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ORNI 7-3063
Conf-670503-15
CESTI RACES
JUN 1 3 1967
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HC. 83.00: MN. 65
THE EFFECTS OF IONIZING RADIATION ON INTERSPECIZIC COMPETITIO..
. B. G. Blaylock
Radiation Ecology Section, Health Physics Division

MASTE
Oak Ridge National Laboratory, Oak Ridge, Tennessee
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ABSTRACT
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:-*...
The effect of acute and chronic radiation on competition was stidies.
laboratory populations of the sibling species Drosophila melanoge... ar.
Drosophila simulans which are similar both morphologically anä genetical
Drosophila simulans was the superior species for thirty weeks in the corne..
populations, but was eliminated by D. melanogaster when the populacions de
exposed to a chronic dose of 4.3 rads/hr. After an acute dose of 2000 reke
D. melanogaster was the superior species for approximately 12 weeks; however,
the reverse was true when the populations were exposed to 10,000 raus.
Drosophila melanogaster was eliminated by D. simulans by the ená oř six zuena.
The frequency of irradiated D. melanogaster or irradiated D. simu len: (2006
rads) decreased rapidly when placed in competition with non-irradiated in
of the opposite species. Under these conditions, irradiated D. melanogaster
recovered after six weeks, but irradiated D. simulans never recovered and was
eliminated in one population after 24 weeks.
1.!.
Population size was affected by the exposure to acute anạ chronic raai-
ation, but most of the populations recovered after six weeks. The popular: 3:13
receiving 10,000 rads were the exceptions; they required twelve weeks to
* Research sponsored by the U. S. Atomic Energy Commission under
26 contract with the Union Carbide Corporation.
DISTRIBUTION QE THIS DOCUMENT IS CANLIMITED
UCN-5807
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huge 13, 1467 ..
capabilities of these species. After nine weeks of competition the popularicu
3
having the largest average size were the ones that received an acute dose o
2000 rads.
INTRODUCTION
When closely related species are placed in competition for food and
space, one species is usually eliminated. This competitive displacement
principle was stimulated many investigations. The voluminous amount o
lit-
erature which has evolved on this subject was recently reviewed by DeBaca
(1966). One species replaces another because it is the more "fit" species
....
under the given conditions. The word "fitness" may refer to individual
.
genotypes, Mendelian populations, a species, or a higher biological category.
Its usage has implied widely different meanings, but invariably "... fitness
must refer to the ability of an organism to leave surviving offspring..."
(Ierner, 1954).
Attempts to measure the fitness of a Mendelian population have been made
by Andrewartha and Birch (1954), "innate rate of increase", and by Carson
(1957, 1958), "biomass" (the wet weight of hatching adults). Claringbola
and Barker (1961) and Barker (1963) used the rate of change in the frequencies
of two species in competition to estimate the relative fitness of Drosophila
populations. Fitness, when applied to Drosophila melanogaster or Drosophila
sim lans in relation to experimental data presented in this paper, represents
the relative frequency of that species in the total adult population.
LEGAL NOTICE
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UCN.8667
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The fitcess of a population is affected by the exposure to ionizing
| radiation. The diferent components of the life-cycle, such as fecundity,
egg hatchability, pupal mortality, etc., are influenced ny radiation. These
components are directly related to the genetic damage suffered by irradiated
populations in the form of dominant lethals, recessive lethals, and deleter-
ious mutations. Dominant lethals produce their effects and are eliminatea
rapidly from a population while recessive lethals and deleterious genes may
persist in the gene pool and affect the fitness of future generations.
Muller (1959) and others have expressed concern about this increase in genet-
ic load and its de leterious effects on the fitness of populations. Contrary
to most findings, Wallace (1958) and Crenshaw (1965) have shown that under
very special conditions irradiation can increase the fitness of a population.
This paper deals with the effects of radiation on laboratory populations
of D. melanogaster and D. simulans in competition. Drosophila melanogaster
and D. simulans are sibling species that were not distinguished from each
other until 1913 (Sturtevant). Their morphology is so similar that only the
males can be separated with reliability. Not only is the external morphology
similar, but there is a corresponding parallelism in their genetics (sturte-
vant, 1920, 1921a, 1921b, 1929). Competition between these two species hicis
been studied by Moore (1952), Herskowitz (1953), Claringbold and Barker (1962),
and recent y by Miller (1964) and Sameoto and Miller (1966).
14
15
MATERIAL AND METHODS
A strain of D. simulans derived from a collection at Solway near Oak
25 Ridge, Tennessee was used. This strain has been mass-cultured in the labora-
26 tory at the Biology Divison, Oak Ridge National Laboratories, for about five

UCN-5867
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years.
In order to distinguish both sexes, a strain of D. melanogaster con-
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taining the "e" ebony sooty marker was used. This strain has also been
maintained by mass culture in the laboratory for several years. Three
day-old flies were collected from mass culture bottles and aged for two days.
Fifty males and fifty females of each species were used to start laboratory
populations which were maintained at 25 1°c and 50 - 60% relative humidity.
The population cages and the technique for counting were described by
Blaylock (1967). Briefly, the total adult population of each species was
counted every 21 days and transferred along with the food cups containing
the larvae and pupae to another cage. One of the three food cups containing
11 | 40 ml of a corn meal agar live-yeast media was changed weekly in the cages
12 which supported approximately 1500 flies.
The source of radiation for the acute exposure of the adults was a co
Gammacell 200 from the atomic Energy of Canada, Ltd. The dose rate was 10
rads/sec. The chronic gamma irradiation was from a 25-curie co source wita
a dose rate of 4.3 rads/nr. Two cages were started for each treatment, as
--
follows:
Numbers 1 and 2 - controls
Numbers 3 and 4 - chronic irradiation of 4.3 rads/hr
Numbers 5 and 6 - acute irradiation of 2000 rads
Numbers 7 and 8 - acute irradiation of 5000 rade
Numbers 9 and 20 - acute irradiation of 10,000 rads
Numbers 11 and 12 -- acute irradiation of 2000 rads to D. simulans only
Mumbers 13 and 14 - acute irradiation of 2000 rada to D. melanogaster
only.
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The acute irradiation in each case was administered to the flies immediately
| before starting the populations.
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RESUITS
The effect of radiation on competition was demonstrated by plotting the
frequency of D. melanogaster in the different populations against time. In
other words, the number of D. melanogaster observed in the populations at
the 21-day counting intervals was divided by the total number of D. meler.o-
gaster and D. simulans observed in the population at that time. The percent
of D. melanogaster in the two control populations and the chronically irradi-
ated populations is shown in Fig. 1. Drosophila simulans was the most abund-
ant species in the control populations during fifteen generations of compe-
tition (2 wkb/generation). In contrast, D. simulans was eliminated in the
chronically irradiated population after 21 weeks.
The results of acute radiation on competition are given in Fig. 2. The
frequency of D. melanogaster in the populations that received 2000 rads re-
mained above 50% for 9 weeks; then the pattern of competition was similar
to that of the controls. The populations that received 5000 rads have not
been in competition long enough to justify comments other than to say that
no drastic effect was found and that both species were capable of reproduction
after the exposure to this amount of radiation. In the two populations that
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received 10,000 rads, D. melanogaster was eliminated after 6 weeks. Acute
doses of 5,000 and 10,000 rads seriously affected the reproductive capabili.
ties f these species.
E
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Figure 3 shows the frequency of D. melanogaster in the populations irat
2 were initiated with only one of the species being irradiated. In populations
| 11 and 12, where irradiated L. simu lans were in competition with nonirradiates
4 D. melanogaster, the percent of D. welanogaster in the populations increased
rapidly and remained above 80 percent in both populations through 24 weeks.
| In the complementary treatment in which irradiated D. melanogaster were in
competition with nonirradiated D. simulans (populations i3 and 14) the pre-
quency of D. melanogaster decreased for the first 6 weeks, but then increased
in frequency. In population 14 the frequency rose above 50 percent after
9 weeks.
11 The effect of radiation on the size of the different populations can be
12 found in Table I. The first colum lists the weeks that counts were made
13 after the populations were started. The next colums give the size of the
adult populations in the cages at each counting interval. The number of each
species in the populations can be obtained by using the percent D. melanogaster
16 found in the first three figures.
The populations usually build up to a maximum size and then the size
fluctuates with time. As expected, the size of the acute and chronically
19 | irradiated populations was less than that of the controls after the first
few weeks. However after six weeks of competition, populations 5 and 6 had
the largest average size. These were started with flies of both species that
received a dose of 2000 rads. The largest population occurred after twelve
23 weeks in population 12, which was started with irradiated D. simulans and
nonirradiated D. melanogaster. In the populations exposed to 20,000 rada, no
reproduction occurred before the first counting interval and many more
D. melanogaster than D. simulans survived. However, after six weeks
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D. melanogaster was eliminated by D. simulans. The lack of reproduction.
in the first three weeks accounts for the slow build-up in populatijn size.
DISCUSSION
In most laboratory population cage competition experiments, Drosophila
melanogaster is superior to Drosophila simulens (Moore 1952; Barker 1963).
However, laboratory conditions can alter the outcome of competition between
these two species; for example, when populations are maintained at 25°C,
D. melanogaster eliminates D. simulans within 80 days, but D. simulans is the
superior species at 15°c (Moore 1952). Although these species are very simi-
lar morphologically and genetically, there are some ecological differences
which may be important under natural conditions. Patterson (1943) observed
that near Austin, Texas, D. simulans is more abundant in the late summer
months while D. melanogaster predominates in the spring.
Under the present experimental conditions when near-constant environmen-
tal conditions were maintained, D. simulans was the superior species in the
control populations (Fig. 1). When the populations were exposed to chronic
irradiation of 4.3 rads/nr, D. melanogaster was clearly the superior species;
D. simulans was eliminated in cage 3 after 15 weeks and in cage 4 after 21
weeks (Fig. 1). The components of fitness of laboratory and natural popula.
tions of Drosophila are adversely affected by the exposure to ionizing radia-
tion (Wallace 1956; Marques and Maciel 1961; Stone et al. 1962). Laboratory
populations of D. melanogaster exposed to chronic irradiation (5.1 x/hr)
maintained a greater frequency of recessive letinals in their gene pool than
did nonirradiated controls (Wallace 1956). The cessation of irradiation was
:
UCN-$8.07.
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| followed by a reduction in the frequency of lethals und an increas: ia tre
viability of the population (Wallace 1956; Mourad 1962, 1964;; Durscenar:yo-
nan 1964, 1965).
A difference in fecundity could explain the elimination of D. simulers
by D. melanogaster in the chronically irradiated populacions. Providing the
viability - egg to adult - was similarly affected by radiation, the ricre
fecund species woulâ be expected to replace its competitor. Sameoto and
Miller (1966) extended Miller's (1964) well-documented investigation of
larval competition between these two species to include factors controlling
productivity. In their stocks no difference was detected in the fecundity of
the two species for the rirst five days, but D. simulans produced signifi-
cantly more eggs after 10 days. Di Pasquale and Santibanez (1960) analyzed
fecundity in several different lines of D. simulans and D. melanogaster;
in general, the production of D. simulans was lower than D. melanogaster
although each line showed its own characteristic.egg production. Chiang
and Hodson (1950) stated that egg production of D. melanogaster within the
first two days made up 80 percent of the adult production in the first gene-
ration and that most of the production was accounted for in the first four
days. A significant decrease in the number of eggs laid per female was
found in D. aimulans by Samooto and Miller (1966) when the density of fe.
males was increased above a certain number; however, this was not the case
with D. melanogaster.
When the populations were initiated with flies that had received an
acute dose of 2000 rads, D. melanogaster was the superior species for about
six generations (2 wks/generation). Later the pattern of competition more
closely resembled that of the controls. The acute dose of 2000 rads
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produced a sreater effect on the fitness of D. simulans than on D. wolunc-
gaster, populations 5 and 6, Fig. 2. This was futher confirmed when irrait
ated D. simulans were placed in competition with nonirradiated D. melaromester,
populations 11 and 12, fig. 3, and nonirradiated D. simulars were placed in
competition with irradiated D. melanogaster, populations 13 and 14, Fig. 3.
The frequency of D. melanogaster with irradiated D. simulans increased rapiche
furthermore the D. simulans had not recovered after approximately ten gencrat
tions. The reciprocal experiment showed a decrease in the frequency of
irradiated D. melanogaster with nonirradiated D. simulans, but a recovery
of the D. melanogaster took place in four or five generations.
The rapid recovery of a population after acute irradiation is due to
the elimination of dominant and semi-dominant deleterious components on the
genetic load. These components which drastically affect the parameters of
fitness, such as fecundity, fertility, and viability, are eliminated in the
first generation or so and would explain the large decrease in the irradiatek
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D. melanogaster and irradiated D. simulans when they were placed in competitt
ion with nonirradiated flies of the opposite species. The irradiated D.
nelanogaster recovered rapidly, but the D. simulans was eliminated in cage
12, and was at a very low frequency in cage ll. This difference in recovery
of the two species could be related to seemingly small biological differ-
ences. An acute dose of 2000 rads or more has a marked depressing effect
on egg production (Touchberry and DeFries 1964). Moore (1952) found that
D. simulans will not oviposit readily where D. melanogaster has ovi posited
previous ly; however, the reverse was not true for D. melanogaster. Narise
(1965) has shown that in competition between D. simulans and D. melanogastes,
UCN 0307
the competitive ability of D. simulans was increased by the relative 1sc-
quency of the parents. The irradiated D. melanogaster demonstristed that the
populations recovered from the effects of radiation, but the irradiatca
D. simulans (which was the superior species in the control population, dia
to
How
not recover.
Te opposite effect of radiation on competition was fouid when both
species were exnos eå to an acute dose of 20,000 rads. Drosophila simulans,
which was eliminated when exposed to chronic irradiation and decreased in
relative frequency when exposed to an acute dose of 2000 rads, eliminatea
D. melanogaster in both populations by the end of six weeks. The explanation
for this was apparent when the population size was examined (Table I). After
three weeks D. melanogaster was the superior species, but apparently no
reproduction occurred. As expected, both species were temporarily sterilized
by the acute dose of 10,000 rads; however in both populations D. simulans
recovered by the sixth week.
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Population size fluctuated from generation to generation; although the
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populations within treatments varied in size, their sequences of fluctuation
were usually similar (Table I). The effects of radiation on population size
can be seen after three weeks; the Irradiated populations were less than the
controls. However, most populations recovered by the end of six weeks.
It is interesting to note that at the end of six weeks the populations that
received an initial dose of 2000 rads had the largest average number. A
radiation-induced increase in fitness was demonstrated in D. melanogaster
(Wallace 1958) and Tribolium confusum (Crenshaw 1965). Their general con-
clusion was that polygenic mutations, most of which would be deleterious in
UCN-$567
the homozygous conditions, produced heterotic effects in the heterozygous
conditions which more than counter-balanced inducer, dominant, deleterious
mutations. Ayala (1966) further advanced this line of investigation with
irradiated large populations (2000 Plies) of Drosophila serrata and
Drosophila birchil which decreased in size for the first few weeks; however, from
the sixth to the fifteenth week they increased in size and production and
maintained their superiority over the controis.
It was evident from these experiments that the exposure to ionizing
radiation can influence the outcore of competition between closely related
species. It also was possible to point out the importance of the different
components that contribute to the fitness of a population. In directly
affecting the different components of the life cycle — fecundity, egg
hatchability, pupal mortality, etc. — radiation has acted as an additional
environmental stress. Unlike other environmental factors which select only
the present genotype, radiation may influence future generations by adding
new genetic combinations to the gene pool.
ACKNOWLEDGEMENT
The author wishes to acknowledge the capable technical assistance of
Mrs. Margaret F. Miller and to thank Dr. E. H. Grell of the Biology Division,
ORNL, for his many helpful suggestions.
UCN-8667

LITERATURE CITED
Andrewartha, H. C., and L. C. Birch. 1954. Distribution and abundance of
animals. Univ. Chicago Press, Chicago.
Ayala, Francisco Jose. 1966. Evolution of fitness. I. Improvement in the
productivity and size of irradiated populations of Drosophila serrata
and Drosophila birchii. 53: 883-895.
Barker, J. S. F. 1963. The estimation of relative fitness of Drosophila
populations. Evolution 17: 56-71.
Blaylock, B. G. 1967. A population cage for counting adult Drosophila.
Drosophila Inf. Serv. 42: 113.
Carson, H. L. 1957. Production of biomass as a measure of fitness of ex-
perimen tal populations of Drosophila. Genetics 42: 363-364.
1958. Increase in fitness in experimental populations result-
ing from heterosis. Proc. Natl. Acad. Sci. U. S. 44: 1136-1141.
_ 1964. Population size and genetic load in irradiated popu-
lations of Drosophila melanogaster. Genetics 49: 521-528.
Chiang, H. C. and A. C. Hodson. 1950. An analytical study of population
growth in Drosophila melanogaster. Ecol. Monogr. 20: 175-206.
Claringbola, P. J., and J. S. F. Barker. 1961. ine estimation of relative
fitness of Drosophila populations. Journal Theoret. Biol. 2:190-203.
Crenshaw, John W. 1965. Radiation-induced increases in fitness in the flour
beetle Tribollum confusum. Science 149: 426-427.
peBach, Paul. 1966. The competitive displacement and coexistence principles.
Annual Review of Entomology 11: 183-212.
UCN-6867
13
1 D1Pasquale, A., and S. Koref Santibanez. 1960. Fecundity in several lines of
Drosophila simlans and Drosophila melanogaster. ATTI A.G.I. V: 94-100.
3 Herskowitz, Irwin H. 1953. The survival and developmental rates of D.
me lanogaster and D. simulans. The American Naturalist 833: 113-115.
5 Lerner, I. M. 1954. Genetic homeostasis. Oliver and Boyd. London.
Marques, E. K. and Clara M. P. Maciel. 1961. Some components of adaptive
values of heterozygous Drosophila willistoni from irradiated natural
populations. Exper. 17: 404-405.
9 Miller, R. S. 1964. Larval competition in Drosophila melanogaster and Di
simulans. Ecology 45: 132-147.
Moore, John A. 1952. Competition between Drosophila melanogaster and
Drosophila simulans. I. Population cage experiments. 407-420.
Mourad, A. M. 1962. Effects of irradiation in genetically co-adapted systems
Genetics 47: 1647-1662.
- _. 1964. Lethal and semilethal chromosomes in irradiated experi-
mental populations of Drosophila pseudoobscura. Genetics 50: 1279-1287.
Muller, H. J. 1959. The guidance of human evolution. Perspectives in
I Biology and Medicine 3: 1-43.
19 Narise, Takashi. 1965. The effect of relative frequency of species in com-
petition. Evolution 19: 350-354.
21 Patterson, J. T. 1943. Studies in the genetics of Drosophila. III. The
Drosophilidae of the Southwest. Univ. of Tex. Pub. No. 4313.
Sameoto, D. D. and R. S. Miller. 1966. Factors controlling the productivity
of Drosophila melanogaster and D. simulans. Ecology 5: 695-704.
UCN-8867
Sankaranarayanan, K. 1964. Genetic loads in irradiated experimental popu-
lations of Drosophila melanogaster. Genetics 50: 131-150.
- 1965. Further data on the genetic loads in irradiated
experiizntal populations of Drosophila melanogaster. Genetics 52: 153-1644.
Stone, Wilson S., M. R. Wheeler and F. D. Wilson. 1962. I. Genetic studies
of irradiated natural populations of Drosophil... V. Summary and discus-
sion of tests of populations collected in the Pacific Proving Ground
from 1955 through 1959. Univ. of Texas Pub. 6205: 1-54.
Sturtevant, A. H. 1919. A new species closely resembling Drosophila
melanogaster. Psyche 26: 153-155.
1920. Genetic studies on Drosophila simulans. I. Intro-
duction. Hybrids with Drosophila melanogaster. Genetics 6: 488-500.
· 1921. Genetic studies on Drosophila simulans. II. Sex-
linked group of genes. Genetics 6: 43-64.
1921. Genetic studies on Drosophila simulans. III. Auto-
somal genes. General discussion. Genetics, 6: 179-207.
_ 1929. The genetics of Drosophila simulans. Carnegie
Inst. of Wash. Pub. 399. 1-62.
Touchberry, R. W. and J. C. DeFries. 1964. Genetic effects of gamma-
irradiation on egg production and adult emergence of Drosophila
melanogaster. Genetics 49: 387-400.
Wallace, B. 1956. Studies on irradiated populations of Drosophila melano-
gaster. Jour. Genetics 54: 280-293.
L 1958. The average effect of radiation-induced mutations on
viability in D. melanogaster Evolution 12: 532-556.
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FIGURE 1. Competition between D. melanogaster and D. simulans in
chronically irradiated and control populations.
UCN.6667
FIGURE 2. Competition between D. melanogaster and D. simulans in
acutely irradiated populations.
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FIGURE 3. Competition between irradiated D. melanogaster and non-
irradiated D. simulans, and competition between non-
irradiated D. melanogaster and irradiated D. simulans.
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(3 0.601
TABLE I
NUMBER OF ADUIT D. MELANOGASTER AND D. SIMULANS IN IRPADIATED AND CONTROL POPULATIONS
Chronic Radiation
tok

Wks.
Control
2000 rads
4.3 rads/hr
3
Acute Radiation
2000 rads
· 2000 rads
5000 rads | 10,000 rads
D. simulans only 1 D. melanogaster only
78 19 10
12
14
1
2
11
13
L
_
W
-
200
200
200
200
200
200
200
200
200
200
200
200
925
700
542
752
613
546
51
1197
908
870
943
1407
1409
851
792
1546
1582
590
1213
782
1218
1712
581
341
1061
925
792
818
1625
1422
849
1468
1120
1461
2004
1399
1697
1243
823
607
1061
1116
1237
1286
770
1175
943
1381
1097
You were i to ū ñ o awo
701
03
1070
885
1035
991
921
800
814
1205
757
1144
1273
847
893
924
770
1404
1237
llll
1519
1151
1049
973
1085
1285
1098
560
1036
1036
720
893
1041
1005

LIST OF FIGURES
.
FIGURE 1. Changes in the frequency of D. melanogaster adults in competition
with D. simulans in chronically irradiated populations.
FIGURE 2. Changes in the frequency of D. melanogaster adults in competition !
with D. simulans in scutely irradiated populations.
FIGURE 3. Changes in the frequency of D. melanogaster adults in populations
started with Irradiated D. melanogaster and nonirradiated
D. simulans, and in populations started with nonirradiated
D. melanogaster and irradiated D. simulans.

UCN.3667
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ORNL-DWG 67-4291

PERCENT DROSOPHILA MELANOGASTER
-
-
-
-
-
-
- -
-
CONTROL POPULATIONS
- CHRONICALLY IRRADIATED POPULATIONS: 4.3 rads/hr
3
6
9
21
24
27 : 30
12 15 18
TIME (weeks)
ORNL-DWG 67-4293

2000 rads TO DROSOPHILA
MELANGASTER ONLY
2000 rads TO DROSOPHILA
SIMULANS ONLY
PERCENT DROSOPHILA MELANOGASTER
3
6
9
21
24
27
30
12 15 18
TIME (weeks)
ORNL-DWG 67-4292

--- 2000 rads
5000 rads
- 10,000 rads
PERCENT DROSOPHILA MELANOGASTER
VAL
9,10
6
3
9
21
24
27
30
12 15 18
TIME (weeks)

END
DATE FILMED
3 / 15 /67
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