tative mating. Intense disruptive selection may lead to assortative mating. HODAY (1967) stated as one of a number of generalizations concerning dis-. Truptive ...
EFFECTS OF POPULATION SIZE AND SELECTION INTENSITY ON RESPONSES TO DISRUPTIVE SELECTION IN DROSOPHILA MELANOGASTER J. S. F. BARKER
AND
L. J. E. KARLSSON
Department of Animal Husbandry, Uniuersity of Sydney, Sydney, N.S.W. 2006, Australia Manuscript received January 21, 1974 ABSTRACT
Disruptive selection for sternoplcural bristle number with opportunity for random mating was done in the four treatment combinations of two population sizes (40 pairs and 8 pairs of selected parents) and two selection intensities (1 in 40 and 1 in 2). In each generation, matings among selected parents were obsserved in a mating chamber, and progeny collected separately from each female parent. In the high number, high selection intensity treatment, divergence between the high and low parts ceased about generation 11. The isolation index increased rapidly to generatioa 3, but then fluctuated to termination ob the population at generation 17. The overall isolation index was significant, indicating a real tendency to assortative mating. The failure of the isolation index to increase after generatiomn 3 was attributed to lower average mating fitness osf high males (due to inbreeding) and reduced receptivity of law females (due to a homoeygoas lethal gene with a large effect on sternoleural bristle number in heterozygotes). In the two lolw number treiatments, isolation indices fluctuated from generation to generation with no obvious trends, and none of the overall isolation indices were significantly different fro” zero. The high number, low selection intensity treatment shoswed very little divergence, and one of the replicates showed, in contrast with expectation and the high number, high selection intensity treatment a significant tendency to disassortative mating. Intense disruptive selection may lead to assortative mating. HODAY (1967) stated as one of a number of generalizations concerning disTruptive selection, that it “can split a population into two parts between which there is considerable reproductive isolation”. This generalization remains controversial because the majority of laboratory experiments on disruptive selection have not lead to reproductive isolation between the two parts of the population. A total of 22 separate experiments utilizing different foundation stocks (all but one D. melanogaster) were reviewed by THODAY and GIBSON(1970). Only two of these (replicate experiments from the same foundation population) showed strong evidence of isolation. Four more experiments have since been reported. GRANT and METTLER (1969) obtained little divergence and no evidence of isolation in selecting f o r induced vertical I-maze activity. COYNEand GRANT(1972) selected for the same character from the same foundation population, but used a modified experimental design. High and low females were placed in separate culture vials after mating, and possible migration between the two parts was Genetics 78: 715-735 October, 1974.
716
J .S. F. BARKER A N D L. J. E. KARLSSON
reduced (but not precluded as stated by COYNEand GRANT)by selecting high and low respectively from the two subpopulations. Significant isolation was found in one of two replicates. ROBERTSON (1970) and BEARDMORE and AL BALDAWI (reported by THODAY 1972) both repeated the experiment of THODAY and GIBSON (1962), using the same character and the same selection method. ROBERTSON obtained no isolation, while the brief report of the experiment of BEARDMORE and AL BALDAWI indicated the development of reproductive isolation by generation 16. The probability of disruptive selection leading to isolation therefore would appear to be quite low, but the controversy centers on the nature of the foundation populations, on aspects of the experimental design to which the results may be sensitive, and on the frequency with which disruptive selection in wild populations may lead to isolation (THODAY and GIBSON1970, 1971; SCHARLOO 1971). For isolation to develop, there must be some relevant genetic variance present in the foundation population. Clearly the development of isolation would be more likely if there were a genetic correlation between the primary character under selection and isolation tendency (i.e., assortative mating). The possibility of such a correlation has been investigated by BARKERand CUMMINS(1969b), with negative results, and by GRANTand METTLER(1969), with positive results. Alternatively, isolation could develop through selection against hybridizationreducing the frequency or success of hybrid matings, o r reducing the fitness (viability or fertility) of the progeny of hybrid matings. The possibility of developing isolation in this way is likely to be enhanced (given that relevant genetic variability exists) by utilizing large populations (ROBERTSON 1970) , and by intense selection for the primary character. The experiment reported here was done to investigate the effects of population size and selection intensity on responses to disruptive selection. We have chosen to use the same foundation population and to select for the same primary character as previously (BARKERand CUMMINS1969a)-that is, the Canberra cage population, in which we found no evidence for a genetic correlation between this primary character (sternopleural bristle number) and isolation tendency. MATERIALS A N D METHODS
Experimental design In comparison with previous studies of disruptive selection, two refinements of technique were used in this experiment. In previous studies, the selected high and selected low flies of both sexes were all placed in a mating bottle for periods of up to two days. The males then were discarded, and die groups of high and of low females put into separate bDttles. Thus information on the relative frequency of the four possible mating types and the progeny productivity of each, together with estimates of the degree of isolation between the high and low parts olf the line, hnd to be obtained in separate supplementary experiments. In our experiment, the selected high and low parents in each line in each generation were put in a mating chamber (with flies identified by wing marking-see below), and all matings were observed and recorded for up to two hours. As all females had not necessarily mated by the end of the observation period, the flies then were transferred to a bottle for a further time, such that the tostal mating period in every generation in each line was 5-6 hours. At the end of the mating period, niales were discarded. Females were placed in individual vials, and discarded four days later. Thus we have data on the relative frequency of the fosur mating types, and the degree of isolation for every generatioa.
DISRUPTIVE SELECTION IN DROSOPHILA
717
The practical implications of these pro'cedures determined to some extent the experimental design. Preliminary tests showed that the maximum number of flies that could be ob'served in a mating chamber was 40 pairs. Available data on progeny productivity per female, together with the recognition that this woiuld decreas2 as selection proceeded, indicated that an average of 40 pairs of progeny per female parent should have been readily obtained over the 15 or SO generations that the experiment was expxted to continue. The high selection intensity was set, therefore, at 1 in 40. A low population size of 8 pairs od parents and a low selection intensity of 50% also were used, so that the four treatments were: 1) High number, high selection intensity (Hr,Hs)-40 pairs selected each generation from 1600 pairs scored (1 replicate only). 2) High number, low selection intensity (HnLs)--iN) pairs selected each generation from 80 pairs scored (2 replicates). 3) Low number, high selection intensity (LnHs)-8 pairs selected each generation from 320 pairs scored (2 replicates). 4,) Low number, low selection intensity (LnLs)-8 pairs selected each generation fro,m 16 pairs scored ( 4 replicates). In all lines, the disruptive selection was symmetrical, so that one half of the selected parents osf each sex were those with the highest scores, and one half those with the lowest scores. This selection was from ail flies scored, i.e., regardless of the culture from which the flies came. When one or more ob several flies with the same bristle number had to be selected, random numbers were used t o ensure against bias toward choosing flies from particular cultures. Initiation of the lines: A sample of 50 pairs of flies was taken from the cage population by egg sampling in 50 vials put into the cage for four hours, and collecting one male and one female virgin progeny from each vial. Fifty single pair matings were set up, using males and females from different egg sampling vials. Six pairs of virgin prcgeny were collected from each vial, and 30 bottles set up, each with 10 pairs of parents, again using males and females collected from different vials. Progeny were collected as virgins fromm these bottles. The first 2COO pairs which emerged were allocated to HnHs. The next 2000 pairs to emerge were allocated to the other three treatments, and kept in storage vials at 20" for an additional week, so that HnHs and the other three treatments were scored in alternate weeks throughout the experiment. Following scodng and selection in HnHs, HnLs and LnLs, either all the flies selected as high parents or all those selected as low parents had the tips of both wings lightly clipped to allow identification during the mating observations. This clipping was alternated between the two sets of selected parents in subsequent generations. In LnHs, each of the eight selected flies of each sex was uniquely identified by using a system of punching small holes in the wings in different positions. Detailed pedigree information therefore was kept for this line. After selection and wing marking, the selected parents were stored for 18-24 hours before the mating observations commenced. Mating observations: Modified ELENSand WATTIAUX (1964) mating chambers were made of perspex with sloping walls to facilitate obmservation. For the small population size lines, chambers were cut in 2.0-cm-thick clear perspsx, with an internal diameter of 1.5 cm at the bottom, and 8.0 cm at the top. The base (3.0 mm white perspex) and the tap (3.0 mm clear perspex) were screwed on to allow easy dismantling for cleaning between obesrvation runs. For the large population size lines similar chambers were made with intsrnal diameters of 9.0 cm at the bottom and 16.5 cm at the top, thus giving approximately the same surface area per fly during mating observations in all treatments. During observation, the chambers were illuminated by a circular fluorescent tube. This surrounded a biconcave lens (12.7 cm diameter, 4.0 dioptre, 2'5.4 cm focal length) which facilitated the recordings of matings, particularly for the large population size lines. Flies were transferred to tlie mating chamber without etherization, with females introduced about 10 minutes before the males. At the end of the observation period, flies were transferred directly to a bottle for the additional mating period, again without etherization. In the HnHs treatment, mating cbservations in every generation commenced at 4:080 p.m. to avoid po'ssible
718
J .S. F. BARKER A N D L. J. E . KARLSSON
effects of diurnal variation in mating behavior (BARKER1962a). For the eight lines of the other three treatments which ran concurrently in the alternate week to HnHs, this possibility had to be balanced against possible age effects. T h e d o r e in each generation, observations of these eight lines were done on the one day in random order, with the first starting at 8 : 0 a.m. Each treatment was to be obsa-ved for one hour or uniil such time that any co'pulation begun within this period had finished. However, for HnHs, the period of observation was increased from generation 5 to two hours. Virgin collection and fitness of the lines: 111 the low selection intensity lines, where only two male and two female progeny were required from each parental female, all progeny were collected from one 12-hour emergence period on the ninth day after mating. Therefore in these treatments, selected parents in every generation were the same age, obviating any effects of age on mating behavior (BARKER1962b, 1967). For the high selection intensity lines, this was not possible, but in an attempt to minimize effects of age on mating behavior, the first two-thirds ob the collected virgins were stored at 20", and the later collections at 25". Vials containing developing progeny were checked on the moming of the eighth day after mating. Very rarely had progeny emerged at this time, but this check insured that all emergences twelve hours later could be collected as virgins. Virgin collection continued every 12 hours. To generation 3, sufficient total progeny were obtained in five collections. The number of collections required steadily increased, and by generation 8, it was necessary to collect until the morning of the thirteenth day after mating. The possible range of ages of selected parents at mating was then 3oL138 hours. In addition, selection could be confounded by parental age effects on the sternopleural bristle number of their progeny (PARSONS1962) and by later-emerging flies having smaller body size and hence smaller bristle numbers (PARSONS1961). By generation (G.) 13 of HnHs, developmental time had increased such that the first virgin collection was 24 hours later than at G.O., so that virgin collection was continued until the morning of the fourteenth day after mating, and this treatment was then put on a 15-day cycle. Scorin,g and selection: For the high selection intensity lines where dams had progeny in excess of the required number, the first emerging 40 male and 40 female prosgeny were scored. For dams with less than 40 progeny of either or both sexes, a dam of the same mating type with excess progeny was randomly allocateJ to make up the numbers. The bristle sco'res of the two lots of progeny were kept separately, so that the mean bristle number of the progeny from each dam could be calculated. Where a dam was infertile, she was allocated to a mating type on the basis of the observed mating frequencies. Then a dam from the cho'sen mating type was randomly used to provide as many progeny as possible, then if necessary ano'ther of the same mating type, and so on. All flies were reared on a dead yeast fortified medium (Medium F of CLARINGBOLD and BARKER1961). The experiments (including mating observations) were done at 25" -C 0.5" and a relative humidity of 65-70% in a room lit for 12 hours per day (6:OO a.m. to 6:OO p.m.). RESULTS
Selection responses: Changes in mean bristle number in the high and low parts (i.e., progeny of high and low female parents) of HnHs are shown in Figure 1, and of H n L s and LnHs in Figure 2. Selection in L n L s was essentially ineffective, as might be expected, and detailed results are not given. A measure of the magnitude of selcction applied is given by the difference in average bristle number between the high and low selected parents in each generation, while the difference between the progeny in the high and low parts gives a measure of the effectiveness of this selection. These differences are given in Figure 3 for H n H s and in Figure 4 for H n L s and LnHs. The divergence in G.l f o r H n H s was 1.88 bristles, which was 25% more than predicted using the observed phenotypic standard deviation of G.0, and assuming a heritability of 0.36 (BARKERand CUMMINS
719
DISRUPTIVE SELECTION I N DROSOPHILA
L 0
2
4
6
8
10
12
14
16
GENERATIONS
FIGURE 1.-Response to disruptive selection in HnHs. Generatioa means are averages of male and female bristle numbers.
1969a) and random mating. The observed divergences in the two replicates of L n H s of 1.51 and 1.54 bristles were, respectively, 6% and 4% more than predicted, while those in H n L s of 0.14 and 0.20 bristles were, respectively, 81% and 71% less than predicted. In all lines, response in the first generation was asymmetric, with greater response for high bristle number. Near-linear responses with this asymmetry continued to G.4 in HnHs, but in L n H s there was essentially no further increase in bristle number in the high parts of the lines. Selection responses in the high part of H n H s became erratic from G.4, and in the low part from G.8 (Figure 1). The average difference between the high and low parts (Figure 3) showed no effective increase from G.ll. This difference, averaged over 9.1 1-G.17 (the measure of total divergence achieved), was 10.62 bristles. In LnHs, the same high selection intensity was remarkably ineffective, and both replicates failed to diverge further from G.3 (Figure 4). The differences between the high and low parts averaged over G.3-G.10 were 2.03 bristles in Replicate 1 and 1.90 in Replicate 2. The low selection intensity in H n L s also was ineffective in increasing divergence after G.2 (Figure 4); the average differences (G.2-G.9) being 0.67 bristles in Replicate 1 and 0.74 in Replicate 2. In the original experiment of THODAY and GIBSON(1962) where significant reproductive isolation developed between the high and low parts of the line, the distributions of bristle number in the two parts became so different that by generation 12 they did not overlap. The percentage overlap of the two distribu-
720
J .S. F. BARKER A N D L. J . E. KARLSSON
19
17
0
z
y
21
I-
-
v)
a m
19
17
0
2
4
6
8
10
GENERATIONS FIGURE 2.-Response to disruptive selection in (a) HnLs and (b) LnHs. Generation means are averages of male and female bristle numbers. Rerlicate 1, - - - - Replicate 2. 0High part, 0 Low part.
tions (percentage of their total area that the distributions had in common) gives a convenient measure of their separation, and is given for each treatment in Figure 5. For treatments other than HnHs, these results emphasize the lack of separation of the high and low parts. For HnHs, the percentage overlap decreased rapidly to G.5, but increased markedly to G.6. This was followed by a slow decrease to G.17, but this final degree of overlap was still greater than that at G.5. Factors affectirig selection responses and divergence: Because mating observations were made in each generation, and progeny collected from individual females, it is possible to interpret various aspects of these selection responses. The patterns of response will be affected particularly by migratiofi between the two parts of a line, by variaticn in frequency of the four possible mating types and by differences among the mating types in fertility and progeny productivity. For the two H n treatments in each generation, selected flies were noted as progeny of either high or low female parents. Those that originated from that
DISRUPTIVE S E L E C T I O N IN DROSOPHILA
721
GENERATIONS
FIGURE 3.-Difference in mean bristle number between the high and low parts of HnHs. Differences between the means of the high and low selected parents are plotted against the generation from which they were selected.
part of the line for which they were selected as parents were classified as “native” (i.e., in the high part of a line, the progeny of high females), while those from the other part of a line were classified as “immigrant”. For LnHs, where all selected parents were individually identified, migration rates were expressed as the numbers of parents of selected flies in each part that had themselves been selected into the other part. Considering each treatment separately:
i) HnHs: I n each of G.l and G.2, three immigrants were selected among low parents and one among high parents. By G.2, the divergence between the high and low parts was sufficient to allow specification of the mating type for each female parent from the mean bristle number of her progeny. In addition to the above immigrants in G.2, one selected individual in the high part was the progeny of a high female X low male ( H L ) ,while similarly one in the low part was the progeny of a low female x high male ( L H ). But from G.3, all selected parents in both parts were the progeny of homogametic matings only. Therefore from G.3 there was no gene exchange between the two parts of the population, and each part was essentially a directional selection line for bristle number with imposed selection against heterogametic matings. I n G.5, the response psttern changed dramatically, with a decrease of 2.4 bristles in the high part (Figure 1). Of 8 observed H H matings of G.4 parents,
722
J .S. F. BARKER A N D L. J. E. KARLSSON
c
15
I
c
/--
a
m 5 PROGENY
I
1
0
2
6
4
8
10
GENERATIONS FIGURE 4.-Difference in mean bristle number between the high and low parts of (a) HnLs and (b) LnHs. Differences for parents plotted as in Figure 3 and replicate d e as in Figure 2.
only one produced progeny (Table 1). But of 12 observed H L matings, 9 produced progeny, with these latter progeny largely determining the mean bristle number of the high part in G.5. All selected parents in the high part in G.5 came from the one H H mating, so that the full-sib matings among them would have restricted the genetic variability available f o r further response. Separate data for each mating type are given in Figure 6, which shows mean bristle number, phenotypic variance and coefficient of variation for male progeny in each generation. Data for females were similar except for a smaller variance in LL. After the restriction at G.5, response in H H continued at a slower rate to G.9, but from G.9 to G.17, no further response occurred. The large fluctuations in mean bristle number of progeny of high females in later generations (Figure 1) also were largely due to reduced numbers of fertile H H matings in some generations (Table I ) , probably a result of accumulated inbreeding. As the numbers of selected offspring from each mating were known, the effective population size of each part in each generation was estimated, and used to estimate the average coefficient of inbreeding (Table 2 ) . The effective population size ( N e in generation t ) was estimated as: 2Nt - 2 k - 1 (&/E)
+
DISRUPTIVE SELECTION IN DROSOPHILA
80
8
O
1
1
HnHs
'
'
~
'
1
'
1
1
'
'
'
'
1
1
LnHs
'
1
723
HnLs
'
LnLs
where N t = number of individuals in generation t, and V k = mean and variance of number of gametes contributed per parent in generation (t-1) (CROWand KIMURA1970, p. 351), while the average coefficient of inbreeding in generation t ( f t ) was estimated as: ft-1 (1 - 2ft-1 f t - 2 ) / 2Ne (CROWand KIMURA1970, p. 102). The coefficient of inbreeding increased far more rapidly in the high part than in the low part. Even in G.0 of the high part, the effective population size was restricted, as one mating contributed 21 of the 40 selected individuals in G. 1. Then, apart from the restriction to one pair at G.4, in G.7 only 4 H H matings contributed selected progeny (with 18 selected from one mating) ; in G.12, only 2 H H matings contributed (27 from one mating) ; in G.14, only 4 H H matings contributed (25 from one mating) ; and in G.15, only one H H mating contributed all 40 selected high parents. Although response in the low part was linear to G.8 (Figure l ) , the mean bristle number of the progeny of LL matings showed a marked decrease from G.5 to G.6, and the variance in all three mating types including low parents dramatically increased in the same generation (Figure 6 ) . One of the low males selected in G.4 had only six sternopleural bristles, the previo'us lowest score being 12 bristles. Progeny scoring in G.5 showed that this male had mated with a low female-the progeny of this female segregating into two distinct phenotypic
+
+
724
J .S. F. BARKER A N D L. J. E. KARLSSON
TABLE 1 Number of observed mutings of each type (Obs.), number of fertile dams (Fert.), and number of these dams whose progeny were selected as parents in the next generation (Select.) for each generation of HnHs Mating type ( ? X
d)
NL
IIH
LL
LH
~
Generation
0 1 2 3 4
5 6 7 8 9 10 11 12 13 14 15 16 17 Average
Obs. Fert. Select.
11 9 9 13 8 9 12 9 12 11 14 13 12 -
8 8 14 17 11.1
8 7 11 1 9 11 4 8 7 8 4
2 10 4 2 12
-
6.8
8 : IO 1 8 9 4 7 5 8 3 2 8 4 1 6 5.7
Obs. Fert. Select.
8 11 10 6 12 9 7 8 7 7 8 2 7 11 13 6 9 8.3
10 8 6 9 6 7 7 6 7 8 4 10 5 9 9 5 7.3
4
Obs. Fert.
14 8 4 6 7 6 7 6 10 8 5 8 8 5 5 5 9 7.0
-
9 7 6 3 9 9 5 8 7 4 5 4 5 3 1 4 -
5.6
Select.
2
O h . Fen.
Select.
5
-
-
10 8 13 13 8 9 9 7 10 11 16 8
11 12 14 I4 11 11 14 8 12 13 15 9 13 13 12 14 -
11 12 7 4 10 9 10 7 10 I2 13 9 9 10 10 13
-
13 10 13 10
10.2 12.3
-
9.8
groups for bristle number, with approximately equal numbers in each. One group resembled their extreme low sire; the other had average bristle numbers similar to those for the progeny of other LL matings at this generation. All selected parents in G.5 (except for three females-one each from three different dams) were the progeny of this extreme low sire. Therefore, there was a substantial increase in the average coefficient of inbreeding in the low part in this generation (Table 2) ,but the number of fertile LL matings did not decrease in later generations (Table l ). The fluctuations in mean bristle number in the low part in later generations (Figure 1) were due to the contribution of the progeny of LH matings -mean bristle number being lowest in those generations where the number of festile LH matings in the previous generation was reduced (Table 1). The fact that it was variation among mating types in number of fertile matings each generation, and in average number of progeny per mating that led to the fluctuations in mean bristle number in later generations in both parts of the line, is demonstrated by Figure 6. This shows that the changes in mean bristle number of progeny in each of the four mating types were relatively regular. The genetic basis of the extreme low bristle number first observed in a single male in G.4 has not been fully analyzed, but it is almost certainly due to heterozygosity for a single gene located in the right arm of the third chromosome. The
DISRUPTIVE SELECTION I N DROSOPHILA
i
725
HnHs : Males
FIGURE 6.-Mean bristle number, phenotypic variance and coefficient of variation for the male progeny of each of the four mating types HnHs. A HH, A HL, LH, H LL.
gene is homozygous lethal, and exhibits a variety of pleiottxophic effects-viz., high proportion of missing macrochaetae on head and thorax, microchaetae (sternopleural, coxal and abdominal) reduced in number and in size, reduction in number of branches on the arista, terminal interruption of the fourth and fifth longitudinal wing veins, and a light straw body color. Because of the continued segregation of this gene, the phenotypic variance of the LL mating type (Figure 6) increased rapidly to G.9, more slowly to G.13, and then remained stable at about 35.0 in males and 23.5 in females. ii) HnLs: Records of fertility 0.f individual females were not kept, but very few
726
J . S . F. BARKER A N D L. J. E. KARLSSON
TABLE 2
Effectiue population size (Ne) and average inbreeding coefficient (f) in each generation of the high and low parts of HnHs LOW
High Generation
0 1
6.66 13.47 9.68 15.40 1.89 14.86 11.86 6.46 10.38 7.73 11.34 5.55 3.50 10.01 4.41 1.89 12.26
2 3 4
5 6 7 8 9 10 11 12 13 14 15 16 17
0
20.72 27.47 22.63 13.47 2.26 12.56 11.49 15.56 15.08 17.79 19.89 20.41 19.34 11.87 20.72 20.19 21.91
0
.075 .lo7 ,151 ,177 .388 .w2 ,426 .469 ,492 ,524 ,546 ,583 .637 ,652 ,690 .762 .769
.024 ,091 .m12 ,096 .288 ,309 .338 .359 ,379 .396 .411 .425 .439 .a2 ,475 .487 .499
failed to produce the required two pairs of progeny throughout the 9 generations. Chance and inbreeding were probably unimportant in this treatment, but migration between the two parts remained high in all generations, with no decreasing trend. The proportions of immigrants were very similar in the two. replicates, TABLE 3
Numbers of parents of selecied flies in each of the high and low parts in each generation that had themselues been selected into the other part-for each replicate of LnHs Replicate 1 Generation
High
Low
1 2
5
1
3 4 5 6 7 8 9 10
1 4 7 1
Average Average %
Replicate 2
-
High
Lav
5
6 2
8
1
2 3 1 3 3 4 2
8
8 3 2 2
1.9 11.9
1.9 11.9
2.1 13.1
3.3 20.6
5 5
DISRUPTIVE SELECTION IN DROSOPHILA
72 7
and averaged 0.35 into the low part, and 0.39 into the high part in each generation. Even though the migration rate into the high part was slightly greater, there was a steady increase in mean bristle number in the total population of both replicates (Figure 2), with total gains over 9 generations of 1.64 bristles in Replicate 1, and 0.89 bristles in Replicate 2. iii) LnHs: Even though selection intensity was high, migration between the two parts continued through the 10 generations. As the eight selected flies in each part were individually identified, migration rate was measured as the number of parents (out of 16) in each generation that had themselves been selected into the other part (Table 3 ) , The average migration rate was higher in Replicate 2, and within this replicate, higher into the low than into the high part. Although this migration would be expected to be antagonistic to selection, selection was most effective in the low part of Replicate 2 (Figure 2). Also, migration between the two parts cannot explain the complete lack of selection response in the high parts of both replicates after G.l, as there was no migration into the high part of Replicate 2 until G.4, and there was no response even in the progeny of H H matings. The number of fertile matings per mating type covered the range from zero to four, thus contributing (as in HnHs) to the fluctuations in bristle number from generation to generation. But on average, there were no differences among mating types in fertility or average progeny numbers, and no tendency for these to deteriorate in any mating type over the 10 generations. I n this treatment, with only four pairs of selected parents in each part in each generation, inbreeding might be expected to accumulate rapidly. As the flies were fully pedigreed, inbreeding coefficients were calculated for all selected individuals (Table 4). In Replicate 1, the inbreeding coefficient was higher in the low part to G.6, then higher in the high part, while in Replicate 2, the reverse occurred. The rate of inbreeding was much higher in the first few generations in Replicate 2, but the average coefficient of inbreeding in G.9 and G.10 was higher in Replicate 1. Neither replicate showed the steady increase in inbreeding observed in HnHs, a1though average inbreeding coefficients in G.10 were higher than at G.10 in HnHs. The fluctuations in inbreeding generally are closely related to migration-the average coefficient of inbreeding decreasing in the progeny when a proportion of their parents came from the other part. Mating observations and sexual isolation: The isolation index of MALOGOLOWKIN-COHEN, SIMMONS and LEVENE (1965) was calculated for each generation of each line (Figure 7). For HnHs, the index was based on the maximum number of matings of each type, determined from mating observations or progeny testing (Table 1 ) . Because of low fertility in some generations, particularly of HH matings, the indices also were calculated using the numbers of fertile matings only. However, the pattern of change in the index over generations remained much the same. Mating observations were not made in G.13, and this generation was not included in analysis of the isolation index. After transfer to the mating chamber in G.13, the flies were very sluggish. Very few matings were initiated and after 10 minutes two males collapsed. All flies were immediately removed
728
J .S. F. BARKER A N D L. J. E. KARLSSON
TABLE 4 Mean and standard deviation of the coefficients of inbreeding of selected parents in each generation of the two replicates of LnHs
3 4
5 6 7 8 9 10 Replicate 2 1 2 3
4 5 6 7 8 9 10
Low
Hieh
Generation
Replicate 1 1 2
0
0 0 .008 f .E22 ,137 t .I21 .I31 f ,049 .lo1 f ,010 .276 t .071 ,446 f 0 .558 f 0 .630 f .029
.031 -C .I56 f .273 f .345 t .351 t .273 t .260 f .420 t .485f
.088 .058 .131 .I50 ,216 .052
.ow ,112
.M
0
0 .250 t 0 .375 k 0 .289 t ,145 .356 f .a33 ,433 t .030 ,295 f .I354 .357 f .026 ,396 f 0 .384 t ,036
G ENERAT IONS FIGURE 7.-Isolation index in each generation for each line. E Replicate 1, A Replicate 2, 0 Replicate 3, A Replicate 4.
,250
0
.094f .I74 .219 t .264 t ,261 f .243 k .W5 -t.485 t ,563 t
.058 ,127 .059 ,037 .059 ,073 .I31
729
DISRUPTIVE SELECTION I N DROSOPHILA
TABLE 5 The isolation indices and mule and feinale mating ratio chi-squares for each generation of HnHs ~~
~~~
~~
Chi-square values (each with 1 degree of freedom) Isolation indcx i standard error
Generation
-0.16 k 0.16 0.00 I 0.16 0.11 k 0.16 0.38 I0.15 0.07 k 0.16 0.05 k 0.16 0.18 k 0.16 0.24 I 0.16 0.08 I0.16 0.21 k 0.16 0.35 k 0.15 0.41 1 0 . 1 4 0.08 I0.16
0 1 2 3 44
5 6 7 8 9
10 11 12 13 14 15 16 17
-
0.14 0.05 0.44 0.20
Overall Pooled
iP
