THE EFFECTS OF ARTIFICIAL SELECTION FOR MATING SPEED IN

Drosophila melanogaster

BY AUBREY MANNINGDepartment of Zoology, University of Edinburgh

IntroductionArtificial selection for behavioural features

can be of considerable interest for two reasons .Firstly it is likely to yield information on thegenetic basis of behaviour, and we still requiremuch more of the basic data of behaviourgenetics. Secondly one of the effects of selectionmay be to produce exaggerations of the normalsystem which reveal, by their interactions withother behaviour patterns, something of theunderlying organization .

Among recent behavioural selection experi-ments have been those of Broadhurst (1958)using emotionality in rats, and Hirsch &Boudreau (1958) using phototactic responses inDrosophila . These, with the application of refinedforms of genetic analysis, have yielded results ofgreat interest from both behavioural and genet-ical standpoints .

Drosophila workers have often noticed thegreat variation in the time taken for virgin fliesto mate when males and females are first mixed .Even when conditions are kept as standard aspossible much variation is left, and the experi-ments to be described are an attempt to selectout lines of Drosophila which mate morequickly or more slowly than normal . It isobvious that no single factor will emerge as themediator of mating speed, but the. effects ofintense selection on the complex of factors in-volved may help to identify some of them andindicate those which are most easily changed .

Materials and MethodsThe base stock of D. inelanogaster I used was

collected by Dr . Forbes Robertson some yearsago and since maintained as a large population ;it is known to be genetically diverse . The simplemethod used for selecting mating speed I alsoowe to Dr. Robertson. 50 pairs of virgin fliesare mass mated in a half-pint milk bottle heldhorizontally in a clamp and with the only lightcoming from a 100W . bulb 18 inches from theclosed end. The flies tend to gather at the lightend and as courtship proceeds pairs can besucked out with a "pooter" as soon as they

82

copulate. This can be done without making themseparate or disturbing the courtship of the otherflies. I kept a record of the rate at which pairsmate for periods ur to an hour or more asneeded .

At the beginning of the experiment two iden-tical groups (A & B) of 50 pairs from the basestock were mated and the first 10 and last 10pairs to mate in each were bred in standardculture bottles . These constituted the parentalgeneration of two fast (FA and FB) and twoslow (SA and SB) mating lines . In subsequentgenerations 50 pairs from each stock weremeasured for mating speed and the 10 fastestpairs bred from each of the F-lines, the 10slowest from each S-line. At intervals during thecourse of the experiments the mating speed of asample from the unselected parent stock wasmeasured for comparison .

The ten pairs of selected flies per bottle wereremoved after 24 hours to limit the number ofeggs laid and the bottles were then given ampleextra yeast suspension . Virgin flies were col-lected on the first, second and sometimes thirdday of hatching and the sexes aged separatelyin small groups in vials, being transferred tofresh food every other day . The day before testingthey were etherized again and sorted into theirgroups of 50 ready for shaking into the testingbottles . Collecting flies over 2 or 3 days elimin-ated any chance of selection for rapid or slowdevelopment but meant that the groups testedconsisted of flies of different ages. For thisreason they were aged in vials until the youngestflies would be 4 days old when tested . There isevidence, (Manning, 1959a) that melanogasterfemales reach a peak of receptivity at 3 days ofage, but that thereafter until perhaps 10 daysold there is only a slight decline in their willing-ness to mate . Flies tested at 4, 5 and 6 days ofage may thus be assumed to be quite homo-geneous in their receptivity and age fluctuationswill not hamper the course of selection formating speed .The flies were reared and the tests made at

25°C.

240

ISO

3

MANNING: EFFECTS OF ARTIFICIAL SELECTION FOR MATING SPEED IN Drosophila melanogasler

The Course of the Selection Response andGenetical Results

Fig. 1 shows graphs of the changes in meanmating times, (measured by a probit trans-formation, see p. 00) for successive generationsof the selected lines and of controls when thesewere measured . The response to selection wasalmost immediate and from F2 onwards the S-and F-lines were quite separate . It continued forsome 7 or 8 generations, but subsequently, withthe possible exception of FA, little change inmating speed was produced . The relativelyenormous fluctuations in mating speed betweengenerations are conspicuous . These usuallyeffect all the linesi n a parellel fashion ; thus F3,10 and 18 show fast speeds, whilst F6, 7 and 13are all slow. It seems certain that these fluctu-ations are due to some common environmentalfactors, the nature of which are still obscure .This problem is of great interest, for it is oftennoted that Drosophila have their "on" and"off" days for courtship and hitherto temper-ature has been often the only well controlledfactor .

Fig. 2 shows a complete record of the mating

speeds during the selection test in F18 . Thedifferences were so great that about 70 per cent .(35 pairs) of the F- lines had mated within 2minutes before even the first of the S-lines haddone so. The unselected controls show an inter-mediate rate, but here, as in most generations,they are closer to the F- lines . In general it wastrue that selection had a greater effect towardsslowpess than rapidity . Rapid mating is probablya Feature contributing to "biological fitness" andone'would expect it to be held fairly close to thepossible maximum by natural selection . Conse-quently this artificial selection for mating speedcomes into line with other experiments on suchfeatures, where selection was less effective "up"than "down" .

The genetical results of this experiment revealnothing unexpected beyond a consistency andsimplicity that might not be predicted . Themeasure used is, after all, the behavioural inter-action between two individuals and is thereforeexposed to a whole range of obscuring factors .

The results are consistent with the accumula-tion in the selected lines of many genes affectingmating speed, These blend their action smoothly

83

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2

3

4

5

6

7

8

9

10 12 13

15 16 17 18 19 20 22 23 24 25

GENERATIONS

Fig . 1 . The course of the response to selection for fast and Islow mating speed. Mean mating time is plotted on alogarithmic scale . Selection was relaxed in generations 11, 14 and 21 .

84 ANIMAL BEHAVIOUR, IX, 1-2

10

15

20

25MINS. FROM START OF TEST

Fig . 2. Graphs of mating speed during the selection test on F18 flies .

in hybridizing F- and S- lines to produce F1flies with roughly intermediate mating speeds,similar to that of controls . Fig. 3 shows themating speed graphs of the F1 hybrids from Fl7parents. The subsequent backcrosses showmating speeds intermediate between the F1 andthe parent stock, with remarkably little overlap,Fig. 4. Hybrids between the two F- and the twoS- lines, (which are also shown on Fig . 3) havemating speeds very similar to the .parents andthus it appears that genes with similar actionshave accumulated in each pair of lines .

For each line in every generation the per-centage of pairs mated during the test wastransformed into a probit and plotted againstthe logarithm of time since the test began . Thistransformation usually gave a good straightline plot, (which was fitted by eye) and thus anestimate of the mean mating speed and thestandard deviation . These measures can at bestonly be approximate, for though each popu-lation shows a roughly normal distribution ofmating speeds, conditions in the tests do not .remain constant over the whole period . Thelast few pairs to mate do so under far lesscrowded conditions than the early pairs . How-ever for the S- lines, where often only a few

pairs had mated within a reasonableobservation time, the probit/log. timeplot is probably the best way ofestimating the mean mating time ofthe 50 pair population .

To estimate the heritability of themating speed, i .e. that proportion ofthe total variance which is due to theaverage effects of genes, the selectionresponse must be plotted against thecumulative selective differential . Thislast is a figure which representscumulatively the total selection press-ure applied through successive gen-erations to produce the observedresponse . It combines the intensity ofselection-the proportion of flieschosen as parents for the next gen-eration-with the variance of thepopulation from which they areselected . Clearly the more variable thepopulation is, the more the mean ofthe selected group will depart fromthe population mean and the greaterthe expected effect of selection . Thusthe selection differential will be largerthe more variable the population isfor mating speed . (For a full acount

of this and other methods of calculating herit-abilities, see Falconer, 1960).

Since I omitted to measure controls over thefirst few generations of selection, when it wasmost effective, probably the best estimate ofheritability is gained by measuring the rate atwhich the two pairs of selected lines divergefrom one another. This is done in Fig. 5 wherethe difference between the mean mating speedof the two F- lines averaged and the two S- linesaveraged is plotted against the mean selectiondifferential accumulated over successive gener-ations. The divergence increases rapidly for 7generations and then begins to fluctuate but withlittle sign of further progress-essentially thesame picture as revealed by Fig . 1 . The slope ofthe regression line fitted to the points during theinitial divergence gives a direct measure of the"realized" or "cumulative" heritability . There isperhaps little reality in the heritability of acharacter which involves the interaction betweentwo individuals. However the consistency ofselection's effect and the fact that the accumul-ated genes appear to affect males and females ina behaviourally complementary fashion, makeit' worthwhile to fit a regression line and simplystate that the heritability is approximately 0 . 30,

MANNING : EFFEC'T'S OF ARTIFICIAL SELECTION FOR MATING SPEED IN Drosophila nielonogaster 85

5

10

MINS . FROM START OF TEST

Fig. 3 . The mating speeds of Fl hybrids from F17 parents . Themale parent's line given first in each case.

Since the response is greater in the direction ofslow than of fast mating, the larger part of thisfigure will be due to the S- lines . In a repeat ofthis experiment it will be more interesting tomeasure the heritabilities of mating speed in thetwo sexes separately by measuring a sample ofeach against controls in every generation .

There is no doubt that considerable vari-ability for genes affecting mating speed stillremains in the selected lines . Reversed selection,i .e. breeding from the fastest mating pairs of theS- lines and the slowest of the F- lines, was begunin F15. Within 7 generations the mean matingspeeds of all four lines were back to levels veryclose to that of controls . This experiment should,perhaps, have been continued until the levels"crossed", but it was already clear that thereverse-selected lines were not the same asbefore in behaviour . Reversed selection can rare-ly restore the genetical status quo for of necessity

15

it is acting upon a changed genotype .Analogously the behaviour too was alteredand, for example, the reverse-selected fastmating lines came to include increasingnumbers of extremely inactive flies whichscarcely moved throughout the test andmated slowly for this reason (see p . 00) .

If variability remains in the selected linesone would predict that on relaxing selec-tion, or rather returning to natural selectionalone, the lines will revert to their previousstate since one is bound to have disturbedtheir naturally selected optimum genotype .Samples of the four lines have now beenbred without artificial selection for 15generations, beginning as with counterselection from F15 parents . These relaxedlines have fluctuated widely in matingspeed, as have the lines with continuedselection, but are still little closer to con-trols than at the outset. The most likelyreason for this stability is not lack ofgenetic variability but lack of naturalselection pressure. The conditions in thestock bottles with good food conditionsare such that selection pressures against thefast and slow mating genotypes are slight .

Phenotypic variability, on the other hand,has been greatly changed as a result ofthe selection. The mating time of 50 pairsof the F- lines is compressed into a periodof 3 to 4 minutes, whilst the same numberof control flies take 10 minutes or more .The S- lines are far more variable, forselection has not produced flies which have

a long delay until the first mating, but whichsubsequently all mate within a short time . Nearlyalways one or two pairs are mated within the firstfew minutes but the rest may be spread out over2 hours or even longer .

General Effects of BehaviourHybridizing the S- and F- lines has shown

that they produce an FI with an intermediatemating speed. It is of interest to see whether asimilar intermediate speed is obtained whenmales from an F- line are courting S- linefemales and vice versa . Accordingly the matingspeeds of such crosses were measured at variousstages from F7 onwards . Fig. 6 shows a typicalresult from F7 and the speeds are indeed inter-mediate, whilst intercrossing the two F- andthe two S- lines in this way produces fast andslow speeds respectively .These results confirm those from hybridiz-

86 ANIMAL BEHAVIOUR, IX, 1-2

Fig. 4 . The mating speeds of F2 back .crosses from the hybrids between F-and S-lines shown in Fig. 3 . Backcrossesto the fast parent are solid lines, those tothe slow parent are broken lines .

ation in showing that the changesproduced by selection have beensimilar in both pairs of lines, andfurther that both sexes have beenaffected. An independent measureof this was obtained by testingmating speeds against an unselectedstock which was equally "foreign"to the normal controls and the se-lected lines . Fig. 7 shows the resultstaking samples for F18, all fliesbeing 3 days old when tested. Theselected lines and controls occupythe same relative positions as whenmated within their own stock,though the S- line females materather faster with the foreign malesthan might be expected .

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The Nature of the Behavioural ChangesA large number of factors might be

responsible for changing the matingspeed of Drosophila . Apart from show-ing that some of them have an inheritedbasis this experiment is of little behav-ioural interest unless it manages to nar-row the field . It became obvious quiteearly in the course of selection that feat-ures other than sexual behaviour werebeing affected . The light was madestrongly directional during the tests toencourage the flies to gather at theclosed end of the bottle . After 3 or 4generations the F- line flies were stillcongregating closely but the S-lines weremuch more spread out and not so at-tracted by the light . This inevitablyleads to slowed mating for contacts be-tween males and females are reduced .Hirsch & Boudreau (1958) have shownhow readily the phototaxis of melanogas-ter responds to selection and it seemsprobable that, initially, selection formating speed can act most readily viathe natural variation in phototaxis andits resultant effects on spacing out inthe test bottles . As selection proceededthese changes were soon obscured asother features with a greater effect be-gan to respond .

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CUMULATIVE MEAN SELECTION DIFFERENTIAL

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Fig . 5 . The divergence between F- and S-lines plotted against the meancumulative selection differential . The regression line fitted to the graph upto and including F7 is shown. Its slope gives a measure of the heritability,approximately 0.30.

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MANNING : EFFECTS OF ARTIFICIAL SELECTION FOR MATING SPEED IN Drosophila melanogaster 87

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MIN$ . FROM START OF TEST

Fig. 6 . The mating speeds which result from mixing males and femalesbetween the lines . The flies are all from F7 .

The most noticeable of these related to thegeneral activity of the flies . By as early as F6 itbecame clear that flies of the S- and F- lines hadvery different patterns of activity . Whilst ageingin the vials S- line flies would react strongly toany disturbance but the F- lines were markedlysluggish. These differences reflected themselvesin the behaviour when testing and had greateffects on the mating speed . When first shakeninto the bottle the S- line flies showed I to 3minutes, or even more, of violent activity, run-ning and flying around in a random fashionunrelated to the direction of the light . Thismeant that males did not begin courting per-sistently for 3 or more minutes . On the otherhand the F- lines showed little disturbancewhen put into the bottles and courtship beganimmediately. It is difficult to define "generalactivity" at all precisely in Drosophila but asmeasured here it means, locomotor activityreleased by any environmental stimuli other thansexual ones. Some of it has the appearance ofbeing "spontaneous" but this is difficult toprove. Activity is highest following introductionto a new environment and might have somethingin common with "exploratory behaviour" invertebrates. Certainly Drosophila species show

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characteristic activity levels . Melano-gaster, for example, is far more activethan its close relative simulans, (Man-ning, 1959b) and stocks of the samespecies often appear to differ in thisrespect .

To get some kind of objectivecriterion of activity differences be-tween the selected lines, flies weretested in an open arena . This wasmade of perspex, l O x l O x 1 cm . deepwith its roof, floor and walls markedoff in 1 cm. squares. It was lit byfairly powerful diffused light fromabove and the flies were introducedsingly into a small box with access bya moveable screen to the middle ofone side of the arena. After a pause ofsome 15 seconds in the starting box,the screen was raised and the num-ber of cm . squares the fly entered,(whether on roof, floor or sides)within 1 minute from crossing thescreen was counted . At 3 days of age5 flies of each sex from each of theselected lines and the control stockwere tested this way for 4 successivegenerations, (F16-19) and Table I

summarises the results .There was a great deal of variation within

lines, at least partly due to inevitable differencesin disturbance when introducing flies to thestarting box. There were also inter-generationdifferences significant between 5 and I per cent.levels in the case of males, but the inter-linedifferences were constant. Comparing the vari-ance between lines the results are similar for bothsexes and show :-1 . The two S- lines showsimilar activity levels, as do the two F- lines, butthat of the S- lines is higher, (P<0 . 001) .

2. Control flies show no significant differencefrom the S- lines in activity .

The averages of squares entered gives a veryclear picture of the differences between the F-and S- lines . S- line flies were moving in thearena for the majority of the minute test, withshort pauses when they cleaned themselves .F- line flies often only moved a few squares pastthe screen and then remained still, though oftencleaning, for the rest of the test .It was mentioned that the most obvious

effect of activity upon mating speed was to in-crease the length of disturbance following in-troduction to the test bottle and thereby delaythe onset of courtship. A more accurate measure

8 8 ANIMAL BEHAVIOUR, IX, 1-2

Table 1. The Number of Squares in the Arena Entered within 1 minute . 20 Flies In each Group, 5 from each of Generations16 to 19 Inclusive.

of this effect was obtained by timing the delaybefore courtship in single pair matings made insmall observation cell, 1 inch in diameter . Herea stock female (equally "foreign" to selectedlines and controls) was placed in a cell andallowed to settle before introducing the male tobe tested. The time from his introduction to thebeginning of courtship (the so-called "lag time")was measured to the nearest 5 seconds for 20matings of 3 day old males of each line andcontrols from F9. The results are summarisedin Table II .

An analysis of variance shows significantdifferences between lines and comparing themeans of samples with the within sample vari-ance shows the following results :

1 . SA males have longer lags than SB,(P<0 .0001). This is due to a few extremely longlags in the former sample.

2. SB males have lags significantly longer thanthose of control males, (P<0 .001).

3. Control and FA males do not differ in thisrespect, nor FA and _ FB, but on these figures

control and FB males are just significantlydifferent, (P<0 .05) .

Thus the actual order of the lines based on theactivity measure, (SA, SB, control, FA, FB) isexactly paralleled by average lag time . Howeverin activity control flies were similar to the S-lines, but in lag time they resemble the F- lines .The chief difference in the two situations is thatthe arena offers no sexual stimuli, whilst lagtime is probably a measure of the time takenfor sexual stimuli from the female to overcomethe effects of "activity stimuli" from the newenvironment of the cell. Accordingly it wasnecessary to see if there are any differences inthe sexual responses of the different lines.This was done by recording the courtship of

the males following the lag-period measure-ments just described . The "licking" movement,in which the male extends his proboscis towardsthe female's ovipositor gives a simple criterionof the courtship. Bastock & Manning (1955)showed that the frequency of licking is positivelycorrelated with the length of bouts of wing

Table II. The Lg-periods Before Courtship In 20 3.day old Males from Each Group In F9.

SA SB Controls FA FB

Mean lag periods(seconds) 73 . 8 41 . 5 19 .5 13 .5 9 . 3

Range 20-240 5-125 5-95 5-50 5-.40

2 .9 37 .5 21 .8 14'6 8 .1

MALES

SA SB

Controls FA FB

Mean squares entered 59 .8 54 .6 49 . 1 24 .8 20 .2

Range 23-99 17-125 4-103 1-51 2-61

s 19 . 5 27 . 8 26 .9 15 .4 18 .8

FEMALES

SA SB Controls FA FB

Mean squares entered 50.3

146 .5 43 . 1 21 .6 13 .2

Range

f 1-101 8-89 13-75 1-36 1-36

s

I 27 . 3 24 .9 18 . 3 13 . 1 11 . 3

MANNING : EFFECTS OF ARTIFICIAL SELECTION FOR MATING SPEED IN Drosophila melanogaster 89

Table III. The Frequency of Licking Expressed as a Fraction of 100 seconds of courtship for 20 males of each Group from F9 .

display and thus provides a measure of "court-ship intensity" as a whole . The females used inthese tests were, as mentioned above, of anotherstock from the unselected control flies and were1-day old when tested. This meant that thoughattractive to males, they were not very receptiveand courtships were of adequate length forrecording. No courtship of less than 100 secondswas counted and a sample of 200 seconds wastaken wherever possible. The results are sum-marized in Table Ill . The amount of lickingis expressed as the average frequency in 100seconds of courtship, (breaks in courtship beingexcluded from the time count) and sirlce themovement itself lasts for about a second themeasure approximates to a percentage of timespent licking.

The test was made initially on males from F9and shows clearly that the S- line males have apoorer, lower intensity courtship than controlsor F- lines . The difference in licking frequencybetween the best S- line (SB) and either F- lineor controls is significant at the 0 .1 per cent. level .A similar test with F17 males gave the sameresult with even less overlap in the range oflicking frequencies between the lines . In F27 afurther check of the two extreme lines, (at thisstage) SA and FA together with controls re-vealed mean licking frequencies of 6 .37, 13 .56and 11 .50 respectively with 16 males per group .Here FA has a significantly better performancethan the control line, P<0 .001 . A small differencein the licking frequency represents a consider-able difference in the amount of wing display inthe courtship and the deficiencies of the S- linemales in this respect are probably quite sufficientto account for their poor success .

The short lag-periods and high frequency oflicking in the courtship of F- line males bothindicate that the sexual response threshold islowered. There were other signs that this was so .F- line males courted with extreme persistenceand frequently spent long periods directing theircourtship to other objects in the observationcells. They courted the peg which seals the en-

trance hole, or courted their own reflection inthe cover slip over the top of the cell, Theserarely proved adequate stimuli for the controlor S- line males .The changes in female sexual behaviour are

more difficult to distinguish from changes inactivity levels. The behaviour of a receptivefemale is to stand still or walk slowly when amale courts, and the F- line females all did this .An unreceptive female may, whilst standing still,flick her wings, kick backwards or extrude herovipositor, but she may also decamp, jumpingor flying right away . S- line females most com-monly took the latter course and there was agreat amount of flying and jumping in the testbottles . With both types of female, it is difficultto say how far their behaviour towards malessimply reflects their differential reactiveness todisturbing stimuli or whether it is a purely sexualresponse. However, comparing repelling move-ments made whilst relatively still, there wascertainly more ovipositor extrusion from S- linefemales, a movement of sexual rejection, pre-sumably representing a raised sexual responsethreshold .

Discussion

We may summarize the behavioural differ-ences between the lines as follows :

1 . The S- lines show a high level of activity .The males have long lag periods before courtshipand a reduced frequency of licking in theircourtship. The males have a correspondinglyreduced success with foreign stock females(Fig. 7a) and the females show lowered recep-tivity and require more courtship before accept-ing stock males (Fig . 7b) .

2. The F- lines have a greatly reduced level ofactivity . The males have short lags before court-ship and a licking frequency equal to, or betterthan that of controls. They have increasedsuccess with stock females and conversely thefemales mate rapidly with stock males.

3. Unselected control flies show a high levelof activity, yet their lags are quite short and the

SA SB Controls FA FB

Mean frequency of licking 9 . 1 10 . 5 14 . 3 t3 .6 13 . 6

Range 1-17 6-15 5-22 6-19 6-28

s 3 . 67 2 .69 4 . 74 3 .25 5 . 09

90 ANIMAL BEHAVIOUR, IX, 1-2

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MANNING : EFFECTS OF ARTIFICIAL SELECTION FOR MATING SPEED IN Drosophila melanogaster 91

males show high intensity courtship . They showintermediate success with stock females, andcontrol females have an intermediate speed ofmating with stock males .

Briefly then, whilst the S- lines show highactivity and low sex, the F- lines show lowactivity and high sex, but the controls show bothhigh activity and high sex . It appears that selec-tion has been most successful in directions basedupon raising the thresholds of performance .The most conspicuous effects are the raising ofthe activity threshold in the F- lines and that ofcourtship in the S- lines . This indicates in turnthat under normal domesticated conditions, in apopulation cage or stock bottle, "natural"selection keeps the levels of general activity andsexual behaviour near to the possible maxi-mum. However smaller changes towards lower-ing reaction thresholds have been produced .The S- lines show signs of becoming hyper-active and the most recent test of male sexualbehaviour showed FA to be "better" than thecontrols .

It certainly appears that the differences inactivity between the selected lines are just asimportant in altering the speed of mating as thedirect changes to sexual behaviour . It is not somuch the level of activity that matters as thespeed with which the fly can make the changeover from activity responses to sexual ones .Shaking into the test bottle provides powerfulactivity stimuli and at the same time brings thesexes together and provides sexual stimuli .To this mixed stimulus situation the S linesrespond with violent activity and though thereare frequent encounters between males andfemales, no sexual behaviour is released forsome minutes. The F- lines with their lowactivity scarcely respond to the disturbance,males begin to court when they first meet afemale and within a few seconds the first pairshave mated . Control flies, though they show ahigh activity level in the arena tests, show only abrief burst of activity when shaken into the testbottle. The switch to sexual behaviour is rapid,almost as quick as in the F- lines, and matingspeed is correspondingly fast . Activity and sexualbehaviour are mutually incompatible and aspeedy change over to the latter must depend onthe threshold for sexual behaviour being low,as it is in both controls and F- lines . The raisedsexual threshold in the S- lines results in agreatly lengthened initial period of activity .Conversely the raised activity threshold of the F-lines reduces this period almost to nothing.

The control flies have established under naturalselection optimum levels of activity and sexualbehaviour which leaves them adequately re-sponsive to both types of stimulus . It will beinteresting to examine the response levels offlies newly caught in the wild .

There is clearly a limit beyond which furtherreduction in activity does not result in fastermating. This point was passed by the reverse-selected fast lines, as mentioned on p . 00. Duringtests FA and FB usually had some 30 or 40pairs mated within 3 minutes, but the remainingfew pairs took much longer. These flies wereoften completely motionless for minutes on endand consequently no sexual encounters tookplace to initiate courtship . Tapping the bottlesometimes produced enough disturbance tobring males and females together again . It isthese exceptionally inactive flies, since they arelast to mate, which are bred from during re-versed selection and they tended to increase innumbers. Hence reversed selection, thougheffective in reducing mating speed, did not re-trace the path to a state like that of the controls .

For an understanding of the organization ofDrosophila behaviour it is important that theartificial selection pressure applied here hasresulted in a separation of those systems con-cerned with "general activity" and sexual be-haviour. It is often assumed that a "vigorous"insect displays this quality in all aspects of itsbehaviour . This is just the argument I used toaccount for the observed differences between thebehaviour of D. melanogaster and D. sirnulans(1959b), and certainly activity and sexual re-sponses appear to decline together when fliesare inbred . Normally natural selection may causethe two to be positively correlated as in mycontrol line . However artificial selection hasproduced very active flies with a less vigorousform of courtship and also very sluggish fliesthat nevertheless show great vigour whencourting . Clearly a simple concept of "vigour"is not sufficient .

The physiological bases of these behaviouraldifferences have not yet been investigated andthis may prove very difficult . One can easilyimagine that the decrease in both activity andsexual behaviour which occurs on inbreeding isdue to a depression of the insect's total meta-bolism, but such cannot be the case here . Thereare various facts which tell against any generalmetabolic changes . The development time of allthe selected lines has remained the same ascontrols throughout the experiment. Dr. Forbes

92

ANIMAL BEHAVIOUR, XI, 1 .2

Robertson kindly measured the egg productionof all four lines, which is likely to be verysensitive to changed metabolic conditions, andfinds no difference from controls. Again, Dr .Margaret Bastock finds-that F- and S tinesperform equally well with controls in forcedflight tests-a good measure of muscularefficiency and endurance. The most likely con-clusion is that the genes accumulated in theselected lines are affecting neural thresholds inmechanisms concerned with the control ofsexual behaviour and of responses to otherenvironmental stimuli .

Summary1. This paper describes an experiment in

which lines of . Drosophila melanogaster wereselected for fast and slow mating speed oversome 25 generations .2. After 7 generations selection had produced

a divergence in mating speed such that the meanspeed of a 50 pair population was about 80minutes in the slow mating (S) lines but only3 minutes in the fast (F) lines. Subsequentlyselection made little progress though there havebeen wide fluctuations in speed, probablyenvironmentally determined.

3. Hybridizing S- and F- lines produces anF1 -with intermediate mating speed . Other testsshow that the behaviour of both sexes has beenaltered in a complementary fashion .

4. As measured in an open arena, the generalactivity of S- line flies is much greater than thatof the F- lines and unselected controls resemblethe S- lines in this respect. Measurements of thesexual behaviour of males show that the F-lines and controls have a higher intensity court-ship than the S- line males .

5. The most conspicuous effects of selectionare to raise the reaction thresholds of sexualbehaviour in the S- lines and of general activity

in the F- lines . To situations involving a mixtureof both sexual and activity stimuli, the S- linesrespond with prolonged activity but the F-lines show scarcely any and begin courtshipimmediately. Controls show a brief burst ofactivity but rapidly change over to courtship .

6. Natural selection will normally lead to thelevels of activity and sexual behaviour beingpositively correlated and at an optimum whichdoes not result in over-responsiveness in eitherdirection . Artificial selection has led to aseparation of the two systems and no concept ofa "vigour" which inevitably affects all behaviour-al levels is adequate .7. There are no signs of general metabolic

changes to the selected lines and it is probablythat the accumulated genes affect thresholds inthe nervous system .

AcknowledgmentsI am most grateful to Dr. Forbes Robertson,

Dr. D. S. Falconer and Dr . Margaret Bastockfor help and advice at all stages of this work.

REFERENCESBastock, M. & Manning, A. (1955) . The courtship of

Drosophila melanogaster. Behaviour, 8, 85-111 .Broadhurst, P. L. (1958). Studies in psychogenetics : The

quantitative inheritance of behaviour in ratsinvestigated by selective and cross-breeding . Bull.Brit. Psycho! . Soc ., 34, 2A (abstract) .

Falconer, D . S. (1960). Introduction to QuantitativeGenetics. Edinburgh & London: Oliver & Boyd.

Hirsch, J . & Boudreau, J . C . (1958) . Studies in experi-mental behavior genetics : I. The heritability ofphototaxis in a population of Drosophila melano-gaster . J. comp. physio!. Psycho!., 51, 647-651 .

Manning, A. (1959a). The sexual isolation betweenDrosophila melanogaster and Drosophila simulans .Anim. Behaviour, 7, 60-65 .

Manning, A. (1959b) . The sexual behaviour of two siblingDrosophila species. Behaviour, 15, 123-145.

Accepted for publication 31st August, 1960.