Indian Journal of Experimental Biology Vol. 37, April 1999, pp. 359-364

Bottleneck effect on intra- and interspecific competition in Drosophila malerkotliana

S.N.Hegde & M.S.Krishna

Drosophila Stock Centre, Department of Studies in Zoology,

University of Mysore, Manasagangothri, Mysore 570 006, India

Received 6 August 1998; revised 24 December 1998

Intra and interspecific competition experiments involving control and bottleneck lines of monomorphic and polymor­phic populations of D.lIlalerkatliana with D.nasuta nasuta have been carried out. Productivity and population size were eva luated in these lines. The results show that control line had higher relative fitness and adaptedness (productivity and populati on s ize) under both intra and interspecific competition than their respective bottleneck lines and D.nasuta nasuta. This indi cates that bottlenecks have an effect on competition in D.malerkatliana. The conseqlience of such competition studies using control and bottleneck lines of D.malerkatliana and D.nasuta nasuta is discussed.

The natural population at times goes through a severe .but temporary reduction in size and in extreme situa­tions population size decreases to a few breeding in­di vidual s. When size reduces, populations lose ge­netic variation. Reduced size thus becomes a bottle­neck for operation of natural selection. Thus the demographic dec lines make it necessary to study their evolutionary implications l.4 . This loss of variability is in terms of reduced number of alleles5

.6

, reduced vari­ance in quantitative traits and lack of adaptive flexi­bility7.8 . Effect of such bottlenecks in the laboratory populations9

.12 has been shown confirming Carson's

theory of founder effect. Carson 13·14 in his theory of

founder effect of speciation has proposed the estab­lishment of a reproductively isolated population trig'­gered by bottleneck followed by several generations without selection. Effect of such bottlenecks upon the quantitative genetic variation on morphometric char­acters of houseflies has been studied by Bryant et al. 10. Goodnightl5 has reported an increase in additive genetic variance following a bottleneck. He suggests that this new additive variance may have arisen through conversion to additive of a position of the epistatic variance present in an ancestral population. Bueno el al. 16 have studied the effect of bottlenecks on the competitive ability of Drosophila pseudoob­scura lines and showed that control line had greafer competitive ability than bottleneck lines.

Many workers have suggested that genetic vari­ance can increase after a bottleneck. Increase in the

genetic components of variation in experimentally bottlenecked populations have been observed in houseflies (Musca domestica) by Bryant and his co­workers1o,11. The consequence of increased genetic variance is its effect on fitness, competitive ability and finally sexual isolation. In most of these studies D.simulans and D.pseudoobscura have 'been em­ployed though the genus Drosophila consists of 2400 species. According to Templeton t7 cosmopolitan spe­cies is not a good choice to test founder flush-crash model. Hence in the present study D.malerkotliana, a semi-endemic species belonging to the bipectinata complex was employed to study effect of size bottle­neck on intra and interspecific competition.

Materials and Methods Establishment of lines (control and bottleneck lines)

In the present experiment, control and bottleneck lines established from chromosomally monomorphic and polymorphic populations of Drosophila malerkotliana collected from Bhogadi, Mysore, (Kamataka) and Varanasi (Uttar Pradesh) and D. nasuta nasuta established from isofemale line (My­sore) were used. Initial outbred population of D.malerkotliana was established from 150 naturally inseminated females collected from wild localities. In F2 generation, bottleneck lines were initiated with offspring from exactly one, four and eight pairs (n -1 p, 4p, and 8p) randomly selected from the above outbred population. The bottleneck lines laid eggs for

360 INDIAN J EXP BIOL, APRIL 1999

a week in one culture bottle containing wheat-cream agar medium and for another week in a second cul­ture bottle. All flies from the bottles were m ixed to­gether and redi stributed among 10 new culture bot­tles , each with 26 flies . After a week these flies were transferred to another cul ture bottle in a second set of 10 new culture bottles. The progeny of these bottles were again mixed together and redistributed to 10 new culture bottles with 26 fli es each. This was re­peated 10 times so that once aga in we had 10 cultures each with 26 flies. Additional generations of flush were al so prepared as above. The density of 13 pairs of flies per culture was maintained through out the flu sh phase, so that competition for limited resources wou ld be sli ght or absent, as required in Carson's model. The control line (outbred population) was also maIn tained in the same manner described above for the bottleneck lines . After 10th flu sh cycle both con­trol and bottleneck lines were used to study chromo­somal constitution and competition. For chromoso­mal ana lysis(data on the frequency of inversions), of contro l and bottle'neck iines of polymorphic popula­tions of first .flu sh cyc le (before starting the experi­ment) and 10th flush cycle was made by squashing 100 larvae from each line using the lacto-aceto··orcein method.

For competition study, eggs were collected sepa­rately at the same time using Delcour's procedure IS

from both contro l and bottleneck lines of both chro­mosomally monomorphic and pol ymorphic popula­tions of D. malerkotliana and D.llasuta nasuta. Eggs ( 100) were seeded in quarter-pint milk bottles and when adults emerged, virgin females and males were isolated within 4 hr of thei r ec losion and were used in the competition experiments . Mean values of 5 sets of both control and their respective bottleneck lines of D. malerkotliana and D.nasuta nasuta were used for the statistical analysis . The experiments were carried out during morning hours (7.00- 10.00 hr) at 22u± I °c..

Experimental procedure for competition Control and bottleneck lines of D. malerkotiiana

and D.nasuta nasuta were used to study the effect of size bottlenecks on competi tion . In the adult compe­tition studies, parameters such as producti vity and population size were eval uated . Control and bottle­neck lines of D. malerkotliana and D.nasuta nasuta were employed in both intra- and interspecific com­petition studies. To ana lyse the productivity and

population size during intraspecific adult competition (pure culture) studies, each population (as mentioned above) was built up with 50 (25 females and 25 males) one week old flies. We have used D.nasuta nasuta In interspecific competItIon with D.malerkotliana. This is because D.nasuta nasuta and D.malerkotliana are fast breeding sympatric species which co-exist in most of the natural environments. In interspecific competition (mixed culture) studies 13 females and 12 males of controllbottleneck line of D.malerkotliana with 13 females and 12 males of D.nasuta nasuta were utilized, the populations were maintained by the serial transfer technique of Ayala 19.

The founder flie s were introduced into. quarterpint -mi lk bottles with fresh medium once in seven days, they were etherized, counted and transferred to fresh bottles when the imagoes began to emerge in bottles where eggs had been laid, they were counted and then added to the bott les contained the adult population. The adult · nies were thus always in one bottle and eggs, larvae, pupae and newly emerged adults were in other bottles. The bott les were di scarded four weeks after the adult flies had been first introduced. Five sets of control and their respective bottleneck lines of D.malerkotliana and D.nasuta nasuta were used. Five rep licates were run for each experiment and their mean values were used to ca lculate productivity and populat ion size for control and bottleneck lines of D.malerkotlianCl 3nd also for D.nasuta nasuta. The pure culture (intraspecific competition) of each spe­cies were mainta ined fo r 12 weeks. The mixed cul­ture (interspec ific competition) was maintained till one of the competing species was elIminated.

Results Table I shows freq uencies (%) of he terozygous in­

versions in control and bottleneck lines of polymor­phic population of D.lIIalerkotliana at first fl ush cycle (before startIng the experiment) and 10th tlush cycle . It was noticed that even at 10th flu sh c cle inversion polymorphi sm persisted in all the lines and variation In frequency of heterozygous inversions was also no­ticed in control and bottleneck lines. Further. some invers ions which were not present in the fi rst fl ush cycle were started appearing in 10th fl ush cycle. Similarly some inversions which were present 111 the fi rst flush cycle disappeared in 10th f1 ush cycle.

Mean and standard error of productivity and population size of control and bottleneck lines in intra

HEGDE & KRISHNA: INTRA & INTER SPECIFIC COMPETITION IN DROSOPHILA 361

and interspecific competitions are given in Table 2. From thi s table, it is clear that in pure culture (in­traspecific competition') adult productivity and popu­lation size increases with increasing bottleneck size of monomorphic populations and decreases with in­creasing bottleneck size of polymorphic populations. Further, control line of monomorphic and polymor­phic populations had hi gher producti vity and popula­tion size than bottleneck lines and D.nasuta nasuta.

In mi xed culture (interspecific competition) aver­age adult producti vity and population size of D.lI1alerkotliana decreases wi th increas ing bottleneck size of monom orphic populations whereas in poly-

morphic population opposite trend was noticed. Similar pattern of adult productivity and population size of D.n.nasufa under mixed culture with control and bottleneck lines of monomorphic and polymor­phic populations were noticed. Average productivity and population size of control line was greater than bottleneck lines in both mono and polymorphic populations .

One way analysis of variance applied to mean pro­ductivity and population size in both pure and mixed cultures showed significant difference between con­trol and bottleneck Tines (Table 2).

Figure 1 shows number of weeks taken by control

Tab lc I- Frcquencics (%) of hcterozygous inversions in control and bottleneck lines of polymorphic populat ions of D. malerkotliana

At First Flush Cyc le A t Tenth Flush Cycle Inversions Control Single pai r Four pair Eight pair Control line SIngle pair Four pair

line line line line line line

liLA 4 2 liRA 0 14 Without in version 35 35 3G 32 20 II LA+II LB 8 8 20 2 40 4 II LA+II LB+II LC 0 30 IG 10 15 IILA+II LB+IILC 24 10 2 10 GO ILR A+II LI3 IG 90 2 1 28 23 G IIR A+IILC 28 IILC 4 15

Tab le 2-ln tra and interspecific competition of control and bott leneck lines. [Values are mean + SE)

Line/Popu lation Intraspecifi c competiti on(Pure culture) D.lllolerkotiiolla

Producti vity Population ~ Ize

I. ft, 'ollol/lorp hic pupllloliollS

M±SE M±SE CL 228 .53±9.88 339.80±G.7 1 IP 183 .19±3.5 3 290.02± 7.59 4P 202.04±4.05 313.26± 10.74 8P L24.53±5.70 3 14.23±10.77 D.II .//(]slIla 8 1.79±205 I 7263±3.59 F-Vaiue I 5 (J .(i(y 20 1. 89'

if Pon'lllorp/Ilc popu lOt lOlls CL 192.67±t .99 3 I8.76±9.57 IP 180.S5±6.02 302.89±509 4P 170.18±3. 32 296. 74±3.03 8P 147 .87±6 29 262.48± 7.27 /). II . 1I0SIIto 8 1 .~3±5 .76 I 7263±3 .59 F-V:l lue 25(1 7 1' 172.95'

CL = Control line : I P = Singl e pair bottlencck line 4P = Four pair bott leneck line; 8P = Eight pair bottleneck line (' val ue = ·'« l.05; h< () .() I; '<0.00 I.

Interspecific competition (Mixed culture) D .II .ll osula

Productivity Popu lation Productivity size

M±SE M±SE M±SE I 54.04±0.G2 247.09±0.G9 24.12±0.62 I 29.92±0.48 237.08±0.48 23 .1 G±06 1 124.13±0.66 202. 19±0.67 I (>.22±0.74 I 12.3 2±0.67 183 . 13_0.74 15n±069

47. IY 19.60'" 550 1'

I56 .85±4.23 247.46±8.32 26Jl8± 1.32 104.28±3.20 165 66±6.00 20. 2±O.82 IIG.06±7.IO I R3.48±4.8 1 22.05±0.63 U8.66±7.68 209.84±5 .44 23 31±O.73

62.32b 99.15 b 9.53b

Eight pai r line

2 5

48 12 8 5

18 2

Population size

M±SE 32.09±0.73 2G. 17±0.65 24 .32±0.24 1991±0.69

1799'

31 43±1 77 23.84±O.66 25 .33IO.83 26.2. ±0.49

4.72'

362 INDIAN J EXP BIOL, APRIL 1999

18

17

16

15

14

13

12

" 10

RIB MONOMORPHIC

~ POLYMORPHIC

CL IP 4 P

LINES BP

Fig. I-Interspecific competition of control and bottleneck lines of mono and polymorphic populations in D. ma/erkorlialla . [CL---Control Line; I P--Single pair bottleneck line; 4P-Four pair bottleneck line; 8P- Eight pair bottleneck line].

and bottleneck lines of D.malerkolliana to eliminate D.n.nasula in mixed culture. From this figure, it is clear that control line of monomorphic and polymor­phic populations took 10 and 12 weeks respectively to eliminate D.nasuta nasuta in mixed culture. On the other hand, bottleneck lines (I p, 4p and 8p) of monomorphic populations took 12, 14 and 16 weeks to eliminate D.n.nasula while bottleneck lines (Ip, 4p and 8p) of polymorphic populations took 14, 15 and 18 weeks to eliminate D.n.nasuta in mixed culture. This suggests that control line of monomorphic and polymorphic populations had better competitive abil­ity than bottleneck lines. Furthermore, monomorphic populations were better adapted than polymorphic populations.

Discussion It is evident from Table 1 that even at IOih flush

cycle inversion polymorphism persisted in control and bottleneck line of chromosomally polymorphic population of D.malerkotliana. Further, there was variation in the frequency of heterozygous inversions in different lines with respect to different inversions . This agrees with the work of Singh and Banarjee20

who while studymg chromosomal variability and in­terracial hybridization in D.bipectinata have found persistence of inversion polymorphism in polymor-

phic strains of D.bipectinata even after their mainte­nance in laboratory for 10 generations. They also found variation in the frequency of heterozygous in­versions in different crosses with respect to different inversions. In the present study the observed variation in frequency of heterozygous inversions may be be­cause of in the original population (firs't flush cycle) individuals might have carried the inversion in homo­zygous and heterozygous state due to initiation of bottleneck heterozygosity must have appeared.

Effect of size bottlenecks on competition could be understood by studying their adaptedness (producti ­vity and population size) during intra- and interspeci­fic competition . The adaptedness of a genotype or a group of genotypes can be measured by utilizing two parameters, viz. productivity and population size in experimental populations t9. Adaptedness refers to the ability of the carriers of a genotype or a group of genotypes to survive and reproduce in a given envi­ronmene l

. In the present study the distinction be­tween control and bottleneck lines, is difficult task and therefore direct competition studies between control and bottleneck lines may not be precisely re­warding. Hence, the investigation on the adult com­petition between control line and D.n.nasuta as much as competition, between bottleneck lines and D.n.nasuta have been made indirectly 'to assess the competition between control and bottleneck lines on one hand and between two sympatric species on the other hand. The authors have estimated the adapted­ness of control and bottleneck lines of D.malerkotliana and D.n.nasuta both under pure and mixed cultures. Productivity is the extent of its repro­ductive potential measured in tem1S of new born flies every week. So it is sum total of various components of life cycle such as fecundity , hatchability, rate of development, viability, longevity and sexual activity of the adults. These factors along with the survivor­ship of adult determines the population size. In the present study in both monomorphic and polymorphic populations adaptedness of bottleneck lines was lower than their respective control line under both pure and mixed cultures. Further, productivity and population size of D.nasuta nasuta under. intra spe­cific competition was lower than that of control and bottleneck lines of both monomorphic and polymor­phic populations. The higher productivity and popu­lation size of control line was due to its higher fecun­dity, viability and fertility (Hegde & Krishna, unpub-

HEGDE & KRISHNA: INTRA & INTER SPECIFIC COMPETITION IN DROSOPHILA 363

Iished data) coupled with faster exploitation of the environment. The population having higher produc­tivity was also having higher population size (Table 2).

The populations which maintain a larger popula­tion size may be said to be performing better from the biological point of view than the one having a small population size . Hence, providing means for com­paring the overall biological performance of one population with aI1other, where both are maintai ned under similar environmental cond itions. In the pres­ent study control line had higher productivity and population size than bo ttleneck lines and D. nasuta nasuta . Hence, control line perfomled better than the bottleneck lines and D.nasuta nasuta. This agrees with the work of Bueno et al. 16 who 'while studying the effect of periodic bottlenecks on the competitive ability of Dpseudoobscura lines found that derived population (bottleneck line) performed worse than the ancestral control ones in competition experiments . Furthermore, they also found that the average number of individuals at the end of the competition was hi gher in ancestral lines than in the derived ones. Based on the adaptedness (productivity and popula­tion size) of control , bott leneck lines and D.nasuta nasuta under study, the relative fitness sequence of control , bottleneck line and D. nasuta nasuta under pure culture (intraspeci fic competition) was CL>8P>4P> I P> D. nasl/Ia nasuta in monomorphic populations while in polymorphic populations the relative fitnes s sequence was CL> I P>4P>8P> D.nasula nasula .

In interspecific competition the adaptedness of one species to the other in a particu lar envi ronment can be assessed by their high productivity and population size . In the present study adaptedness of control and bottleneck lines under mi xed culture (interspecific competition ) reveal that among the control and bot­tleneck hnes, con trol line having higher productivity and popula tion s ize than bott leneck lines of mono­morphic and pol ymorphic populations. Hence control line is superior to the other bottleneck lines. On the other hand , among the bottleneck lines single pair bottleneck li ne of monomorphic population and eight pair bottleneck iine of po lymorphic population had the lowest productivi ty and population size is consid­ered to be inferior in its competiti ve abi li ty (Table 2) . Si ngle pair bottleneck line of monomorphic popula-

tion showed decreased competitive ability by having lower productivity and population size than control line and other bottleneck lines . This decreased com­petitive ability of si ngle pair bottleneck line may be attributed to two reasons(i) it may be due to inbreed­ing depression(ii) it may be due to more complex ef­fect of founder event. In polymorphic population sin­gle pair bottleneck line had relatively higher com­petiti ve ability than other bottleneck lines (Table 2).

Apart from the increase in the productivity and population size of control line, it is quite interesting to note that control line of both monomorphic and polymorphic populations eliminated D.nasuta nasuta quite earlier than the bottleneck lines under mixed culture . Bottleneck lines took more time to eliminate D.nasuta nasula in interspecific competition of both monomorphic and po lymorphic populations (Fig. 1) . By considering all these factors , the authors opine that control line is superior in having producti vity, larger population size, and quicker eliminating ca­pac ity than bottleneck lines. D.nasuta nasuta is an inferior competitor having the lower productivity and population size (Table 1) and hence gets eliminated during competition (Fig . 1) . This confirms the con­cept of competitive exclusionn which states that two species competing for the same and limited resources cannot coexist in the same loca lity, and one or the other species wil l be eliminated sooner or later. The Gause concept has been tested by various workers and in each instance one of the competing species was eliminated eventually23.24. Further, the observed duration for control and bottleneck lines to eliminate D.nasuta nasuta also support the Gause principle . The competiti ve ab ility is the sum total of numerous fac tors that in exceedingly complex way2~ . So the

competitive potentialities of control line and bottle­neck lines change, and it is a reflection of the com­petitive interac tions o f the control and bottleneck lines concerned . So pure and mixed cultures of con­trol and bottleneck lines wi th D.nasuta nasula repre­sent two types of competition and hence thei r abiliti es must be assessed 111 both the situations to get an idea of their competiti ve fitness.

The surviva l and dominance of a species depends both on its inter and intra specific competi tive ab ili ­ties. Of these, the authors opine that interspecific competitive fitne ss IS a major issue which regulates the structure and composition of the populations. The species/popul ation with better interspecific competi -

364 INDIAN J EXP BlOL, APRIL 1999

ti ve potenti alities can not only increase its numbers but al so can cause a reduction in the size of its com­petiti on and thus it can enshine the adaptive competi ­ti ons and integrity of its populations. This is true in the present study al so (Tabl e 2), the evaluation of adaptedness (producti vity and population si ze) re­vea ls that fitness seq uenc · of control and bottleneck lines during interspec ifi c competition was CL> IP> 4P>8P in monomorphi c populations, CL>8P>4P> I P in polymorphi c populatIOns. These studies thus sug­gest the crfect of bottl enecks on competition.

Acknowledgement The aut hors are gra teful to Chairman , Department

of Studi es in Zoo logy, University of Mysore for fa­cil ities and UGc, ew Delhi f'or fin ancia l assi stance.

References I

2

Mayr E. SI 'SICIJ/IIlics IIlJd l~/(' 'origilJ of species (Co lumbia l lni l'l'!"s Il Y Prcss, Ncll' Y~lI'k) 1942. ( 'arson II L. SciclJ cc, I ()S ( I ')70) 14 14.

.I ( ' ~ r s on II L. SPC("{llliol/ (i lld /h e Fo ul/der Pril/ ciple Sladler

(" 'IIC/ . Symp., 3 (I (71) 51. -+ Powcll.1 R. F: \'{)III/ iol/ . 32 (1<)78) 4()5._ :, \IC \11 , Maru yama T, & Chakraborthy R, Evoili/ioll, 29

(1(>75) I.

() Maru yama T. & Fucrst P, GCI/ Nics, III ( 1985) 675 . 7 Frank el 0 H. 8:. Sou lc M E, COl/ser vo/ion alld c I'olu/ioll

(ClIllbmlge Unil'crsit y Press, Cambridgc) 198 1.

8 Beardmore J A, in gene/ics and conservation , edited by Schonewald Cox , Chambers C M, MacBryde S M & Thomas L (The Benjamin/Cummings Publishing Co. Inc.) 1986, 125.

9 Ringo J M, Wood D, Rockwell R & Dowse H, Am Na /, ( 1985 ) 642.

10 Bryant E H, Combs L M & McComrnas S A, Gene/ics, 114 ( 1986a) 121 3.

II Bryant E H, McCommas SA & Combs L M, Gell e/ics, 114 (1986b) 11 9 1.

12 Bryant E H, Meffert L M & McCommas S A, Am Na /, /36 (1990) 542.

13 Carson H L, in Popilialioll b io logy and evolu/ion, edited by Lewontin, R.C. (Syracusc Uni versi ty Press, Syrac use, New York ) 1%8, 123 .

14 CarsonHL, AIII Na/, 109( 1975)83. 15 Goodni ght C J, Evohtlion, 41 ( 1987) 171. 16 Bueno J A L, Moya A & Gonzales-Candclas F, Heredity , 70

(1 993) 60. 17 Templeton A R, Gell etics, 94 ( 1980) 10 1 I. 18 Delcour J, Dros. II/ /Or. Sen'. , 44 ( 1969) /33. 19 Ayala F J, Cell c/ ica, 5 1 ( 1%5) 527. 20 Singh B N & Bancrjec R, Cy/obios, 82 ( 1995 ) 21 9. 2 1 Dobzhansky Th , Evolutionary biology, ed it.ed by Dobzhan­

sky Th , Hecht M K & Stecre W C (Appleton-Century-Craft, New York) Vo L 2, ( 1%8) I .

22 Gause G F, Th e struggle fo r existcll ce (Mac Mill an Co., CII'

York ) 1934. 23 Lemer I M & Dampscr E R, Proc. Nat . Acad. Sci., USA. 48

(1962 ) 82 1.

24 King C E & Dawson P, Evoili/io l/ary biology, VoL I, edi ted by Dobzhansky Th , Hccht M K & Steere W C, (A pp leton . Century-Crafts, New York ) 1972 .