Speed in Drosophila Pseudoobscura'

THE GENETICS OF DROSOPHILA MATING BEHAVIOR. 11. THE GENETIC ARCHITECTURE OF MATING SPEED IN DROSOPHILA PSEUDOOBSCURA'

SEYMOUR KESSLER Department of Psychiatry, Stanford University School of Medicine, Stanford, California 94305 Received January 17, 1969

HERITABLE variation affecting mating behavior has been demonstrated to be present in several species of Drosophila. Two components of mating behavior, the mating speed and the duration of copulation, have received the greatest attention and have yielded heritability estimates in the various populations studied, ranging from 0.30 to 0.61 (MANNING1961; PARSONS1964; FULKER 1966) Ior the former and from 0.15 to 0.20 for the latter (MACBEANand PARSONS 1966). Mating speed has been shown to be an important component of fitness in Dro- sophila. SPIES and LANGER(1964) found that the carriers of the most common chromosomal arrangement in the 1959 collection of D. pseudoobscum at Mather, California, tend to show relatively higher mating frequencies than the carriers of arrangements that were less commonly found. In D. mlanogaster (FULKER 1966) and in D. robusta (PRAKASH1967) it has been shown that fast mating males not only display shorter mating latencies for a given mating, but mate more frequently and produce a larger number of offspring than slower mating males. The nature of the genotypic variance affecting the mating speed in D. melanogaster has been studied, utilizing a diallel design, by PARSONS(1964) and by FULKER(1966). The genetic architecture of mating speed in D. metanogaster suggests a past evolutionary history of strong directional selection for rapid mating (FULKER1966). In the present study, the genetic architecture of mating speed in D. pseudoobscura will be examined utilizing strains produced through selection for fast and for slow mating speeds. An analysis of the behavioral con- sequences of the selection has been reported elsewhere (KESSLER 1968).

MATERIALS AND METHODS The starting population was obtained from the intercross of three wild-type strains of D. pseudoobscura from widely separated localities; Chichicastenango (Guatemala), Mara (British Columbia), and Mather (California). Virgin flies were collected from the stock cultures, females and males separated, placed into half-pint culture bottles and aged for 7 days at 19°C. Fifty females were confined for one hour with 50 males in an empty glass tube, measuring 15 cm in length and 2.5 cm in diameter. The tube was stoppered with a cotton plug and secured in a clamp attached to a ring stand. The observation period was considered as having begun as soon as the tube was secured. All copulating pairs were removed by means of an aspirator, the first 10 pairs

Dedicated to my teacher, Professor Th. Dobzhansky on the mcasion of his seventleth birthday

Genetics 62: 421433 June 1969 422 SEYMOUR KESSLER being saved and used as parents to begin the Fast (FA) mating line. The subsequent mating pairs were discarded. When the observation period ended, the nonmaters were etherized, counted, and 10 pairs chosen at random were placed together to begin the Slow (SA) mating line. In each generation the first 10 pairs of flies to mate in the FA line were used as parents for the subsequent FA generation and 10 pairs chosen at random from the nonmaters in the SA line were chosen as parents for the subsequent SA generation. Ten pairs of unselected flies from the foundation population were placed together to begin the Control (C) line and in each generation 10 pairs, chosen randomly from the C line, served as parents for the subsequent C generation. All culture bottles were subcultured on alternate days and maintained in incubators at 25°C. The observations were conducted between the hours of I:OOP.M. and 5:00~.~.(PST), using artificial overhead light and at room temperature. An observation on one selected line was always made concurrently with one on the other selected line, so that both were exposed to more or less common environmental conditions. F, hybrids between the FA and SA lines were obtained by the intercross of flies in the 15th generation of selection. The F, and the subsequently obtained F, and backcross progenies were tested to determine their speed of mating as described by KESSLER(1968). Briefly, 45 females and an equal number of males between 7 and 12 days of age were placed together in an empty glass tube for 30 min. All copulating pairs were removed by means of an aspirator and were discarded. At the end of the observation period, the nonmaters were etherized and counted. F, progeny was obtained a second time from the intercross of FA and SA flies in the 17th generation of selection and the behavioral tests were repeated. In all, the following number of observations of each type were made: In the 17th generation, the first 10 pairs to mate in the SA mating observation were saved and used to start the Slow-reversed line (Srev). In the FA observation, the last 3 pairs to mate were added to the 7 nonmating pairs to begin the Fast-reversed (Frev) line. In each subsequent generation, 50 pairs in each line, aged for at least seven days, were observed for one hour as described above. The first 10 pairs to mate in the Srev line were used as parents for the subsequent Srev generation and 10 pairs of nonmaters or a combination of the nonmaters and the last few mating pairs to make a total of 10 pairs were used as parents for the Frev generation. Ten pairs of flies from the 17th generation progeny in each of the selected lines were used to begin a Fast-relaxed line (Frel) and ‘il Slow-relaxed line (Srel). In each generation, a sample of 50 pairs in each line was observed for one hour. Single pair matings were observed on the progenies of the 18th generation and again on the progenies of the 20th generation. One virgin female and one male were placed in a plastic tube measuring 4.9 cm x 0.9 cm by means of an aspirator and observed for one hour or until a copulation resulted. In all, 40 observations of each type were made of the parental, F, and backcross generations, 25 of the F, and 35 control observations.

RESULTS Response to selection: The mating frequency in terms of percent mated in each generation is shown for all lines in Table 1. Regression of the deviation in mating frequency from that of the control level (percent mated in selected line-percent

Number of Females hIales 15th generation 17th generation uairs of flies 12 4 733 12 4 735 6 5 960 6 5 12 12 1,074 12 15 1,245 12 15 1,189 MATING SPEED IN DROSOPHILA 423

d 424 SEYMOUR KESSLER mated in control line) on generations yielded regression coefficients of +0.017 * 0.003 and -0.024 0.004 for the FA and SA lines, respectively, over the first 20 generations of selection. Both regression coefficients are significant at the 1% level indicating that significant progress in both directions of selection has been achieved. The mean proportion of flies used as parents in each generation was 21.6 * 0.8% for the FA line and 35.5 f 3.1% for the SA line over the first 20 generations of selection. Thus, despite the relatively weaker intensity of selection applied in the SA line, relatively better progress has been achieved in that direction than in the FA line. To assess the changes occurring in mating speed during the course of the obzervation period the number of matings in each 5-min interval was transformed as in the method of SPIESS,LANGER and SPIES ( 1966) and KESSLER(1968) and the mean mating index for each observation was determined. Values of 20, 10, 7, 5, 4, and 3 were assigned in that order to the six 5-min intervals of the 0 to 30-min period, a weight of 2 to the 30 through 60-min period and a weight of 1 to the > 60 (nonmating period). The products of the number of matings in each of the time intervals with its appropriate weight were summed and then divided by the number of pairs of flies involved to obtain a mean mating index for each observation. The mean mating indices in each generation are shown in Table 1. The logs of the ratios of the mean mating indices of the selected and control index selected line lines in each generation (log ) were calculated for the FA and index control line SA lines and are shown in Figure 1. The response to selection in both the FA and SA lines was rapid and was marked by considerable intergeneration fluctua- tions, which, presumably, are largely due to environmental effects. In Figure 2, the deviations of the mating indices of the FA and SA lines are plotted on a log scale. Maximum separation of the two lines appears to be achieved between 3 and 5 generations of selection. Beyond the 5th generation, little or no further separation of the two lines is evident. In terms of the deviations in mating frequency from the control level (Table 1) , regression coefficients of +0.0637 i: 0.0234 and -0.0674 * 0.0298 were found for the FA and SA lines, respectively, over the first five generations of selection. In terms of the mating indices (Fig- ure l), regression coefficients of $0.0710 f 0.0294 and -0.0869 * 0.0491 were found for the FA and SA lines, respectively, over the same period. Despite the differential selection pressure, response to the selection over the initial generations was symmetrical in the two lines. Realized heritability: By plotting the mean mating indices for each selected line against their respective cumulative selection differentials, estimates of the realized heritability were obtained (FALCONER1955; 1960). These values are, after 5 generations of selection, 0.1927 * 0.0878 and 0.0617 f 0.2513 for the FA and SA lines, respectively. The mean realized heritability is 12.7%. It is noteworthy that relative to their respective means, the variances in the SA line are larger than those in the FA line. This is not unexpected considering the selection technique; the SA parents may be selected for phenotypic characteristics less directly related to mating speed than the FA parents. The relatively higher heritability in the FA MATING SPEED IN DROSOPHILA 425

FIGURE1.-Response to selection for fast and for slow mating speed in Drosophila pseudoobscura.

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Gene rati on s FIGURE2.-Divergence between the FA and SA lines (mating index FA-mating index SA) over the course of selection for mating speed. 426 SEYMOUR KESSLER line indicates that the phenotype as observed in the FA parents is a more reliable guide to the offsprings’ performance than that in the SA line. Reversed and relaxed selection. In the Frev line, there is a rapid return to the control level both in terms of the mating frequency (Table 1) and of the mating index (Figure 1) . In the Frel line, however, a shift towards the control level is not evident in terms of a reduction in the mating frequency, but a modest decre- ment in the mating index from that in the FA line appears to have been obtained. In both the Srev and the Srel lines, an initial shift towards the control level was followed by a rapid return to that of the SA line. Little progress was made subsequently in achieving control levels of mating frequency, although, in comparison to the SA line, the Srev and Srel lines both show a net increase in mating frequency. In terms of the mating index, there is no obvious evidence of a return to the control level in either the Srev or Srel lines. It thus appears that, to some extent, fixation of genes affecting the mating speed, as reflected in the mating index, may have occurred in the SA line although the possibility remains that a return to the control level may have been achieved if relaxation was continued further. In the FA line, gains achieved during the course of the selection tend to be lost when the selection pressure is eased or reversed, suggesting that selection for heterozygote gene combinations may have occurred. Genetic analysis: The cumulative mating frequencies for the P,, P,, F,, F, and backcross generations are shown in Figure 3. No evidence of maternal effects was found in the F, generation, the reciprocal crosses between P, and P, producing hybrids with mating frequencies not significantly different from one another. The F, data were therefore pooled. The position of the F, population relative to the parental generations clearly indicates dominant effects of the genes determining fast mating speed. The expected positions of the segregating populations are shown in Figure 3. Assuming that gene effects are additive, the expected positions of the F,, BC, and BC, populations are calculated as % (E,+ E,) , 1/2 (PI+ F,) and % (p2+ F,) , respectively. The F, and BC, populations show mating frequencies reasonably close to their respective expected values. The BC, population, however, shows a lower frequency of mating than would be expected. In the upper portion of Table 2, the generation means are shown in terms of the percent mating, the mean mating index and the logarithmic transformation of the mean mating index. The untransformed means were tested using MATHER’S (1949) scaling criteria and were found to be inadequate in order to proceed with the genetic analysis. MATHER(1949) gives formulae by which the means of the various generations may be tested in order to determine whether the assumptions of the additivity of gene effects and the absence of environment-genotype interactions hoid. Several transformations were tried and none were found to be entirely adequate. The logarithmic transformation of the mean mating index was found to satisfy the requirements of MATHER’SB and C scales and the test S2 max/S2 min (OWEN 1962) showed that the variances of the nonsegregating populations were homogeneous (P I0.01). This transformation was, therefore, employed for the genetic analysis. MATING SPEED IN DROSOPHILA 427

70 -

60 -

50 -

-0 t3 t - Z40 8

30 -

0 5 IO 15 20 25 30 Predicted Minutes FIGURE3.-Cumulative frequency of mating in percent during 30-minute massed mating observations.

The relative positions of the various generations with respect to each other are maintained despite the scale transfonnation and the dominant effect of the genes carried by the PI population is again indicated. Comparison of the two backcross variances on the logarithmic scale shows that the variance of the BC,, the backcross to the recessive parent, is significantly larger than that of the BC, [ (F,,,,, = 2.47) 0.01 5 P I0.051, thus providing further evidence for the relative dominance of P, over P,. The lower portion of Table 2 summarizes the calculated values for the second order components of additivity (V,) dominance (V,) and environment (V,). The dominance ratio (2VD/VA)xis greater than unity, indicating that there may be a preponderance of genes in the PI with overdominant effects. Be- haviorally, however, no manifestation of the overdominance is evident in the Fl generation. The heritability, h2 (VA/VA + VD+ V,) , is of the order of 25% and the number of genes n ( $& (PI- p,),/2VA) is 170. Single pair matings: A parallel analysis was carried out on the single pair data. Nonmating flies were assigned a mating latency of 61 min following the procedure 428 SEYMOUR KESSLER

TABLE 2 Frequency of mating in percent, mating indices and estimates of the genetic components of variance in massed mating observations

Number of Percent Mean Logof mean Generation observations mated mating index matmg index Pl 16 77.4 t 1.7 11.87 & 0.52 1.07 f 0.02 p2 16 19.7 t 2.0 2.23 & 0.53 0.33 f 0.03 Fl 22 76.0 f 1.7 11.11 2 0.52 1.04 i 0.02 F2 24 51.8 f 3.6 7.04 t 0.56 0.81 t 0.04 BC, 27 59.4 t 3.2 8.72 t 0.61 0.92 f 0.03 BC, 27 43.9 t 3.5 5.60 & 0.54 0.69 2 0.05 MATHER'S Scales A -34.5 & 6.8 -5.54 * 1.42 -0.27 & 0.06 B - 7.9 & 7.4 -2.14 t 1.31 +0.02 t 0.10 C -41.7 t 15.1 -8.16 t 2.57 -0.23 f 0.17 V, (additive component of variance) +0.0004 V, (dominance component of variance) +0.0006 V, (environmental component of variance) +0.0006 hz (heritability) 0.25 n (number of genes) 170.0 suggested by PARSONS(1964). The mean mating latencies for each generation are shown in Table 3. The observed means for the segregating populations accord well with the expected means. The mean of the F, population is intermediate to that of the parental populations although it is shifted closer to that of the PI suggesting partial dominance of genes in the PI line. It is of interest to note (Table 3) that with the exception of the P, population, the estimated mean mating times obtained in the massed matings are consistently shorter than the corresponding means for the single pair matings. Because of the method used to account for truncation, the estimated mean mating times for the single pair observations are overestimated. The variances in both the F, and F, populations are increased relative to those of the parental generations. Although the BC, and BC, variances are not significantly different, the latter is slightly larger than the former. In the second order genetical components, the estimate of VDis negative and is therefore meaningless. MATHER(1949) ascribes negative estimates of VD to sampling error. Since the variance of the F, population is derived empirically, it can be taken as an estimate of VA + VE if we assign VD a value of zero (Vr, = VA + VD + VE). VA then be- comes f2.48, hz 17.0% and n 37.0.

DISCUSSION That genetic factors influence mating speed is abundantly clear; considerable MATING SPEED IN DROSOPHILA 429

TABLE 3

Mating latencies and estimates of the genetic components of variance in single pair mating observations

Estimated mean mating Number of Mean latency Matmg latency speed in massed Generation observahons observed (mm) expected (nun) mahng (mm) Pl 40 15.8 !c 3.1 8.4 p* 40 42.9 f 3.0 44.0 Fl 25 21.6 !c 4.2 29.4 9.0 F2 40 30.6 2 3.8 29.1 14.0 BCl 40 25.1 f 3.5 18.7 11.0 BC, 40 33.1 f 3.6 32.3 17.0 Control 35 28.0 t 4.2 MATHER’S Scales A +12.8 f 8.8 B + 1.6 !c 8.9 C $20.6 & 17.9 v* 3.75 VD -1.27 VE 12.07 evidence has been derived from the study of single gene effects, through strain comparisons and through selective breeding (see MANNING1967 for review). By means of selection, MANNING(1961) produced fast and slow mating strains of D. metanogaster. The response to selection was rapid and bidirectional, the sexual behavior of both sexes apparently being affected. Other comparisons between MANNING’Sstudy and of the one reported here are discussed by KESSLER(1968). Selection for mating speed practised in one sex at a time in D.melanogaster proved to be less successful, although response toward slow mating was obtained in the male lines (MANNING1963). In D.simulans, MANNING(19m68) obtained response to selection for slow mating speed, but modification of behavior was shown to occur in the females and not in the males. Transimplantation of the corpora allata from the slow mating females to control ones was found to induce precocious sexual receptivity in the host animals whereas transimplantations in the reverse direction appeared to have no effect. These results suggest that selection for slow mating in D.simulans may have altered target organ responsivity to the endocrine (s) produced by the corpora allata. Mating speed in Drosophila is a complex behavioral character which does not easily lend itself to a genetic analysis. This is so for several reasons. Measure- ment of the character is based on the integrative performance of two individuals; it is, in other words, an interaction characteristic rather than one associated with an individual. The genetic analysis of quantitative characters, as elucidated by MATHER(1 949) and by FALCONER( 1960) and as applied to behavioral characters by BROADHURST(1960), PARSONS(1 967), ROBERTS( 1968) and others presupposes that one is measuring an individual characteristic. Other difficulties arise because of the assumptions generally made concerning the equality of intrapair contribu- 430 SEYMOUR KESSLER tions to the total phenotypic variance and the absence of interpair effects. Con- sidering the gross observable differences between the behavior of the two sexes during Drosophila courtship, it is doubtful that these assumptions are correct. Indeed, it has been shown that females of the strains of D.pseudoobscura studied here contribute almost six times more to the total variance of mating speed than males (KESSLER1968). Furthermore, PETIT( 1958) and EHRMAN(1966) have found that mating success is frequency dependent, rarer male genotype carriers tending to show a proportionally greater amount of mating. In their experiments, the kinds of genotype carriers are held constant, but interpair (i.e., social) rela- tionships are altered by changing the relative proportions of the different types. In the present experiments, it was found that the mean mating latencies were consistently shorter in the mass matings than in the single pair observations (Table 3), suggesting that some form of social facilitation (THORPE1963) affecting mating speed occurs in aggregates of Drosophila flies. Presumably, in nature the presence of a social facilitating mechanism would be a significantly adaptive feature of Drosophila mating behavior since mating is believed to occur on or around feeding sites, where groups of flies gather (SPIETH1968). SPIESS(1968) however, found little effect of population density on mating frequency in D. pseudoobscura except for AR/PP karyotype carriers. Despite the departures from the assumptions made concerning mating speed in the genetic analysis, the present experiments do shed some light on the genetic architecture of mating speed in D.pseudoobscura. Response to selection for mating speed has been obtained. Maximum separation of the FA and SA lines was achieved by five generations of selection and remained relatively stable over the duration of the experiment. Crosses between the FA and SA lines produced hybrids with mating speeds indicating dominance o€ the genes producing fast mating speed. Reversal or relaxation of selection showed tendencies to return to control levels in the FA line whereas in the SA line, mating latencies tended to remain unchanged. The data suggest that genes with nonadditive genetic effects are present in the FA line, whereas in the SA line, the gene effects are relatively more additive in nature. The decrease of the mating speed in the backcross to the dominant parent appears in both the massed and single pair matings and pre- cludes a simple Mendelian explanation. The gene combinations produced through selection in the parental populations appear to maintain coadapted properties in the F, but as recombination and gene segregation are permitted to occur, they display a progressive lack of coadaptation in the BC,, F,, and BC2 generations which results in progressively slower mating speeds. Three sources of information were used to analyze the genetic architecture of mating speed; the massed mating observations, the single pair observations and the response to selection. In the massed matings, the potence ratio suggests that genes with overdominant effects are associated with rapid mating speed. The behavioral data (Figure 2), however do not appear to support this conclusion, suggesting that the estimates of h2 and of n may be unreliable when calculated from these data. In the single pair observations, the effects of interpair interactions are removed. MATING SPEED IN DROSOPHILA 43 1 The estimates of h2and n are considerably smaller than that found in the analysis of the massed matings. The rapidity of the response to selection and the failure to obtain further separation of the FA and SA lines beyond the 5th generation suggests that relatively few genes affecting the mating speed are involved. FULKER(1966) has estimated that in males of D. metanogaster at least 5 blocks of genes are present that determine the speed of mating. The estimates of h2derived from the response to selection over the initial generations are difficult to interpret because of the magnitude of the variances. Nevertheless, the central tendencies of these estimates, ranging from between 6 and 19%, are smaller in magnitude than those found by other investigators. For characters associated with fitness, lower rather than higher h2 estimates would be expected (ROBERTSON1955; MATHER1966). The genetic architecture of mating speed in D.pseudoobscura, as revealed by KESSLER(1968) and by the present experiments appears to be complex. Selection for mating speed in D. pseudoobscura has been shown to have differential effects in the two sexes; the females appear to respond to selection for slow mating whereas the males appear to respond to selection for rapid mating (KESSLER1968). When compared to the two selected lines, the mating speed shown by the control population was more similar to that of the FA line. When each sex in the control line was compared separately, with respect to mating speed, to their counterparts in the selected lines, it was found that the C and FA females were not significantly different from one another whereas the C males resembled those oi the SA line. Taken together with the present study these experiments suggest that the relatively fast speed of mating in unselected populations of D. pseudoobscura is maintained as a consequence of genes with dominant effects in the females. Thus, at least for the females, the genetic architecture of mating speed appears to reflect an evolutionary history (BREESEand MATHER1960; MATHER1953) of directional selection for fast mating. In D. melanogaster FULKER(1966) fwnd that rapid mating speed was maintained by genes with dominant effects in males. FULKER, however, studied only male mating speed. The differences between his findings and those reported here may be due to species differences or to conditions related to the particular strains of flies or differential methods utilized. With respect to the unselected males of D. pseudoobscura, the data of KESSLER (1968) suggest that some mechanism or mechanisms are operative to oppose the maximal expression of mating speed. Two mechanisms, or a combination of both, may be operative; counterselection in the males for genes producing slow mating speed or sex-limited expression of the genes, which in the females, would produce rapid mating. In both cases, intermediate rather than maximal speeds of mating would be exhibited, suggesting that mating speed in the unselected males may have been subjected to stabilizing or normalizing natural selection. The integrative effects of the differential genetic architectures for mating speed in the two sexes would be the maintenance of behavioral flexibility and genetic heterogeneity and the production of less than maximum, but nonetheless, relatively rapid speeds of mating. Whether these findings represent a broad generalization with respect 432 SEYMOUR KESSLER to D.pseudoobscura or are limited to the experimental material used here, would, of course, require further study.

The author wishes to express his thanks to DR. DAVIDD. PERKINSfor providing the necessary space and equipment needed to initiate the present study, to DR. HELENKRAEMER for statistical advice, to PROFESSORL. L. CAVALLI-SFORZAfor his interest and helpful suggestions, and to MISS SALLYTHOMASSON for her typing skill. This research was supported by NIMH grants MH 8304. and MH 14364.

SUMMARY Selection for fast and for slow mating speed in D.pseudoobscura was carried out for 24 generations. Maximum separation of the two lines occurred within five generations of selection and remained relatively stable thereafter. Estimates of the components of the genotypic variance were obtained from massed and from single pair matings. The mean realized heritability over the first five generations of selection was approximately 12.7%. Optimal speeds of mating appear to be maintained by the presence of differential genetic architectures in the two sexes.

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