated with X chromosome recombination
in either translocation heterozygotes or homozygotes.
Because recombination frequencies normally vary with maternal age, the
observed responses to translocations during the 0 to 8 day test period might be
sampling artifacts due to alteration in the maternal age effect unaccompanied
1434
J. H . WILLIAMSON
TABLE 1
The effectsof heterozygous and homozygous autosomal translocations on X chromosome
recombination determined from crosses of y2 cv wy car/y2 sc v f.scV1
females to B; bw s t males
Autosomal
constitution
of females
Number of
sons scored
sc-cu
cu-u
TI/+
TI/T1
2993
5750
1274
10.5
10.8
5.4'
2003
4698
2485
+/+
+/+
T6/+
T6/T6
+/+
6/+
T16/T16
+/+
T18/+
TI 8/T18
+/+
T33/+
T33/T33
Percent recombination1
__
-
U-wy
wy-f
f-car
18.5
20.7'
15.6'
8.4
9.1
9.1
14.0
15.1
14.2
10.5
14.6'
12.6'
19.5
22.0'
16.2'
8.9
8.9
8.9
1874
3411
1091
10.3
10.9
7.5;
20.9
18.7
15.2'
5124
6526
1169
9.5
8.4'
5.8'
4885
7617
1445
10.6
10.6
9.9
car-scvl
Total
6.9
8.7'
7.6
4.4
5.7'
4.2
62.7
70.0'
56.0'
13.3
15.1
12.2
7.3
8.1
5.6'
4.6
5.3
3.9
64.1
74.0'
59.4'
7.5
8.0
7.0
12.6
13.1
14.2
7.9
7.5
5.4;
3.0
4.6'
3.8
62.2
62.8
53.1*
19.9
17.7;
13.1'
8.4
7.4'
6.2
13.8
13.2
11.3
7.1
5.6'
5.9
4.3
3.7;
3.6
63.1
56.0'
45.9'
19.0
17.6
18.9
8.0
8.6
7.3
13.8
12.8
13.7
7.3
6.9
6.2
4.0
3.7
2.8'
62.6
60.3'
58.8;
-
* The 95% confidence limits of ratios of these values to their respective control values do not include 1 .OO.
TABLE 2
Effectsof autosomal translocations and age of females on X chromosome recombination
Percent total recombination
Autosomal constitution
of females
+/+
T6/+
T6/T6
+/+
T18/+
T18/T18
Age in days
@3
70.5
(235)t
73.6
(4011
57.3'
(4.30)
63.1
(179)
68.6
(382)
49.3;
( 172)
3-5
5-7
7-9
9-1 1
0-1 1
60.8
66.8
(436)
76.0'
(736)
63.0
(1103)
63.8
(1624.)
75.0'
(3193)
59.0*
(3894)
60.0
(338)
64.3
(697)
50.0'
(330)
58.5
(1221)
61.9
(2481)
49.2'
(1421)
( 3271
77.9'
( 764)
58.2
(9251
59.7
( 2681
61.7
(543)
50.0'
(328)
~~
~
~~
~~~~
~~
* The 95% confidence limits of ratios of these values to their respective control values do not include 1.00.
i Number of males scored.
1435
TRANSLOCATIONS A N D R E C O M B I N A T I O N
by changes in overall recombination frequencies. This possibility was examined
for T6 and T I 8 using females of the three constitutions described above but transferred to fresh culture bottles at 3,5, 7 and 9 days of age. I n both cases the control
data (Table 2) showed typical effects of maternal age on recombination; the age
effects were less pronounced in the data from translocation-bearing females. It
should be noted, however, that in T6/+ females recombination was consistently
greater in all broods than in the controls, while in T6/T6 and T18/T18 females
recombination was consistently lower than in the controls. Thus the effects of
translocations on X chromosome recombination are continuous over the tested
reproductive span of the female. For no apparent reason, recombination in T18/+
females was not decreased as expected from previous experience.
The average numbers of progeny from translocation heterozygotes varied from
0.70 to 1.03 those of the standard homozygotes while the average numbers of
progeny from translocation homozygotes varied from 0.71 to 1.08 those of the
standard homozygotes. These data indicated that nonrandom recovery of the
strands of various ranks among viable zygotes might account for the observed
effects of translocations on X chromosome recombination. To examine this possibility, fecundity and hatchability were determined by the method of PARKER
(1959) using females aged three days before mating. Canton-S females served
as controls (+/-I-); translocation heterozygotes were obtained from crosses of
Canton-S females and T(2;3) /T( 2;3) males; translocation homozygotes were
obtained from subcultures of T(2;3)/T(2;3) stocks. Females heterozygous for
autosomal translocations were more fecund, while the homozygotes were less
fecund than the controls as judged by the numbers of eggs laid during two successive test days (Table 3). The observed frequencies of zygotic mortality in the
eggs from T/+ females were much greater than those from the
females
as was expected on the basis of aneuploid segregations. On the other hand, the
proportions of unhatched eggs from the T/T females were not significantly
+/+
TABLE 3
Fecundity and egg mortality of translocation heterozygotes and homozygotes as
compared with meld-type females
.\utosomal constitution
of females
+/+
TI/+
Tl/TI
T6/+
T6/T6
TI 6/+
T16/T16
T18/+
T18/T18
T33/+
T33/T33
Number of
eggs scored
Number of eggs/
female/day
Percent
unhatched eggs
1223
1133
1175
964
350
14-50
272
1273
459
1291
231
33.9
56.7
30.9
42.3
26.9
55.8
27.2
57.9
32.8
64.6
11.6
4.9
41.8
2.9
36.9
4.9
36.4
11.0
42.2
5.0
37.5
5.2
1436
J. H. WILLIAMSON
different €rom the control value; therefore nonrandom recovery cannot explain
the decrease in X chromosome recombination observed in the homozygotes.
To characterize further the effects of autosomal translocations on X chromosome recombination, double translocation heterozygotes were synthesized by
mating y z sc U f . scvl;
females and y9sc u f scv1;T(2;3)/SM5,Cy females
to p cu wy car; T(2;3)/vgD males with each parent in the latter cross carrying
a diff went translocation. These crosses produced females carrying both multiply
marked X chromosomes and either +/ugD (controls) or +/T(2;3), and T(2;3)/
ugD or T(2;3)/T(2;3); females of these genotypes were mated singly to 3 to 5
y w B males. Recombination of X chromosome markers (Table 4) in the translocation heterozygotes was as expected; the decrease in T33/+, although not significant, was of the expected magnitude. Seven of the nine tested combinations of
translocations significantly increased recombination in the X chromosome. The
combined effects of the translocations were approximately additive, except in
those cases involving T33 where a superadditive interaction was suggested.
+/+
DISCUSSION
The independence of maternal age effects and the effects of both heterozygous
and homozygous translocations (Table 2) corresponds to the observations of
RAMELand VALENTIN(1966) with Zns(2L+2R),Cy and recombination in chromosome 3. Although there was a distinct maternal age effect on recombination,
the interchromosomal effects of the inversions were maintained throughout the
13 day test period. REDFIELD
(1955) noted a somewhat greater effect of heterozygous autosomal inversions on recombination in short distal regions of X in
females 2 to 6 days of age as compared with females 7 to 11 days of age. The
difference was small and in both age groups strong interchromosomal effects were
exerted. Consideration of these observations leads to the conclusion that the effects
of translocations, as well as of inversions, on recombination in heterologous chroTABLE 4
Interactions of translocations on X chromosome recombination
Percent total recombination
Autosomal
combinations
W D
-I-
T1
T6
T16
TI8
55.3
(1171)f
60.5'
(1993)
68.5'
(1201)
76.0*
(1408)
58.9
(1080)
71.3'
(1414)
74.7'
(956)
61.5*
(704)
62.8
(105)
T6
T I6
...
..
T I8
~~~~~
...
~
* The 95% confidence limits of ratms of these values to the control values do not include I 00.
t Number of males scored.
T33
TRANSLOCATIONS A N D RECOMBINATION
1437
mosomes cannot be explained by shifts in maternal age effects on recombination.
Nonrandom recovery of multiple exchange strands, as opposed to real increases
in exchange frequencies, has been proposed by COOPER,ZIMMERING
and KRIVSHENKO (1955) as one cause of interchromosomal effects of heterologous inversions. The proportions of unhatched eggs from T/+ females are large enough for
nonrandom recovery to be operative, but the causative mechanism would have
to exclude lower rank strands in T1/+ and T6/+, be unimportant in T16/+
and exclude higher rank strands in T18/+ and T33/+. Such an unlikely combination of events suggested to HINTON
(1965) that the high proportion of unhatched eggs, attributable to aneuploid segregations, is unrelated to the interchromosomal effects of heterozygous translocations. For homozygous translocations. higher rank strands would have to be excluded in all five tested cases, and
minimal zygotic lethality upwards to 23% would be required to produce the
observed decreases in X chromosome recombination. The observed frequencies
of zygotic mortality (Table 3) are too low to allow such a mechanism to operate
in this class of females. These considerations are consistent with REDFIELD’S
(1957) contention that nonrandom recovery is not a sufficient explanation of
increased recombination in the presence of heterologous inversions. The elimination of sampling artifacts associated with maternal age effects or with zygotic
lethality as explanations for altered recombination frequencies makes probable
the interpretation that the effects of rearrangements are exerted directly on one
or another of the exchange processes.
The correlation between the interstitial distances of 2;3-translocation heterozygotes and their effects on total X chromosome recombination which was pointed
(1965) has been confirmed for the five translocations tested in
out by HINTON
this study. SCHULTZ
and REDFIELD
(1951) suggested that crossing over is a function of the difficulty homologs experience during synapsis and that heterozygous
rearrangements which create pairing difficulties within the affected bivalent
would also increase the frequency of interactions of that bivalent with heterologous chromosomes. These interactions would result in increased crossing over
in the heterologs. HINTON
(1965) extended this hypothesis to include structural
rearrangements which would reduce the interactions of heterologs during pairing
and result in reduced map distances in the heterologs. If the rearrangement
facilitates pairing, duration of the pairing process may be shortened. Alternatively, the rearrangement may allow a more compact pairing configuration relative to the norm and thus allow fewer interactions with heterologous chromosomes. According to this cause-and-effect interpretation, the correlation between
recombination response and interstitial distance of translocation heterozygotes
should not be exhibited by translocation homozygotes. The data from the five
translocations tested clearly support this expectation.
None of the existing hypotheses concerning interchromosomal effects predict
decreased X chromosome recombination in females homozygous for autosomal
translocations. Since the autosomes of such females are structurally homozygous,
it may have been predicted that recombination in X would not be different from
that in the controls. Indeed, RAMEL(1962) found that in T ( l ; 4 ) f l / T ( l ; 4 ) w m
1438
J. H. WILLIAMSON
females autosomal recombination was not significantly different from the control
value. It is difficultto compare the interchromosomal effects of homozygous autosomal translocations with those reported for homozygous X chromosome inversions. Marked decreases in autosomal recombination were obtained by SCHULTZ
and REDFIELD(1951) using Zn(l)sc8/Zn(Z)sc8 females and by RAMEL(1962)
using Zn(Z)ysP/Zn(l)ygP
females. However, SUZUKI( 1963) observed increased
autosomal recombination for these two inversions and four others among eight
X chromosome inversions tested as homozygotes; the other two had no interchromosomal effects. He suggested that disruption of chromosome orientation,
namely loop formation caused by distally displaced heterochromatin, initiates
the interchromosomal effects of homozygous X chromosome inversions. There
is no reason to believe that the autosomes of the five tested translocation homozygotes undergo loop formation during synapsis because none have distally displaced heterochromatic segments.
When one considers regional effects (Figure 2), fairly obvious differences
between responses to the five translocations are apparent. More striking than
these differences, however, are the indications that for each translocation the
response along the length of the X chromosome in T/T females is similar to that
of the respective T/+ females. These specific relationships may be attributed to
position effects ( STEINBERG
and FRASER
1944) of the translocation breakpoints,
to disrupted chromosome continuity other than position effects (LINDSLEY
1965)
or to different genic modifiers of recombination (HINTON
1966) carried by the
translocations. Indeed, the net effects of the translocation homozygotes on X chromosome recombination may be due to such causes.
Females carrying two 2;3-translocations possess no large autosome of standard
sequence, being in this aspect comparable to the translocation homozygotes. Conversely, the combinations of two translocations involved structural heterozygosity
of various degrees. The obvious conclusion to be drawn from the tests of double
heterozygotes (Table 4) is that none of the combinations responded as the previously tested homozygotes. When pairing configurations were sketched assuming perfect pairing, each configuration included a loop. Comparison of the number of polytene subdivisions involved within the loops and total X chromosome
recombination indicated a nonlinear relationship. In general, loops involving
approximately twenty polytene divisions were associated with the greatest increases in X chromosome recombination; larger or smaller loops were associated
with lower increases in recombination. The data are not conclusive, but such an
and REDFIELD(195 1) hypothesis if
association is consistent with the SCHULTZ
one assumes that loops of intermediate proportions create greater difficulties than
either smaller or larger loops.
As RAMEL(1962, 1965) and SUZUKI(1962, 1963) have suggested, none of
the existing hypotheses concerning interchromosomal effects on recombination
can explain all of the observations. The effects of autosomal translocations on
X chromosome recombination support this conclusion. As the variety of interchromosomal effects on recombination expands, the position that more than one
mechanism is involved becomes ever more tenable.
TRANSLOCATIONS A N D RECOMBINATION
1439
I wish to thank DR. C. W. HINTQNfor his guidance during the course of this study and for
his helpful criticisms during the preparation of this manuscript.
SUMMARY
Five 2;3-translocations, of which two increased, one did not affect and two
decreased X chromosome recombination as heterozygotes, all were found to decrease X chromosome recombination as homozygotes. The interchromosomal
effects could not be explained by differential recovery of strands of the various
ranks among viable zygotes, and they were not due to alterations of maternal age
effects on recombination. None of the tested combinations of translocations decreased recombination as do the translocation homozygotes. The combined effects
of two translocations appear to be a function of the degree of structural heterozygosity involved. These results suggested that more than one mechanism is
required to explain the variety of known interchromosomal effects on recombination.
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