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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 be- havior, 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. melano- gaster 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 popu- lation 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 subse- quent 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 back- cross 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 direc- tion 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.
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