Bottleneck Effects on Genetic Variance for Courtship Repertoire

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1 Copyright by the Genetics Society of America Bottleneck Effects on Genetic Variance for Courtship Repertoire Lisa M. Meffert Department of Biology, University of Houston, Houston, Texas Manuscript received February 21, 1994 Accepted for publication September 2, 1994 ABSTRACT Bottleneck effects on evolutionary potential in mating behavior were addressed through assays of additive genetic variances and resulting phenotypic responses to drift in the courtship repertoires of six two-pair founder-flush lines and two control populations of the housefly. A simulation addressed the complication that an estimate of the genetic variance for a courtship trait (e.g., male performance vigor or the female requirement for copulation) must involve assays against the background behavior of the mating partners. The additive environmental effect of the mating partner s phenotype simply dilutes the net parent-offspring covariance for a trait. However, if there is an interaction with this environmental component, negative parent-offspring covariances can result under conditions of high incompatibility between the population s distributions for male performance and female choice requirements, despite high levels of genetic variance. All six bottlenecked lines exhibited significant differentiation from the controls in at least one measure of the parent-offspring covariance for male performance or female choice (estimated by 50 parent-son and 50 parent-daughter covariances for 10 courtship traits per line) which translated to significant phenotypic drift. However, the average effect across traits or across lines did not yield a significant net increase in genetic variance due to bottlenecks. Concerted phenotypic differentiation due to the founder-flush event provided indirect evidence of directional dominance in a subset of traits. Furthermore, indirect evidence of genotypeenvironment interactions (potentially producing genotypegenotype effects) was found in the negative parent-offspring covariances predicted by the male-female interaction simulation and by the association of the magnitude of phenotypic drift with the absolute value of the parent-offspring covariance. Hence, nonadditive genetic effects on mating behavior may be important in structuring genetic variance for courtship, although most of the increases in genetic variance would be expected to reflect inbreeding depression with relatively rare situations representing the facilitation of speciation by bottlenecks. URELY additive genetic models predict the loss of been reported among experimentally inbred lines of a P additive genetic variance with inbreeding (WRIGHT number of invertebrate species (EHW 1969; HAY 1977; FALCONER 1989), but theoretical consideration of 1976; POWELL 1978; RINGO et al. 1986; MEFFERT and nonadditive genetic structuring (dominance and epista- BRYANT 1991; WEINBERG etal. 1992; GALIANA et al. 1993), sis) indicates that population bottlenecks can increase but founder-flush effects on the genetic structure for additive genetic variance (ROBERSTON 1952; GOODNIGHT courtship repertoire have not been investigated. Be- 1988; GIMMELFARB 1989; TACHIDA and COCKERHAM 1989; WILLIS and ORR 1993). Hence, nonadditive genetic effects have been inferred in inbreeding studies on morphometric or fitnesstraitswhenincreasesinadditive genetic variance or patterns of interpopulational differentiation refuted neutral expectation (LINTS and BOUR- GOIS 1982; WADE and MCCAULEY 1984; BRYANT et al. 1986a,b; LOPEZ-FANJUL and VILLAVERDE 1989; CARSON 1990; WADE 1991; BRYANT and MEFFERT 1993). Thus, the genetic revolutions proposed for founder-flush populations in speciation theory MA^ 1954; TEM- PLETON 1980; GIDDINGS et al. 1989; CARSON 1990) may be characterized by increased additive genetic variance for reproductive isolating mechanisms affected by nonadditive genetic processes (see MAYR 1988). Divergent courtship and premating isolation have Address for correspondence: Department of Biology, University of Houston, 4800 Calhoun, Houston, TX Genetics (January, 1995) cause of the obvious association of mating behavior with total fitness, additive genetic variance for courtship is expected to be minimized in equilibrium populations and potentially structured by nonadditive genetic ef- fects (FISHER 1958; ROSE and CHARLESWORTH 1980; TEMPLETON 1980; LYNCH and SULZBACH 1984; SPENCER et al. 1986; CARSON 1990). Inbreeding depression effects, asymmetrical selectional responses, and diallel analyses have indicated directional dominance for mating propensity, courtship performance and mate recognition in Drosophila (.g., PARSONS 1967; MANNING 1968; AVERHOFF and RICHARDSON 1974; KAWANISHI and WATANABE 1981; SHARP 1984; RINGO et al. 1986; HOIK- KALA and LUMME 1987; WELBERGEN and VAN DIJKEN 1992). However, estimates of epistasis are comparatively more difficult (GEIGER 1988; BARTON and TURELLI 1989) and have not been conducted for components of mating behavior. In fact, estimation of even the additive component

2 366 L. M. Meffert of genetic variance for a courtship trait (e.g., the vigor of performance by the male) is problematic. In general, behavior traits typically have high phenotypic variances due to environmental noise contributions to the expression within individuals (e.g., male mating success is related to the gross effects of age on general ambulatory activity, MEFFERT and BRYANT 1992). Assays of courtship behavior are particularly complicated because any trait must be evaluated against the background behavior of the mating partner (here, the female s requirement for the intensity of the male s pursuit), and the potential exists for the net courtship performance to be affected by interactions between the two sexes (sensu BASTOCK 1967; BATESON 1983). Genotype-environment interactions have been theorized to maintain genetic variance by hindering selectional response, particularly through negative parent-offspring covariances, and to contribute to interdemic differentiation (VIA and LANDE 198 7; GILLESPIE and TURELLI 1989; WADE 1990). GRIFFING (1982) proposed that negative selectional responses can occur for one or more life-history traits, like viability or fecundity, when the interactions among individuals within a group create genotype-environment effects. Thus, if the expression of a courtship trait is dependent upon the environmental background of the mating partner s phenotype, genotype-environment processes may strongly affect the genetic structure for mating behavior. In previous studies on bottleneck effects on quantitative traits in the housefly, nonadditive effects were inferred when morphological traits in founder-flush lines exhibited increases in additive genetic variance and significant disruption of genetic integration (BRYANT et al. 1986a; BRYANT and MEFFERT 1988, 1993). Rebounds from inbreeding depression, phenotypic divergence in courtship repertoire and patterns of assortative mating suggested that similar genetic perturbations occurred for fitness traits (BRYANT et al. 1990; MEFFERT and BRY- ANT 1991,1992). This current study extends these findings as a more direct assessment of bottleneck effects on the genetic variance for fitness traits which have the potential to affect the process of speciation in populations that can escape local extinction from inbreeding depression (DOBZHANSKY 1951; PATERSON 1980; MAYR 1988; OTTE and ENDLER 1989). Specifically, the efficacy of bottlenecks to increase evolutionary potential in courtship is evaluated through assays of additive genetic variances (through parentdaughter and parent-son covariances) and consequent phenotypic responses to drift (and/or natural selection) in an independent of set bottlenecked and control lines, A genotypeenvironment model applied to courtship is proposed as a possible explanation for negative parent-offspring covariances (implicating a form of epistasis), and tests for concerted phenotypic divergence in the bottlenecked lines are used as indirect evidence for directional dominance. This study thus provides indirect theoretical and empirical support of nonadditive genetic structure of courtship repertoire that can result in increased evolutionary potential in bottlenecked populations, although only relatively rare cases would be expected to reflect the kind of catalysis of speciation promoted by founder-flush theory (see RICE and HOSTERT 1994). MATERIALS AND METHODS A single sample of houseflies (-100 females) was taken from a local field population to establish an initially outbred control (ancestor) in the laboratory. After three generations, six separate bottlenecked lines were derived from the progeny of two (randomly selected) male-female pairs and allowed to flush to the standard husbandry size of at least 1000 pairs. At the same time, the control was separated into two replicates (large populations of roughly 1000 pairs that had not experienced bottlenecks) to account for the effects of sampling, genetic drift, and inadvertent laboratory selection through the course of the experiment (25 generations) that were independent from the founder-flush events. After three generations of population flush for the bottlenecked lines, an average of 100 families (200 courtships) of each of the six bottlenecked lines and two controls were videotaped for evaluation of the covariance of mating repertoire between 50 sons and their parents and another 50 daughters and their parents (FALCONER 1989). Collection of the videotapings of 1610 successful courtships (those achieving copulation) required six generations (roughly 350 h of the concurrent use of five camcorders). The parents for the genetic variance estimates were drawn from a large sampling of eggs from the stock population cages with the following controls for potential environmental and experimental effects on behavior: (1) body size: larval rearing at egg density at 80 per 18 g CSMA larval medium and temperature of 27, (2) virginity: sexing under light COB anesthesia within 24 hr of eclosion and housed separately by sex, (3) age: videotaping at the age of 7 days post-eclosion, (4) circadean cycles: rearing in a ight:dark cycle with all video- tapings made between 9AM and 2PM and (5) mating compatibility: evaluating only those courtships that resulted in copulation within 40 min. Although there were significant differences among lines in the percentage mating under this latter constraint, differences in the genetic variance estimates were not simply a matter of precision (see FALCONER 1989) because the sample sizes were balanced across lines and because the genetic variance estimates were not related to the percentage of parents copulating within 40 min. The exact values for the sample sizes and mating percentages are given in the APPENDIX. Offspring were tested under the conditions above against a randomly selected mate from the same generational pool. The probability of mating with a sibling was generally much less than 10% as the pool of potential mating partners for any videotaping day was created with roughly equal contributions from each of the families that were emerging ( t5 individuals per family) with at least two families contributing to the pool (usually >10 families). To assess the relationship between the magnitude of the genetic variance estimate and the degree of subsequent phenotypic response to drift and/or natural selectional pres sure (inadvertent selection in the laboratory environment), all eight lines were maintained at the standard size of at least

3 Genetics Quantitative of Courtship pairs for 10 generations after completion of the genetic variance estimates. Using the controls on environmental and experimental effects that are noted above, phenotypic shifts in the 10 traits were assayed for each line by the absolute value of the difference between the phenotypic means for the 100 parents used in the genetic variance estimates (spanning 5 generations) and the means for an average of 50 malefemale pairs recorded 10 generations later (also spanning 5 generations). Concurrent videotaping of thebottlenecked and control lines in every generation ensured homogeneity across lines for unidentified effects. RESULTS Quantitative genetic models or mating behavior: A number of factors complicate the estimations of the additive components of mating repertoire. Most behavior traits have strong environmental or experimental noise contributions to the variation within an individual s performance (e.g., the effect of age on ambulatory activity and consequent mating success). Such environmental noise components will be exaggerated in genetic variance estimates through the calculation of parent-offspring cross-products, so this study uses relatively rigorous controls for at least some factors, as described above. Nevertheless, assays of courtship behavior must still involve either artificial control on the phenotypes of the mating partners or some consideration of the random influences of the opposite sex on the net courtship intensity. Random effects would be more likely in nature; thus, measuring a parent-offspring covariance for male performance vigor should involve testing fathers and their sons against randomly selected female partners. For example, the relationship between fathers and sons that secure copulations after relatively brief periods of courtship activity (ie., fast mating speed) must be evaluated against some random sampling of females that vary in the minimum (or mean, see LANDE 1981; ARNOLD 1985) amount of general courtship activity required for the acceptance for copulation. Similarly, a parent-offspring estimate of the genetic variance for the female requirement for the minimum overall performance necessary for copulation (or mean value) requires detecting a relationship between mothers and daughters that require long precopulatory periods as they are exposed to a pool of males that vary in their genetic aptitudes to meet such requirements. Parent-offspring covariances were simulated among an array of allele frequencies for male vigor and female choice for additive and interactive models. Following similar theoretical treatments of quantitative genetics and mating behavior (see O DONALD 1980; ARNOLD 1985; FALCONER 1989), a single representative locus with two alleles that behave additively was modeled for each of the male and female traits. Monte Carlo draws were performed for the genotypes of both parents and the offspring s mate, each based upon the allele frequencies in the population. The genotype of the off- spring was then based upon the known genotype of the parent of the same sex and a random sampling for an allele from the parent of the opposite sex (which is not expressing the behavior due to sexual dimorphism). For an additive model of courtship, the net intensity of performance was derived by I=M+F whereby 6 M, and Fdenote the net intensity, male vigor and female choice phenotypes, respectively. For simplicity, the environmental variance is assumed to be neglible and random across sexes, so the phenotypic values for male vigor and female choice are equal to their respective genotypes. The resulting parent-offspring covariance is analogous to a midparental-offspring covariance. No genetic variance for male vigor would be detectable if the males simply performed to the minimum levels required by the females (ie., males are absolutely plastic) or otherwise failed to copulate due to their genetic inadequacies. Thus, some level of additivity (with or without the interaction term below) is necessary to account for the presence of genetic variance in potentially both the male performance vigor and the corresponding female choice. For an interactive model, the net courtship intensity was derived by I=M+F+ [M-FJ whereby [ ] indicates the absolute value. In this way, the component of species-specific mate discrimination involves the exaggeration of the average courtship performance by the degree to which the distribution of males in the population fail to match the pool of female requirements, such that gross incompatibility could reduce the likelihood of copulation. Each simulated genetic variance (100 for each pairing of allele frequencies across male and female traits) was based upon 50 families (the sample size for the empirical estimates), and each variance estimate was tested by 100 bootstrap samplings for detection of a significant difference from zero for either a positive or a negative parent-offspring covariance. In both simulations (Figure 1), an estimate of genetic variance is diluted by the environmental noise of the phenotypic variance of the mating partners, and the presence of genetic variance is detected more easily when the additive genetic variance increases as the population shifts toward intermediate allele frequencies (as expected). The saddle shape in the additive simulation (Figure la) involves increased environmental noise contributed by the mating partners as their phenotypic (genetic) variance increases. Negative parent-offspring covariances were not found in the additive simulation but occurred in the interaction model when there was inflated additive genetic variance (with intermediate allele frequencies) along with a high likelihood of re-

4 368 L. M. Meffert FIGL.REI.-Monte Carlo simulations o f genetic variance estimates for the additive (a) and interactive (b) models. P is the allele frequency o f one of the two alleles for the trait to be expressed (~.g.,male performance vigor), andr is the allele frequency of one of thc two alleles for the companion trait in mating partners (P.R., female requirement for copulation). Surfaces show the percentage o f variance estimates with a significant difference from zero in bootstrap testing for either a positive (+) or negative (-) value. productive incompatibility between random male and female pairings. Active assortative mating does not occur in the simulation because all randomly generated male-female pairs mate, yet effective assortative mating occurs when the probabilityof compatibility between a male's vigor andthe female's requirement is high. Thus, negative parent-offspringcross-products occur when the offspringfromabbreviatedcourtships between highly compatible parentsface a high probability of mating with an incompatible partner that protracts courtship. Innumerable interactive models can be conceived, yet the model presented here allows positive genetic variances in both sexes or a positive variance in one sex with a contrasting negative value in the companion trait, as suggested by the empirical data below. Courtship in the housefly: Housefly courtship is depicted in Figure 2. The male performs an abortive form ofwalking (CREEP) in stalking the female (Figure 2a), coming to the pointof interdigitating his legs with hers (CLOSE) andmakingtappingmovements (TAP) to FIGURE5.-Diagrams of housefly courtship repertoire. (a) Pre-mounting behavior. (b) Post-mounting behavior. (c) C o p dation. See text for a description of housefly courtship. which thefemale may respond with similar fencing (TOUCH). Aborted mountingbehavior prior to copulation also occurs (MOUNT).Immediately prior toc o p ulation (Figure 2b), the male mounts the female and flutters hiswings (BUZZ) while lunging forward toward the head of the female (LUNGE), enabling him to lift (LIFT) her forelegs with his. HOLD is interruption of BUZZ by the male holding his wings in a position that is roughly 180" from their resting orientations. When mounted, the female thrusts herwings 90" to her body and perpendicular to the substrate(wing OUT). Mating speed (SPEED) is the amountof time elapsed from the pointof the introductionof the flies to the chamber to the initiation of copulation (log, transformedfor normality). For assays of courtship repertoire, the last 2 min of precopulatory activity (reviewed in real time) and the entire postmounting bout prior to copulation(-2 sec reviewed at SO frames/sec slow motion) were analyzed for each mating. The data were collected by the Observer event recording software for the PC (NOLDUS 1990) and analyzed by SAS matrix language (SAS INSTITUTE, IN<: ) on the mainframe at theuniversity of Houston. The total duration of the execution of each of the postmounting behaviors (BUZZ, LUNGE, LIFT, HOLD andwing OUT) was standardized by the durationof the observationperiod and multiplied by 10 to bring the values to a commensurate scale with the premounting behaviors. For conformation to normality, the premounting behaviors required log,. transformationsforthedurations of theperformanceand

5 Genetics Quantitative of Courtship 369 observation periods, followed by reciprocal transformation for MOUNT, CREEP, TAP and TOUCH but arcsine transformation for CLOSE (SOKAL and ROHLF 1969). Although standardization by the observation period may potentially obscure results, significant genetic variance estimates were not simply attributable to the magnitude of the parent-offspring covariance for the observation period. Tests for significance were based upon bootstrap samplings with the 95% (or 99.2% in protected tests; see below) confidence bounds determined from a distribution of 1000 genetic variance estimates from the parentoffspring data from the two control populations ( EFRON and GONG 1983). In pooling blocks within lines and in pooling the two controls, block and strain effects were removed (by centering around the block means and pooling only deviations) to obviate the artifactual inflation of genetic variance estimates. Such potential block effects were also removed in the bootstrap simulations. Empirical estimates of additive genetic variance for courtship: A priori tests were performed on each genetic variance estimate from each bottlenecked line against the respective value for the pooled controls. At the 95% levelof confidence, allsixof the founderflush lines exhibited significant differentiation from the pooled controls for parent-offspring covariances involving mating speed or courtship repertoire, withonly WING OUT failing to discriminate the founder-flush lines from the controls (Figure 3). The average Pearson correlation coefficients (72) among the courtship traits for the phenotypic means, parent-son covariances and parent-daughter covariances were 0.07, 0.12 and 0.12, respectively (for analyses pooled across all lines; the average Spearman correlation across the parent offspring-covariances was 0.13). Thus, the potential for spurious significant effects due to intercorrelation is low; nevertheless, a measure to account for testing replicate bottlenecked lines (see LYNCH 1988) is a significance level adjusted for the number of replicate strains, yielding a type I error rate of (= 0.05/6). In such protected tests, all six bottlenecked lines still showed significant divergence from the controls in a least one parent-offspring covariance for mating speed or courtship movement (involving BUZZ, CLOSE, CREEP, HOLD, LUNGE, MOUNT and TOUCH). Elevated genetic variances in the bottlenecked lines were found for the intensity of male courtship performance (.g., increased parent-son covariances for BUZZ and HOLD in line B3) and for the minimum (or mean) performance required by the female (e.g., increased parent-daughter covariances for BUZZ and HOLD in line B2). The founder-flush populations also exhibited inflated genetic variance for the ability of the male to elicit fencing by the female (e.g., increased parent-son covariances for the female TOUCH behavior in line B5). Three bottlenecked lines had significantly in- creased additive genetic variance (P < 0.008) for the minimum level of overall mating activity required by females for copulation (Figure 3; see parent-daughter covariances for SPEED in lines B1, B3 and B5). Differentiation between the controls for the levels of genetic variance for the female performance vigor for TOUCH (see parent-daughter covariances for TOUCH, Figure 3) suggests that the genetic variance for female vigor in this movement is subject to drift or selectional processes in the laboratory environment that can produce differentiation among lines apart from bottleneck effects. The concerted phenotypic divergence of the bottlenecked lines from the pooled controls provides indirect evidence of directional dominance (and inbreeding depression) in 7 of the ll characters. The heading for each panel in Figure 3 provides the significance level for the ANOVA performed to detect a treatment effect of the bottleneck protocol us. the control on the phenotypes of the parents. Nevertheless, even though HOLD, LIFT and WING OUT showed significant trends for directed bottleneck effects on phenotypic means, the analyses on each trait involved at least one case of a bottlenecked line with significant (P < 0.05) divergence in the opposite trajectory. Negative parent-offspring covariances are theoretically feasible in genotype-environment models (VIA and LANDE 1987; GILLESPIE and TURELLI 1989) and in the male-female interaction model of courtship (Figure lb), yet empirical negative genetic variance estimates are generally interpreted as artifactual. However, four of the six bottlenecked lines exhibited significant (P < 0.008) negative parent-offspring covariances (see SPEED, BUZZ, CLOSE, LUNGE and MOUNT in Figure 3), suggesting that they might be biologically real. There is a propensity for parent-daughter covariances to be positive such that all of the bottlenecked lines showed inflated values for mating speed (three are significant at P < 0.008) and the values for the courtship traits have a significant departure from a random distribution around zero (68% are positive: x' = 9.8, P < in analyses pooled across lines). In contrast, a significant (P < 0.008) parent-son covariance is noted for SPEED, and the values for the courtship traits are evenly distributed around zero (x2 = 0.20, not significant). Thus, the positive parent-daughter covariances provide evidence of additive genetic variance for female requirements for male movements and for the female vigor to perform TOUCH and WING OUT. Conversely, the variation in parent-son covariances suggests that either the mean genetic variance across the male movements (and the males' manipulation of the TOUCH and WING OUT behavior in the females) is zero or that some process is decrementing the parent-son covariances to the point of creating negativevalues, despite any latent levels of genetic variance. The

6 mf, L. M. Meffert SPEED: P = F1 l i!=$j BUU; P - = * * * * 0.5 o m duohlrr # ClC C1 C ClC ClC HOLD;P = ~ P LUNGE; = ## ** 0.2- I## i '... i i : # ' -1.0 ClC C1 C ClC MOUNT: P = TAP; P = TOUCH; P = ;-$"q P OUT; WING = ClC C1 C ClC C1 C FIGURE 3,"Parent-offspring covariances for mating speed and the 10 courtship elements in the control (C1 and C2) and the bottlenecked (Bl-B6) lines. and M, parent-son and parentdaughter estimates,respectively.asterisks (*) and pound (#) symbols denote significant deviations from the pooled controls for the 95 and 99.2% confidence levels, respectively. The head of each panel bears the significance value for the ANOVA for the detection of a concerted bottleneck effect on the phenotypic means of the parents, such that a significant value would implicate directional dominance. The values for the controls are segregated from those of the bottlenecked lines in each panel simulation the on male-female interaction in courtship (Figure lb) presents one such possible mechanism. In order to test the extent to which the parent-offspring assays reflect levels of additive genetic variance, phenotypic shifts in the 10 courtship traits were assayed in the bottlenecked lines after 10 generations of drift and/or "natural" selection (inadvertent selection in the laboratory environment). Figure 4 shows that the degree of genetic perturbation for a courtship trait predicts the magnitude of phenotypic drift, whereby each bottlenecked line exhibited significant shifts in at least 2 of the 10 of the courtship phenotypes. The degree of phenotypic divergence is explained by a simple linear relationship with the level of the parent-daughter covariance (Figure 4a), yet a second-order relationship is necessary to account for increased evolutionary potential relative to the magnitude of a negative parent-son covariance (Figure 4b). Thus, even though many of the deviations from the controls for either positive OT negative parent-offspring covariances were not significant (Figure 3), they still reflected enhanced evolutionary potential for significant shifts in courtship repertoire. These patterns cannot be attributed to some peculiarity of a subset of traits (e.g., the method for transformation to normality) because 8 of the 10 courtship phenotypes contributed to the significant effects. Moreover, the parabolic function (Figure 4b) supports a prediction of the interaction model that a schism in male performance and female requirement (creating negative parent-offspring covariances) should drive the reestablishment of mating compatibilities if the population is genetically competent for a selectional response (see BOAKE 1986; KANESHIRO 1989). The difference between the relationships of drift with the parent-daughter and parent-son covariances is probably due to more negative values being found in the latter. DISCUSSION The neutral expectation of a decrement in additive genetic variance as a function of a bottleneck is refuted

7 Quantitative Genetics of Courtship 371 w31 o.ol P = PARENT-DAUGHTERCOVARIANCE PARENT-SONCOVARIANCE FIGURE 4.-Relationship of the genetic variance estimate with the magnitude of phenotypic drift. The figures depict the absolute values of the differences between the phenotypic means for the two testing periods (separated by 10 generations) relative to parentdaughter covariances (a) and (b) parent-son covariances. The analyses are pooled over the 60 values (10 traits assayed in each of the 6 bottlenecked lines) that conform to normality. Significance values are given for the linear and secondorder polynomial regressions. The dashed horizontal line indicates the upper confidence bound for a significant (P < 0.05) phenotypic shift after 10 generations of drift. when founder-flush lines exhibit higher levels than those of the ancestral control, and the simplest explanation is that bottlenecks serve to increase the frequencies of deleterious alleles that were at low levels in the ancestral population (ROBERSTON 1952; CHARLESWORTH et al. 1982; LYNCH 1991, WILLIS and ORR 1993, see Figure la). In such a scenario, bottlenecked populations would be plagued by inbreeding depression that would compromise the importance of differentiated courtship. Indeed, the tests for concerted phenotypic divergence due to bottlenecks implicated directional dominance in 7 of the 11 mating characters. Thus, major a factor in the founder-flush increases in additive genetic variance appears to be the increased frequencies ofrecessive alleles that are likely to be deleterious. Nevertheless, significant divergence from the controls in levels of genetic variance was not strictly related to traits that were apparently structured by dominance. Specifically, the two characters with the highest number of significant (P < 0.008) bottleneck effects on parent-offspring covariances (SPEED and BUZZ, Figure 3) did not exhibit dominance via directed phenotypic differentiation. Furthermore, the trait with the strongest evidence of dominance (WING OUT) produced no detectable bottleneck effects on genetic variance. Therefore, some other mechanism(s), besides dominance, might also be affecting this system. I propose that genotype-environment processes in courtship might be involved in the decrementing genetic variance estimates to the point of producing negative parent-offspring covariances, masking inflated evolutionary potential for phenotypic response to drift or selection. Mechanistically, male-female interactions in Dipteran courtship are implicated when faster matings are characterized by simpler repertoire and when courtship becomes more elaborate with serial abortive mat- ing attempts (e.g., BASTOCK 1967; AVERHOFF and RICH- ARDSON 1974; MEFFERT and BRYANT 1991; GROMKO and MARKOW 1993). Specifically, males appear to compensate for high female thresholds when they would have succeeded with simpler courtships to more receptive females. Similarly, the acceptance thresholds of females may increase when females become agitated by males that fail to satisfy their courtship requirements rapidly. Negative parent-offspring covariances and selectional responses have been predicted when parental and offspring populations are assessed in different environments or when life-history traits (like viability or fecundity) are affected by intraspecific interactions (GRIFFING 1982; VIA and LANDE 1987; GILLESPIE and TURELLI 1989). In the simulation of this study, negative parentoffspring covariances in mating behavior characterize a population's schism between the distribution for the male performance phenotype and the pool of female choice requirements (see BOAKE 1986; KANESHIRO 1989). Such a phenomenon could explain dissassortative mating found in the previous housefly study (MEF- FERT and BRYANT 1991) and related experiments on Drosophila (AHEXRN 1980; RINGO et al. 1986; GALIANA et al. 1993). Sexual dimorphism creates a genotypeenvironment interaction between male performance vigor and the female requirement, yet because the interacting loci are subsets of the same genome, such genotype-environment interactions on the individual level could operate epistatically on a population level. This component of genotypegenotype interaction is strongest when the population suffers a disjunction in mating compatibilities between males and females; therefore, such negative parent-offspring covariances could reflect a form of epistatic inbreeding depression. Genotypeenvironment interactions and epistasic-like processes in mating behavior could thus define evolutionary con-

8 372 L. M. Meffert straints for speciation through the camouflage of additive genetic variance within populations and the contribution to interdemic differentiation of reproductive isolating mechanisms (see MAYR 1988). Behavioral plasticity in both sexes is symmetrical in the interaction model presented, but an asymmetry for more rigidity in the female relative to the male could explain the bias for negative parent-son covariances found in this system. Strict asymmetry (ie., plasticity in only one sex) might be applicable for quantitative genetic models of female choice for male morphology (e.g., LANDE 1981; IRKPATRICK et al. 1990). LYNCH (1988) has suggested that the expectation of high variance among lines for genetic variance estimates discredits the inference of nonadditive genetic effects when inbred lines fail to show the reduction of additive genetic variance predicted by neutral models. Linkage disequilibrium is predicted to contribute to the variance among lines, and the housefly would have relatively strong linkage effects due to a low number of chromosomes and the lack of crossing-over in males. However, linkage would be expected to dissipate in such large populations over the 12 generations elapsed for establishment of the ancestral stock, derivation of the founder-flush populations and videotaping for the parent-offspring assays.with regard to the expected variance among lines, all six bottlenecked lines showed significant differentiation from the controls after adjustment of the type I error rate to account for replicate bottlenecked lines. Furthermore, statistical pooling of replicate controls should have incorporated sampling and drift processes on the means and confidence limits of genetic variance estimates in order to lend a conservative bias for the detection of bottleneck effects. There was no apparent coordination of founder-flush effects on the genetic architecture for male vigor, female choice or reproductive compatibility. For example, the increased parent-son covariance for BUZZ (line B3, Figure 3; which waspossibly associated with increased assortative mating) contrasted the negative value for the same trait (suggesting reproductive incompatibility) in other lines (lines B1 and B6). Similarly, components of the courtship repertoire within lines showed independence for divergence toward either positive or negative parent-offspring covariances (e.g., negative and positive values for parent-daughter covariances for CLOSE and LIFT, respectively, in line B3, Figure 3). Consequently, the average parent-offspring covariance across traits or across lines yields no net bottleneck effect. Hence, founder-flush perturbations do not allow access to the universe of courtship phenotypes but still have the potential to promote shifts in a subset of courtship repertoire that could create novel multivariate form. Founder-flush events profoundly disrupt the genetic architecture for courtship by creating apparently ran- dom shifts in the additive genetic variances for components of male performance vigor and female choice, which translate to increased evolutionary potential to respond to drift or selection. In a sense, additive genetic variance for assortative mating, per se, may have been increased in populations that experienced disjunction a of mating compatibilities, creating negative parent-offspring covariances. These results contribute to the growing body of theoretical and empirical evidence that purely additive theory is inadequate for defining the genetic architecture of traits that are important to the integrity of species (ROBERTSON 1952; LINTS and BOUR- GOIS 1982; BRYANT et al. 1986a,b; GOODNIGHT 1988; GIMMELFARB 1989; TACHIDA and COCKERHAM 1989; CARSON 1990; LOPEZ-FANJUL and VILLAVERDE 1990; WADE 1991; BRYANT and MEFFERT 1993). Thus, the nonadditive genetic processes of dominance, epistasis and genotype-environment interactions might critically influence the structure of genetic variances in equilibrium populations. Nevertheless, the inflated additive genetic variances for courtship repertoire and disrupted male-female compatibilities should be unstable, especially in those traits that exhibit inbreeding depression. The relatively rare population that can survive the processes of selection against deleterious recessive alleles (BARTON and CHARLESWORTH 1984; CHARLESWORTH and ROUHANI 1988; LYNCH 1991), realignment of epistastic associations (WRIGHT 1977; WADE and GOODNIGHT 1991) and resolution of mating incompatibilities might experience a Wrightian shift in components of courtship repertoire that could be important in defining a species. I am very grateful to E. H. BRYANT for unrestricted access to his laboratory and to my brother, D. J. MEFFERT, for his artwork in Figure 2. I thank N. H. A. TERRY and E. H. BRYANT for their comments on earlier drafts and A. J. Moow for his observations on the models. Patient and provocative critiques by M. SIATKIN and three anonymous reviewers were invaluable for the final revision. This work was supported by grants from the National Science Foundation (L. M. MEF- FERT: BSR ; E. H. BRYANT: BSR and BSR ). LITERATURE CITED AHEARN, J. 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