EQUILIBRIUM ANALYSIS OF SEXUAL SELECTION IN DROSOPHILA MELANOGASTER

Several models for sexual selection, both by male-male competition and female choice, predict that a character which covaries with mating success should be near an equilibrium where the intensity of sexual selection opposes viability selection. This prediction was used to design experiments for estimating the intensity of sexual and viability selection on wing length in a recently captured population of Drosophila melanogaster. Observations of matings by males color-marked for wing length indicated that the standardized sexual selection differential on wing length was 0.24 under a wide range of effective sex ratios. After estimating the heritability of wing length to be 0.62, the expected standardized response due to sexual selection was calculated as 0.15 (SE = 0.15). The response due to viability selection was then estimated by comparing wing lengths of progeny of flies that had been randomly mated, thereby preventing sexual selection, with progeny of flies that had been allowed to acquire mates in a mass-mating chamber. The results support an equilibrium model in that the standardized response due to viability selection (?0.31, SE = 0.08) was opposite in sign and similar in magnitude to the estimated response due to sexual selection. Observations of females orienting in front of males which differed in wing length indicated that the mating advantage accruing to long-winged males was not due to female choice. Instead, male-male competition in which the larger of two randomly chosen males succeeded in mating, explains the observed sexual selection. An experimental analysis of genotype-environment interaction revealed that larval density had a nonlinear effect on mean wing length within sibships. If a population is displaced from equilibrium, therefore, the evolutionary trajectory of mean wing length will depend both on the intensity of selection and the environment in which that selection is operating.     

DIRECTIONAL AND STABILIZING DENSITY-DEPENDENT NATURAL SELECTION FOR PUPATION HEIGHT IN DROSOPHILA MELANOGASTER

Six populations of Drosophila melanogaster have been kept at extreme population densities, three high and three low, for 175 generations. Larvae from the high density populations pupate 50%-100% higher than larvae from the low density populations. At high larval test densities there is both a directional and a stabilizing component to selection, with viabilities ranging from 0.14 to 0.992, depending on the choice of pupation site. The directional component is stronger on the populations which have evolved at low densities, while the stabilizing component is stronger on the populations which have evolved at high densities. There is no indication that the evolution of this trait, in response to density, has altered its phenotypic plasticity.     

LATITUDINAL CLINE IN DROSOPHILA MELANOGASTER FOR KNOCKDOWN RESISTANCE TO ETHANOL FUMES AND FOR RATES OF RESPONSE TO SELECTION FOR FURTHER RESISTANCE

We have introduced a device for selecting Drosophila for increased resistance to very high concentrations of ethanol fumes. This device has enabled us to: 1) select quickly and easily over a thousand flies at a time, and 2) score the knockdown time of every fly in the distribution, while causing very little injury to the flies. A sample of nine west coast populations of Drosophila melanogaster showed a significant trend toward higher knockdown resistance in more northern populations. A population's level of knockdown resistance was virtually uncorrelated with its alcohol dehydrogenase (Adh) allele frequencies. Five of the above nine populations were then subjected to selection for further knockdown resistance. Each population was divided randomly into four groups of 256 flies: two lines to be selected, and two lines to remain unselected as control lines. In every generation each selected line was measured for knockdown resistance, and the last quartile of flies to be knocked down was saved to continue the selection cycle. Population sizes of the selected and unselected lines were all maintained at 256. Realized heritability, based on the responses to selection of the first four generations, was calculated for each selected line. The five populations were significantly heterogeneous for heritability estimates; the average heritability of the five populations pooled was 0.143 ± 0.019. Over the course of twelve generations, the ten selected lines increased their knockdown times by an average factor of 2.40. Before selection, the five populations were heterogeneous for knockdown resistance, and resistance was greatest among the most northern populations. The amount of change of knockdown resistance over the course of selection was also correlated with latitude: the most southern population increased its knockdown time by a factor of 2.23, and the most northern population increased it by a factor of 2.55. After ten generations of selection, the cline of knockdown resistance was about 4.5 times as steep as that before selection. Small phenotypic differences among populations before selection were thus exaggerated by the action of selection. The differences among populations in their rates of response to selection were attributed to genetic differences that existed before selection. The pattern of change of Adh frequencies over the course of selection was very inconsistent, both among and within populations. From this inconsistency of change of Adh alleles with selection, and the lack of correlation between Adh frequencies and knockdown resistance before selection, we concluded that Adh frequency changes could not have had much effect on the responses of the selected lines.     

RESISTANCE OF GENETIC CORRELATION STRUCTURE TO DIRECTIONAL SELECTION IN DROSOPHILA MELANOGASTER

The genetic covariance and correlation matrices for five morphological traits were estimated from four populations of fruit flies, Drosophila melanogaster, to measure the extent of change in genetic covariances as a result of directional selection. Two of the populations were derived from lines that had undergone selection for large or small thorax length over the preceding 23 generations. A third population was constituted using flies from control lines that were maintained with equivalent population sizes as the selected lines. The fourth population contained flies from the original cage population from which the selected and control lines had been started. Tests of the homogeneity of covariance matrices using maximum likelihood techniques revealed significant changes in covariance structure among the selected lines. Prediction of base population trait means from selected line means under the assumption of constant genetic covariances indicated that genetic covariances for the small population differed more from the base population than did the covariances for the large population. The predicted small population means diverged farther from the expected means because the additive genetic variance associated with several traits increased in value and most of the genetic covariances associated with one trait changed in sign. These results illustrate that genetic covariances may remain nearly constant in some situations while changing markedly in others. Possible developmental reasons for the genetic changes are discussed.     

EVOLUTIONARY EFFECTS OF SELECTION ON AGE AT REPRODUCTION IN LARVAL AND ADULT: DROSOPHILA MELANOGASTER

Two sets of three replicate lines of Drosophila melanogaster were artificially selected by reproduction at either a ‘young’ or an ‘old’ age. The pure lines, the hybrids between the lines within a selection regimen and the base stock from which the lines were derived were compared for longevity, early and late fertility, development time, larval viability and adult thorax length. Comparison of hybrid with pure lines showed some evidence for inbreeding depression in the lines from both selection regimes. Comparison of hybrid lines with the base stock did not provide evidence for any trade-off in either males or females between early fertility on the one hand and late life fertility and longevity on the other. Nor was there any clear evidence of a trade-off between pre-adult and adult fitness components. There was evidence of inadvertent selection for rapid development in both selection regimens, especially in the females of the ‘young’ lines, and this complicated the interpretation of the responses and correlated responses to selection. An improvement in adult performance in the ‘old’ line males relative to the base stock appeared to be attributable to reversal of mutation accumulation. Comparison of the hybrid ‘young’ and ‘old’ lines with the base stock did not support the idea that the superior longevity and late life fertility of the ‘old’ lines relative to the ‘young’ lines could be accounted for by the effects of mutation accumulation in the ‘young’ lines. The results point to the need to compare selected lines with their base stock when deducing responses and correlated responses to selection and to avoid unintentional selection. In this type of experiment, larval density should be standardized during selection, and adults should not be under pressure for rapid maturation.     

SELECTION ON COPULATION DURATION IN DROSOPHILA MELANOGASTER: PREDICTABILITY OF DIRECT RESPONSE VERSUS UNPREDICTABILITY OF CORRELATED RESPONSE

Estimates of heritabilities and genetic correlations for seven reproductive attributes had previously been obtained from parent-offspring regression (Gromko, 1987, 1989). Copulation duration was shown to have a heritability of 0.23 and to be genetically correlated with courtship vigor (r_A = ?0.41) and with fertility (r_A = 0.27). These observations form the basis for the prediction of direct and correlated responses to selection for increased and decreased copulation duration, which are reported here. The direct response corresponded closely to prediction, but the correlated responses did not provide consistent qualitative fit. A hypothesis is proposed to explain this difference in predictability of direct and correlated response to selection. The major postulate is that the different polygenes involved in the direct response to selection for copulation duration have different pleiotropic effects.     

POLYMERIZATION OF SILICA IN ACIDIC SOLUTIONS: A NOTE OF CAUTION TO PHYCOLOGISTS1

Concentrated stock solutions of sodium metasilicate (Na₂SiO₃.9H₂O), prepared in deionized-distilled water and acidified to pH 2 as suggested by McLachlan (1973), were found to be polymerized; stocks kept at their alkaline pH of dissolution were not polymerized. Depolymerization studies using the acidified stock added to deionized-distilled water, 3% NaCl or artificial saltwater medium showed that complete depolymerization took longer than 2 days. Studies of Si uptake, using the marine diatom Thalassiosira pseudonana (Hust.) Hasle and Heimdal, showed that use of an acidified Si stock for experiments can lead to anomolous results. It is recommended that concentrated Si stocks be stored at their pH of dissolution and not be acidified prior to addition to media. In instances where the pH must be adjusted prior to addition (e.g. in some Si-uptake experiments), it may be necessary to take into account polymerization / depolymerization effects.