A COMPARATIVE STUDY OF INTER AND INTRALOCUS
RECOMBINATION IN DROSOPHILA.
by
DOROTHY JANE STUART SCHOLEFIELD
.Sc. (Hons.), University of British Columbia, 19
A THESIS SUBMITTED IN PARTIAL FULFILMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
in the Department of
ZOOLOGY
We accept this thesis as conforming to the
required standard
THE UNIVERSITY OF BRITISH COLUMBIA
December, 19 65 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of
British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that per• mission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives„ It is understood that copying or publi• cation of this thesis for financial gain shall not be allowed without my written permission.
Department of
The University of British Columbia Vancouver 8, Canada
Date Qt*- - i -
ABSTRACT
The effect of different treatments on crossing over between and within genes at the tip of the X chromosome of Drosophila melanogaster was studied to determine whether exchange in the two regions occurs by different mechanisms.
In response to autosomal inversions, y -radiation, and heat shock, crossing over of both types was altered in the same direction and to a comparable extent. This would be expected if there were only one crossover mechanism involved. There was some difference in response of inter- locus and intralocus crossing over after mitomycin C injection but, since the effect on interlocus crossing over in two separate regions was not consistent, the significance of this result is questionable. Although double crossing over involving the two interlocus regions was very rare doubles involving an inter and an intralocus region were recovered. The association of exchanges within a gene with a crossover between genes might indicate that there are two noninterfering mechanisms, or that multiple exchanges occur in short effectively paired regions. A further experiment designed to detect switch regions is outlined. - ii -
TABLE OP CONTENTS
PAGE
INTRODUCTION 1
METHODS and MATERIALS 3
Series 1. Autosomal Inversions 6
Series 2. Cis-Trans Effect 7
Series 3. y -Radiation 7
Series 4. Heat Shock 10
Series 5. Mitomycin C Injection 11
RESULTS 12
DISCUSSION 28
SUMMARY 31
LITERATURE CITED 32 - iii -
LIST OF TABLES
TABLE PAGE
1 Phenotype scored in each crossover class. 5
2 Crossover values in each region in females heterozygous for different autosomal inversions. 21
3 Crossover values in each region in females with different marker combin• ations. 22
4 Crossover values in each region in radiated females. 23
5 Crossover values in each region in heat shocked females. 24
6 Crossover values in each region in mitomycin C and saline injected females. 25
7 Cumulative sterility values of mitomycin C and saline injected females. 26
8 Summary of expected and observed double crossover types in each test series. 27 - iv -
LIST OF FIGURES
FIGURE PAGE
1 Mating procedure for obtaining sibling females with different combinations of inversions. 9
2 Ratios of crossover values in each region for each treatment (P^) to
their respective control value (P0). 16
3 Number of females with values in each crossover percentage class for Region 1, Series 1. 18
4 Number of females with values in each crossover percentage class for Region 3, Series 1. 20 - v -
ACKNOWLEDGEMENT S
I wish to thank particularly Dr. David T.
Suzuki for his encouragement, inspiration and guidance while this study was being carried out.
I am grateful to Dr. Harold Batho and Mr.
Ken Yuen of the British Columbia Cancer Institute, for their assistance in radiating the flies.
I wish to thank all the members of the laboratory for the help they have given at various stages of the work, particularly Miss Susie Hayashi and Mr. Geoffrey
Foster for their extensive help in scoring. - 1 -
INTRODUCTION
Classically, crossing over was considered to be
an event occurring between genes and exhibiting high
positive interference within short map distances. By
increasing the resolution in crossover analysis, Lewis
(1945) and Green and Green (1949) established that
intragenic recombination did occur but at a frequency
too low to have been detected previously. This finding has been corroborated many times in Drosophila and in microorganisms. High negative interference has in many
cases (Pritchard, 1955 and 1960; Streisinger and Franklin,
1956; and Chase and Doermann, 1958) been associated with
the phenomenon of intralocus recombination. These observations contradict basic assumptions of the classical
crossover model.
In order to resolve this contradiction, Lewis
(19 63) distinguishes between two classes of events which would occur by different mechanisms: classical interlocus
crossing over which is characterized by positive inter•
ference in short regions, and intralocus crossing over which
is associated with negative interference in short regions.
On the other hand, Pritchard (1955 and 1960) invokes an hypothesis of short effectively paired regions within which multiple exchanges readily occur. If markers are
included within such a region, double and triple crossovers - 2 -
could be detected but if markers surround a switch region only odd number exchanges would be detectable and these would appear as single crossovers. Thus he accounts for the correlation of intralocus recombination with high negative interference, as well as classical interlocus crossing over by a single mechanism.
The present study, involving two recessive pseudoalleles at the Notch locus and closely linked outside markers, was carried out in an attempt to determine whether a One or a Two mechanism hypothesis applies to crossing over in Drosophila.
Certain treatments are known which alter inter- genic crossover frequencies. If there is only one mechanism by which both intralocus and interlocus crossing over occur, then intralocus crossover frequencies should be altered in the same way and to a similar extent as interlocus crossover frequencies by each treatment. If there are two different mechanisms then one treatment might not necessarily affect recombination of each type in the same way. - 3 -
METHODS and MATERIALS
Crossing over at the tip of the X chromosome of
Drosophila melanogaster was studied using the following markers (followed by map distances as listed by Bridges and Brehme, 1944): apricot - wf; (1.5), facet-notchoid -
fano (3.0±), split - s£l (3.0±) and ruby - rb (7.5).
Welshons (1958) showed that fano and spl were separable,
fano being localized 0.03 units to the left of spl. Separate
Series were run testing four treatments known to affect
crossing over: presence of autosomal inversions,
y-radiation, heat shock and mitomycin C injection. In
each Series, recombination in the intralocus (fano - spl)
and interlocus(wa - fano and spl - rb) regions was measured
concomitantly.
Individual wa + + rb/+ fa110 spl + or wa + spl +/
+ fano + rb females 12-18 hours old were testcrossed to
three wa fano spl rb/Y males in shell vials. After six
days (Brood I) the parents were discarded or transferred
to a second vial. The flies were again transferred after
three days (Brood II) and discarded after six days (Brood
III). All offspring of each female were recorded separately
for each brood.
The phenotypes of each class of crossover
chromosome are shown in Table 1. Three of the double Key to symbols:
w - white s - smooth
+ - wild r - rough
o - orange d - delta i rb - ruby e - easy I
c - by chance TABLE 1
PHENOTYPE SCORED IN EACH CROSSOVER CLASS, w + + rb
fa no spl +
CROSSOVER TYPE GENOTYPE PHENOTYPE RECOGNITION eye eye colour texture wing
a Non w + + rb w s + no + fa spl + + r d single: 1 wa fano Spi + o r d e (inter) + + + rb rb s + e
wa + spl + o r + e (intra) + fano + rb rb s d e
wa + + + o s + e (inter) + fano spl rb rb r d e double: 1,2 wa fano + rb w s d c (inter:intra) + + spl + + r + c
1,3 wa fano spl rb w r d e + + + + + s e (inter:inter) wa + spl rb — w r + c 2,3 + fa no + + + s d e (intra:inter) - 6 -
crossover (DCO) classes (wa fa spl rb, + + + +, and
+ fano + +) could be recognized at all times. The other three DCO classes can be distinguished from the parental classes only by a detailed examination of all wild or white-eyed flies. Since the significance of DCO events was not anticipated at the beginning of the study, time was not taken to look for these three DCO classes and as a result they were recognized only by chance. All three of these classes are of the intra:interlocus type.
The lines in Table 1 indicate the parental class with which these DCOs would be confused.
Crosses were made at 24° ± 2° C on a Standard
Cal Tech Drosophila medium.
Series 1. Autosomal Inversions
Steinberg (193 6) found that autosomal inversions increase crossing over in the X chromosome. The regions affected to the greatest extent are at the tip and adjacent to the centromere (Schultz and Redfield, 1951).
In this study the interchromosomal effect of the inversions
SM-1 and Ubx"*"^^ on crossing over at the tip of the X was determined. SM-1 is a series of inversions of the second chromsome, homozygous lethal and associated with a 130 dominant curly wing phenotype. Ubx is a series of - 7 -
inversions of the third chromosome, also homozygous lethal and associated with a dominant enlargement of the halteres.
Sibling females heterozygous for the X chromosome markers and carrying different combinations of the inversions were recovered as shown in Figure 1. Crossing over was studied only in Brood I.
Series 2. Cis-Trans Effect
Altenburg and Browning (19 65) found that exchange values within a cistron are different when pseudoalleles are linked in cis or trans. In order to test the possible cis-trans effects on exchange within the Notch locus, crossing over was also measured in the trans arrangement in wa + spl +/+ fano + rb females in
Brood I.
Series 3 . -Radiation
Mavor (1923) showed that radiation decreases crossing over in the we - m region of the X chromosome, whereas Muller (1925) found that radiation increases crossing over around the centromeres of chromosomes II and III. Crossing over in the distal or medial regions FIGURE 1: Mating procedure for obtaining sibling females with different combinations of inversions. r no , , + fa spl + . -1 +- ?? SM-1 Ubx ,. no , . • <5&~ + fa spl + + Pm Sb .
Select
110 + + rb + fa spl + SM-1 Ubx w X , — , — « w + + rb +
Autosomal SM-1 SM-1 Ubx + + Ubx + Genotype + + + of Females
Score X chromosome in testcross offspring - 10 -
of the autosomes is not affected significantly. Suzuki
(1958) obtained similar results.
Heterozygous females with fano and spl linked in cis were radiated with 4,000 rads of X-rays from a 6,000 curie Theratron Cobalt^0 "bomb". As controls crossing over in unirradiated siblings was scored simultaneously.
Since Plough (1924) and Mavor and Svenson (1924) showed, that the effect of radiation on crossing over is detected at least six days after treatment, only Broods II and III were scored.
Series 4. Heat Shock
Plough (1917) demonstrated that crossing over in the centromere regions of chromosomes II and III was increased by heat or cold shock. However the X chromosome
(Plough, 1921) was refractory to the treatment. The increase on the autosomes was observed in broods collected after the eighth post-treatment day. Mavor and Svenson (1924) obtained similar results. Mitchell (1958) was able to distinguish between gene conversion and crossing over in
Neurospora on the basis of response to heat shock of the protoperithecia, gene conversion varying significantly while crossing over remained unaltered. Towe and Stadler
(19 64) showed that crossing over was higher in Neurospora - 11 -
incubated at 18°C than in Neurospora incubated at 25°C.
Heterozygous females were placed in an incubator at 40 t 0.5°C for 20 minutes. They were then mated after a recovery period of approximately two hours at room temperature. Controls were sibling females kept at room temperature. Only Broods II and III were scored.
Series 5. Mitomycin C Injection
Mitomycin C (MC) is an antibiotic which is known to inhibit DNA replication, without interfering with
RNA or protein synthesis (Shiba et al, 1959), possibly by forming crosslinks between the DNA helices (Szybalski and
Iyer, 1964). Holliday (1964) and Suzuki (1964) found that
MC increases mitotic crossing over in Ustilago and
Drosophila respectively.
Heterozygous females were injected with a solution of MC dissolved in 0.7 N saline at a concentration of
100 jug/ml. The females were injected dorsally between the 4th and 5th tergites until noticeable swelling occurred.
No attempt was made to determine the exact volume injected since Carlson and Oster (1962) have shown that varying amounts of fluid leak out after injection. Control females were injected with 0.7N saline. Broods I, II and III were scored. - 12 -
RESULTS
For each Series, crossover frequencies in
Regions 1, 2, and 3 were calculated. Since a Chi-square
analysis indicated that the values from the various
controls were not homogeneous, the control data were
not lumped and each set of test results was compared to
its own control. The ratios of the test values (P^) to
their respective controls (Pc) are plotted in Figure 2.
The autosomal inversions, SM-1 and Ubx,
increased crossing over in all three regions (Table 2).
Qualitatively, the increase was similar in the interlocus
and intralocus regions, SM-1 having a greater effect than
Ubx, and the effect of SM-1 and Ubx together being greater
than either alone (Figure 2A). Each increase, except
that in Region 2 with Ubx, was significant at the .01
level.
For each female, the crossover value for Regions
1 and 3 was calculated and the frequency distribution of
females in each crossover percentage class plotted
(Figures 3 and 4). There was no increase in clustering of crossover events with the inversions, the interchromosomal
effect of the inversions being expressed in all females.
When the pseudoalleles fano and spl were arranged
in the trans configuration, wa and rb were also linked in
trans. Crossing over in the intralocus region (Region 2) - 13 -
was not significantly different when fano and spl are in
cis or trans. However the crossover frequencies in the
two interlocus regions (Regions 1 and 3) were significantly,
although only slightly, increased (Table 3, Figure 2A).
Presumably some variation in the rest of the genotype was
responsible for this difference even though the wa + spl +
and + fano + rb stocks were generated from crossover
products of a wa + + rb/+ fano spl + female used in an
earlier Series.
Crossing over in the interlocus regions is
decreased by radiation, the effect being more pronounced
in Brood III than Brood II (Table 4, Figure 2B). Crossing
over in the intralocus region in the third brood is also
decreased although not significantly. The apparent increase
in this region in the second brood is not significant and
undoubtedly the result of the small numbers scored in the
control.
Heat shock had no real effect on crossing over
either in the intralocus or interlocus regions (Table 5,
Figure 2C), the small decrease in Region 3, Brood II being barely significant at the .05 level. The low
number of control flies scored makes a comparison of the
test crossover percentage in Region 2, Brood III to the
control value of no significance. The frequency of
0.31% is similar to the frequency of 0.29% for the control
of Series 1 (Table 2). - 14 -
The effect of mitomycin C injection is less straight forward (Table 6). Crossing over in Region 1
(interlocus) is decreased in all three broods (Figure
2D). Crossing over in Region 3 (interlocus) is decreased in Broods II and III but is increased in Brood I. In the intralocus region there is no effect on crossing over in either of the first two broods but a very significant decrease in Brood III. This decrease appears to be real since the total number of flies scored in both the test and control Series was large. The great increase in sterility of females injected with MC (Table 7) can be taken as an indication of the biological activity of the antibiotic used.
Assuming no interference, the expected number of double crossovers of the three types - 1,3(interlocus: interlocus); 1,2(interlocus:intralocus); and 2,3(intralocus: interlocus) - was calculated for each Series. These values along with the observed numbers of double crossovers are presented in Table 8. Whereas positive interference is essentially complete with respect to interlocus:interlocus doubles, interference is apparently absent or negative with respect to intralocus:interlocus doubles. As mentioned previously, all interlocus:interlocus doubles would have been observed, but only a minimum estimate of the number of intralocus: interlocus doubles is possible because of the scoring procedure. FIGURE 2: Ratios of crossover values in each region for each treatment (Pj) to their respective
control values (PQ): A) autosomal inversions and cis-trans effect, B) Y -radiation, C) heat shock, D) mitomycin C injection. A A CyUba O—O Cy +
J I 1 1 L
O O BROOO II Q © BROOD I A A BROOD III O O BROOD 11 A A BROOD III
7&
CROSSOVER REGIONS - 17 -
FIGURE 3: Number of females with values in each crossover percentage class for Region 1,
Series 1 plotted to two different scales.
N, the total number of females in each group, for + + is 297, for + Ubx is 254, for Cy + is
255, and for Cy Ubx is 249. The arrows indicate the mean crossover percentage values for each inversion class. — 19-
40 + + 80 Jl 20 4<^
P*" . CO 1 < U +Ubx 80H I co 40
U P—n rn 5 o-
Cy+ 8O1 40 1 CyUbx 8O
20 40- I
T" l""r O-l- i . ? LO 20 3.0 0/ 4.0 SO 6.0 70 03 6 CROSSOVER PERCENTAGE CLASSES - 19 -
FIGURE 4: Number of females with values in each crossover percentage class for Region 3,
Series 1 plotted to two different scales.
N, the total number of females in each group, for + + is 297, for + Ubx is 254, for Cy + is
255, and for Cy Ubx is 249. The arrows indicate
the mean crossover percentage values for each
inversion class.
TABLE 2
CROSSOVER VALUES IN EACH REGION IN FEMALES HETEROZYGOUS FOR
DIFFERENT AUTOSOMAL INVERSIONS.
CROSSOVER REGIONS INVERSION
GENOTYPE 1 2 3 TOTAL # % # % # %
+ + 324 0.85 11 0.029 1131 2.95 38306 to + Ubx 442 1.40** 18 0.057 1378 4.38** 31477
SM-1 + 5 74 1.76** 22 0.071** 1652 5.33** 31000
SM-1 Ubx 676 2.73** 29 0.117** 1851 7.48** 24758
** values differ significantly from control at the .01 level TABLE 3
CROSSOVER VALUES IN EACH REGION IN FEMALES WITH DIFFERENT MARKER COMBINATIONS.
CROSSOVER REGIONS X-CHROMOSOME GENOTYPE 12 2 TOTAL # % # % # %
+ fano spl + 428 0.87 17 0.034 1470 2.98 49293 wa + + rb
4- -Fano 4- T-T-I
a 711 1.03** 27 0.039 2329 3.39** 68753 w + spl +
** values differ significantly from control at the .01 level TABLE 4
CROSSOVER VALUES IN EACH REGION IN RADIATED FEMALES.
CROSSOVER REGIONS
BROOD TREATMENT 12 3 TOTAL # % # % # %
0 rads 55 0.85 0.030 190 2.94 6463 II 4000 rads 322 0.75 16 0.037 960 2.24** 42927
0 rads 55 0.91 0.033 210 3.48 6039 III 4000 rads 172 0.46** 0.019 770 2.08** 37038
** values differ significantly from control at the .01 level TABLE 5
CROSSOVER VALUES IN EACH REGION IN HEAT SHOCKED FEMALES.
CROSSOVER REGIONS
BROOD TREATMENT 12 3 TOTAL # % # % # %
24° ± 2°c 102 1.07 6 0.063 315 3.30 9544 II 40° ± 0.5°C 389 0.95 22 0.054 1208 2.95* 40886
24° t 2°c 81 1.15 0 - 249 3.53 7056 III 40° t 0.5°c 352 1.10 10 0.031 1033 3.22 32042
* value differs significantly from control at the J35 level TABLE 6
CROSSOVER VALUES IN EACH REGION IN MITOMYCIN C AND SALINE INJECTED FEMALES.
CROSSOVER REGIONS
BROOD TREATMENT TOTAL # % # % # %
Saline(0.7N) 158 0.84 5 0.027 437 2.33 18731
MC(100wg/ml) 78 0.68 3 0.026 337 2.93** 11488
Saline 199 1.01 6 0.030 565 2.87 19702 II MC 220 0.69** 10 0.031 772 2.43** 31767
Saline 225 1.19 6 0.037 651 3.45 18858 III MC 259 0.77** 3 0.009** 951 2.82** 33709
** values differ significantly from the control at the .01 level TABLE 7
CUMULATIVE STERILITY VALUES OF MITOMYCIN C AND SALINE INJECTED FEMALES.
BROOD NUMBER OF TREATMENT I II III FEMALES INJECTED # % # % # %
Saline (0.7N) 1 1.49 1 1.49 2 2.98 67 ro
MC (lOOjig/ml) 22 10.28 40 18.69 84 39.25 214 TABLE 8
SUMMARY OF EXPECTED AND OBSERVED DOUBLE CROSSOVER TYPES IN EACH TEST SERIES.
DOUBLE CROSSOVER TYPE
3T CONDITION 1,3(inter :inter) 1,2(inter :intra) 2,3(intra : inter) TOTAL #e #o #e #o #e
+ + - 12.8 1 0.15 2 0.50 49293
+ Ubx - 25.1 1 0.27 - 0.87 41266
SMI + 2 47.3 - 0.55 1 1.68 46041
Sml Ubx 1 72.8 - 1.09 4 3.05 34722
Trans 1 24.0 1 0.25 2 0.84 68753
0 rads 0 3.5 0 0.03 1 0.10 12502
4000 rads 4 10.8 2 0.15 1 0.50 79965
24° C 0 6.2 0 0.06 0 0.20 16600
40° C 1 22.8 0 0.32 1 0.97 72928
Saline 0 17.1 0 0.18 2 0.53 57291
MC 2 14.9 0 0.11 0 0.41 76964 - 28 -
DISCUSSION
Intergenic exchange responded differently to the various treatments as expected from previous work. The increase in crossing over in response to autosomal inversions, the decrease in response to radiation, and the lack of response after heat shock, each was paralleled by an alteration of similar direction and extent on intralocus exchange caused by the same treatment. After mitomycin C injection, although there was a significant decrease in both intralocus and interlocus crossing over in Brood III, in Broods I and II there was no consistent relation to the control. Since there was an even greater difference in response between the two interlocus regions than between the interlocus and intralocus regions, the significance of this set of results is questionable. That the main effect is in Brood III is consistent with the conclusions that MC primarily affects mitotic crossing over in the odgonia (Suzuki, 1964).
The conclusion that the direction and extent of alteration of crossover frequencies is the same in intergenic and intragenic regions for the treatments used agrees with the observations of Watson and Burdick (1964) at the m-dy locus. These results are compatible with the concept that crossing over occurs by one mechanism.
However, this does not provide critical proof since the - 29 -
similar alteration of two different mechanisms by all three treatments, although unlikely, cannot be ruled out.
Since double crossovers were observed, positive interference is not complete in this short genetic region.
If the degree of interference is correlated with the size of genetic regions, the majority of double crossovers should have been of the 1,3(inter:interlocus) type.
Even though more than half of the DCOs of the 1,2 and
2,3(intra:interlocus) type were not detected (see Table
1), a greater proportion of the doubles observed were of this type. It is apparent that while positive interference is essentially complete when only intergenic regions are considered, there is high negative interference when intragenic regions are also studied. These observations fulfil the prediction of Lewis' Two mechanism hypothesis, but are not inconsistent with Pritchard's One mechanism hypothesis since both are attempts to explain negative interference. The final decision will depend on establishing whether regions of high multiple exchange do occur in Drosophila and are simply undetectable by most crossover analyses since the markers would be separated by a greater distance than can be included in a switch region.
Assuming that switch regions do occur, it is necessary to increase the resolution of recombination analysis so that they can be detected. Since Pritchard1s - 30 -
hypothesis is based on detection of multiple exchanges
within a locus, a parallel system in Drosophila could
provide sufficient refinement. Such a system could be
as follows:
Co w N rb —v—
fano spl + +
It is thus predicted that the percentage of intra:interlocus
doubles should exceed that of inter:intralocus and inter:
interlocus doubles in the following order: 2,3 ^ 1,2 or
3,4 > 1,3 or 2,4 ^> 1,4. As the size of the genetic
region is decreased, there should be an increased frequency of recovery of a multiple exchange event occurring within
an effectively paired region. - 31 -
SUMMARY
Crossing over in individual Drosophila melanooraster females with the genetic constitution wa + + rb/+ fano spl + was studied in response to several treatments. The percentage of crossing over in the intralocus region fano - spl was not altered by a cis or trans arrangement of the pseudoalleles. Autosomal inversions increased crossing over in interlocus and intralocus regions, ^ -radiation decreased crossing over in both types of regions, and heat shock had no effect on crossing over in either type. The effect of mitomycin C injection varied from brood to brood and region to region in the two interlocus regions. The prediction that both types of crossing over would be altered similarly by various treatments if there is only one mechanism of crossing over is supported by these data.
An analysis of the double crossover products in the regions studied reveals negative interference when one of the exchanges occurs within a gene, whereas interference is essentially complete for interlocus:interlocus doubles.
This could be explained by a Two mechanism hypothesis, or by a One mechanism hypothesis if there are effectively paired regions, as described by Pritchard (1960). An experiment designed to detect possible switch regions is outlined. - 32 -
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Bridges, C. B., and K. S. Brehme, 1944. The mutants of Drosophila melanogaster. Carnegie Inst. Wash. Publ. 552.
Carlson, E. A., and I. I. Oster, 1962. Comparative mutagenesis of the dumpy locus in Drosophila melanogaster. II. Mutational mosaicBm induced without apparent breakage by a monofunctional alkylating agent. Genetics 47:561-576.
Chase, M., and A. H. Doermann, 1958. High negative interference over short segments of the genetic structure of bacteriophage T4. Genetics 43: 332-353.
Green, M. M., and K. C. Green, 1949. Crossing over between alleles at the lozenge locus in Drosophila melanogaster. Proc. Natl. Acad. Sci. U.S. 35: 586-591.
Holliday, R., 1964. The induction of mitotic recombination by mitomycin C in Ustilaqo and Saccharomyces. Genetics 50:323-335.
Lewis, E. B., 1945. The relation of repeats to position effect in Drosophila melanogaster. Genetics 30: 137-166.
Lewis, E. B., 1963. Genes and developmental pathways. Am. Zoologist _3:33-56.
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Mavor, J. W., and H. K. Svenson, 1924. A comparison of the effects of X-rays and temperature on linkage and fertility in Drosophila. Genetics 9.:588-608. - 33 -
Mitchell, H. K., 1958. Crossing over and gene conversion in Neurospora. pp. 94-113. The Chemical Basis of Heredity. Edited by W, D. McElroy and B. Glass. The Johns Hopkins Press, Baltimore, Maryland.
Muller, H. J., 1925. The regionally different effect of X-rays on crossing over in the autosomes of Drosophila. Genetics 10;470—507.
Plough, H. H., 1917. The effect of temperature on crossing over in Drosophila. J. Exptl. Zool. 24:147-209.
Plough, H. H., 1921. Further studies on the effect of temperature on crossing over. J. Exptl. Zool. 3_2:187-202.
Plough, H. H., 1924. Radium radiations and crossing over. Am. Naturalist 58:85-87.
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Schultz, J., and H. Redfield, 1951. Interchromosomal effects on crossing over in Drosophila. Cold Spring Harbor Symp. Quant. Biol. 16:175-197.
Shiba, S., A. Terawaki, T. Taguchi, and J. Kawamata, 1959. Selective inhibition of formation of deoxyribonucleic acid in Escherichia coli by mitomycin C. Nature 183:1056-1057.
Steinberg, A. G., 1936. The effect of autosomal inversions on crossing over in the X-chromosome of D. melanogaster. Genetics 21:615-624.
Streisinger, G., and N. C. Franklin, 1956. Mutation and recombination at the host range genetic region of phage T2. Cold Spring Harbor Symp. Quant. Biol. 21:103-109.
Suzuki, D. T., 1958. Irradiation effects on crossing over between genes exhibiting complete interference on the X chromosome of Drosophila melanogaster. B.A. Honors Thesis. Amherst College. - 34 -
Suzuki, D. T., 1964. Effects of mitomycin C on crossing over in Drosophila melanogaster. Genetics 51: 635-640.
Szybalski, W., and V. N. Iyer, 1964. Crosslinking of DNA by enzymatically or chemically activated mitomycins and porfiromycins, bifunctionally "alkylating" antibiotics. Federation Proc. 2_3:946-95 7.
Towe, A. M., and D. R. Stadler, 1964. Effects of temperature on crossing over in Neurospora. Genetics 49: 577-583.
Watson, J. E., and A. B. Burdick, 1964. The effects of heterologous rearrangements on inter-locus and intra-locus recombination in Drosophila melanogaster, Genetics 50:739-744.
Welshons, W. J., 1958. A preliminary investigation of pseudoallelism at the Notch locus of Drosophila melanogaster. Proc. Natl. Acad. Sci. U.S. 44: 254-258.