Stochastic selection in large and small populations

We describe two models of stochastic variation in selection intensity. In both models the arithmetic mean fitness of all genotypes is equal; in both models the geometric mean fitness of the heterozygous genotype is greater than that of both homozygous genotypes. In one model the correlation between the fitnesses of the homozygous genotypes is +1; in the other it is −1. We show that the expected time to absorption of an allele in a finite population is significantly retarded for all initial gene frequencies in the former model. The expected time to absorption of an allele in the latter model is retarded only at extreme initial gene frequencies; at intermediate initial gene frequencies the expected time to absorption is accelerated. We conclude that the criterion for polymorphism based on the geometric mean of the heterozygote being greater than that of both homozygotes provides only limited information about the fate of gene frequency.     

Disease, frequency-dependent selection, and genetic polymorphisms: experiments with stripe rust and wheat

Abstract.— Pathogens have the potential to maintain genetic polymorphisms by creating frequency-dependent selection on their host. This can occur when a rare host genotype is less likely to be attacked by a pathogen (frequency-dependent disease attack) and has higher fitness at low frequency (negative frequency-dependent selection). In this study, we used wheat genotypes that were susceptible to different races of the pathogen Puccinia striiformis to test whether disease created frequency-selection on its host and whether such selection could maintain polymorphisms for resistance genes in the wheat populations. Four different two-way mixtures of wheat genotypes were planted at different frequencies in both the presence and absence of disease. Disease created frequency-dependent selection on its host in some populations. Unknown factors other than disease also created frequency-dependent selection in this system because, in some instances, rare genotype advantage was observed in the absence of disease. Although the pathogen created frequency-dependent selection on its host, this selection was not sufficient to maintain genetic polymorphism in the host populations. In all cases where frequency-dependent selection occurred only in the diseased plots, one of the two genotypes was predicted to dominate in the population and the same genotype was predicted to dominate in both the presence and absence of disease. Only in cases where frequency-dependent selection was not caused by disease was there evidence that genetic polymorphisms would be maintained in the population. The frequency-dependent selection described in this study is a consequence of epidemiological effects of disease and differs from the time-lagged frequency-dependent selection resulting from coevolution between hosts and parasites. The impact of this direct frequency-dependent selection on the maintenance of genetic polymorphisms in the host population is discussed.     

Field tests of density-and frequency-dependent selection in Erigeron annuus (Compositae)

Selection may maintain genetic diversity in natural populations if the physical or biotic environment is variable over space and-or time. Because density and genotype frequencies can be heterogeneous, and because genotypes may differ in competitive ability, both density-and frequency-dependent selection have been considered to be potentially important evolutionary processes. To address the possibility that intraspecific interactions among plants are a source of fitness variation in Erigeron annuus, we conducted field experiments over 2 yr that were designed to examine the potential of population density, genotype frequency, and their interaction to act as selective agents. In both experiments, apomictic genotypes of Erigeron were paired. Seedlings were planted into plots that differed in density and the identity of minority and majority genotype. There was evidence for a differential effect of density among genotypes for only one year's experiment, suggesting that density-dependent selection is either weak or temporally variable. Genotype frequency had no effect on fitness in either year, and thus there was no evidence for frequency-dependent selection. In addition, the lack of a frequency ;ts density interaction demonstrates that resource partitioning, one mechanism for frequency dependence, is not strong among Erigeron genotypes. If frequency-dependent selection does occur in this species, it is either too weak to detect even in large field experiments, or occurs only in the presence of a selective agent (e.g., pathogens) that was lacking in our experiments.     

Ecological genetics of the Bromus tectorum (Poaceae)-Ustilago bullata (Ustilaginaceae) pathosystem: A role for frequency-dependent selection?

? Premise of the study: Evolutionary processes that maintain genetic diversity in plants are likely to include selection imposed by pathogens. Negative frequency-dependent selection is a mechanism for maintenance of resistance polymorphism in plant-pathogen interactions. We explored whether such selection operates in the Bromus tectorum-Ustilago bullata pathosystem. Gene-for-gene relationships between resistance and avirulence loci have been demonstrated for this pathosystem. ? Methods: We used molecular markers and cross-inoculation trials to learn whether the SSR genotypes of the host exhibited resistance to co-occurring pathogen races, whether host genotypes within a population had equal disease probability, and whether a common resistance locus and its corresponding avirulence locus exhibited predicted allele frequency changes during an epidemic. ? Key results: Five of six putative resistance loci that conferred resistance to co-occurring pathogen races occurred in common host SSR genotypes. Some common genotypes within populations were more likely to be diseased than others, and genotype frequencies sometimes changed across years in patterns consistent with frequency-dependent selection. Observed changes in frequency of resistance and virulence alleles during an epidemic provided further support, but evidence was inconclusive. ? Conclusions: Frequency-dependent selection may operate at endemic disease levels in this pathosystem, but is difficult to detect because many susceptible plants escape infection. Most pathogen isolates were virulent on most host genotypes, minimizing the apparent importance of frequency-dependent selection even during epidemics.     

Frequency-Dependent Selection at the PGM-1 Locus of DROSOPHILA PSEUDOOBSCURA

Frequency-dependent fitness was studied at the Pgm-1 locus of Drosophila pseudoobscura with respect to two fitness components: rate of development and larva-to-adult survival. The Pgm-1 locus is very polymorphic with only two alleles, Pgm-1¹⁰⁰ and Pgm-1¹⁰⁴, occurring at high frequencies. For each of these two alleles, 20 homozygous strains were obtained from a sample of 1,140 wild-inseminated females. First-instar larvae of the two genotypes were combined in a set of eight different frequencies: 0.0, 0.10, 0.25, 0.40, 0.60, 0.75, 0.90, and 1.0. Frequency-dependent fitness effects were observed for the two survival-related fitness components examined: larvae of the less common genotype develop faster and have a higher probability of survival than larvae of the more common genotype. The rate of survival at intermediate genotypic frequencies is similar to that in pure cultures. If selection acted solely as frequency-dependent effects on survival-related components of fitness, the equilibrium frequency of the Pgm-1¹⁰⁰ allele would be 0.615 for a two-genotype system, which fits an observed frequency range for this allele in nature between 0.55 and 0.71. Experimentally created linkage disequilibrium was excluded from the experiment by using a large number of independent strains. It is nevertheless possible that the frequency-dependent selection may not affect the Pgm-1 locus per se, but may reflect a linkage disequilibrium present in the natural population. Even if this were the case, the frequency-dependent selection could affect the frequency of the Pgm-1 alleles in nature.     

ALLOZYMIC DIFFERENTIATION IN RESPONSE TO LABORATORY DEMOGRAPHIC SELECTION OF DROSOPHILA MELANOGASTER

Drosophila melanogaster populations that exhibit constrasting life histories as a result of laboratory selection were compared at several potentially relevant enzyme loci. Selection regimes included postponed reproduction, accelerated development, and intermediate generation time. Each selection regime was represented by fivefold replicated populations maintained for between 50 and 500 generations. For each population, allele frequencies were calculated from frequencies of electrophoretically distinguishable allozymes of alcohol dehydrogenase, α-glycerol-3-phosphate dehydrogenase, phosphoglucomutase, and CuZn-superoxide dismutase. Based on allozyme frequency changes consistent across replicate populations, two of the studied loci responded to both selection for postponed reproduction and selection for accelerated development. The responses to contrasting selection regimes were in opposing directions, suggesting antagonistic pleiotropy.     

Genotype-by-Environment and Epistatic Interactions in Drosophila Melanogaster: The Effects of Gpdh Allozymes, Genetic Background and Rearing Temperature on Larval Developmental Time and Viability

The possible role of temperature as a component of natural selection generating the latitudinal clines in Gpdh allele frequencies in natural populations of Drosophila melanogaster was examined. Effects of rearing temperature (16 degrees, 22 degrees and 29 degrees) and of Gpdh allozymes (S and F) on larval developmental time and viability were measured. Eight genetic backgrounds from each of three populations (continents) were used to assess the generality of any effects. Analyses of variance indicated significant temperature effects and allozyme-by-genetic background interaction effects for both characters. Viability showed significant genetic background effects, as well as significant temperature-by-allozyme and temperature-by-allozyme-by-population interactions. In general, the S/S genotype was significantly lower in viability than the F/F and F/S genotypes at extreme temperatures (16 degrees and 29 degrees), with no significant differences at 22 degrees. However, each population had a slightly different pattern of viability associated with temperature, and only the Australian population showed a pattern that could contribute to the observed cline formation. Although the same two interactions were not significant for developmental time, examination of the means showed that the S/S genotype had a slightly faster rate of development at 16 degrees than the F/F genotype in all populations (by an average of 0.25 day or 1.1%). The low temperature effect on developmental time is consistent with the clines observed in nature, with the S allele increasing in frequency with higher latitudes. The results for both viability and developmental time are consistent with the interpretation of Gpdh as a minor polygene affecting physiological phenotypes, as indicated by previous work with adult flight metabolism. Finally, it is proposed that the temperature-dependent antagonistic effects of the allozymes on viability vs. developmental time and flight metabolism may be the underlying force giving rise to the worldwide polymorphism.     

Variation in the direction of selection applied by pentenol to the alcohol dehydrogenase locus in Drosophila melanogaster.

This paper describes selective effects of pentenol-impregnated media on six genotypes at the alcohol dehydrogenase (Adh) locus in D. melanogaster. In the laboratory population studied, developmental times of pre-adults homozygous for an alcohol dehydrogenase "null" allele increased with increasing pentenol concentrations. The developmental times of the other five genotypes, which produced active alcohol dehydrogenases, increased slightly at pentenol concentrations up to 0-0033%, but above this concentration they decreased markedly. In fact on 0-067% pentenol, the highest concentration tested, developmental times of these five genotypes were between 9 and 24 h less than their developmental times on media lacking pentenol. The magnitude of the reduction in developmental time differed significantly between genotypes and was positively correlated with alcohol dehydrogenase activity. Pentenol had toxic effects on adults and significant differences were found between survival percentages of adults of different genotypes on pentenol-impregnated media. These survival percentages were negatively correlated with alcohol dehydrogenase activities. Therefore selective differences between genotypes in adult survival were negatively correlated with those in developmental times. The variations in the direction of selection are discussed in terms of their possible biochemical basis and their effects on the maintenance of Adh polymorphisms.     

Selection on structural allelic variation biases plasticity estimates

Wang and Althoff (2019) explored the capacity of Drosophila melanogaster to exhibit adaptive plasticity in a novel environment. In a full‐sib, half‐sib design, they scored the activity of the enzyme alcohol dehydrogenase (ADH) and plastic responses, measured as changes in ADH activity across ethanol concentrations in the range of 0–10% (natural variation) and 16% (the novel environment). ADH activity increased with alcohol concentration, and there was a positive association between larval viability and ADH activity in the novel environment. They also reported that families exhibiting greater plasticity had higher larval survival in the novel environment, concluding that ADH plasticity is adaptive. However, the four authors now concur that, since the study estimated plasticity from phenotypic differences across environments using full‐sib families, it is not possible to disentangle the contributions of allele frequency changes at the Adh locus from regulatory control at loci known to influence ADH activity. Selective changes in allele frequencies may thus conflate estimates of plasticity; any type of “plasticity” (adaptive, neutral, or maladaptive) could be inferred depending on allele frequencies. The problem of scoring sib‐groups after selection should be considered in any plasticity study that cannot use replicated genotypes. Researchers should monitor changes in allele frequencies as one mechanism to deal with this issue.     

Effect of acetone feeding on alcohol dehydrogenase activity in the olive fruit fly,Bactrocera oleae

The purpose of this study is to demonstrate a clear connection between the presence of acetone in larval diet and alcohol dehydrogenase (ADH) activity in laboratory raised populations of Bactrocera oleae. ADH activity of B. oleae is depressed in acetone-impregnated diets. At the same time the change of activity is accompanied by a change in the relative proportions of the multiple forms of ADH. The bulk of activity in the most cathodally migrating form is lost, and all the activity becomes localized in the less cathodally migrating forms of the enzyme. Moreover, ADH activity, expressed in vivo, appears to drop after exposure to acetone, as shown by the fact that larvae become less sensitive to pentenol poisoning. Our results show clear selective differences imposed by acetone on three homozygous genotypes involving the ADH alleles F, S and I in B. oleae. The directions of these differences were found to vary with the fitness component under test. Acetone treatment seems to affect developmental time and larva's viability as well as allele frequencies of ADH under artificial rearing. The effect of acetone on the maintenance of ADH polymorphism in artificially reared populations of B. oleae is further discussed.     

Dynamics of deletion genotypes in an experimental insect virus population

Defective viruses, that are deficient in certain essential genes, are maintained in the population by trans-complementation, exploiting the gene products of complete genotypes in co-infected cells. This process becomes prevalent only when cells are frequently infected by several virus particles, and only then will the fitness of defective viruses be subjected to frequency-dependent selection. Deletion variants that are not infectious per os are present in a multicapsid nucleopolyhedrovirus (SfMNPV, Baculoviridae) that infects the fall army worm, Spodoptera frugiperda. These variants enhance the pathogenicity and, therefore, the likelihood of transmission of the virus when co-infecting cells with complete genotypes, resulting in occlusion bodies (OBs) that may contain both genotypes co-occluded. Mixtures of complete (B) and defective (C) variants in ratios of 90% B+10% C, 50% B+50% C and 10% B+90% C were used to inoculate by injection S. frugiperda larvae. Viral OBs extracted from diseased insects were subjected to four or five successive rounds of per os infection. Following successive passages, genotype frequencies in all three experimental populations converged to a single equilibrium frequency comprising approximately 20% of deletion genotype C and approximately 80% of complete genotype B. This mirrors the relative proportions of deletion (22%) and complete (78%) genotypes observed in the wild-type SfMNPV population. The pathogenicity of experimental populations at the final passage was not significantly different from that of the wild-type isolate. In contrast, OBs of all genotype mixtures were significantly more pathogenic than OBs of genotype B alone. A population genetics model, in which virus populations were assigned linear frequency-dependent transmissibility values, was in remarkably close agreement to empirical data. Clearly, non-infectious deletion variants can profoundly affect the likelihood of transmission and the genetic structure and stability of virus populations.     

Frequency and density-dependent selection on life-history strategies--a field experiment

Negative frequency-dependence, which favors rare genotypes, promotes the maintenance of genetic variability and is of interest as a potential explanation for genetic differentiation. Density-dependent selection may also promote cyclic changes in frequencies of genotypes. Here we show evidence for both density-dependent and negative frequency-dependent selection on opposite life-history tactics (low or high reproductive effort, RE) in the bank vole (Myodes glareolus). Density-dependent selection was evident among the females with low RE, which were especially favored in low densities. Instead, both negative frequency-dependent and density-dependent selection were shown in females with high RE, which were most successful when they were rare in high densities. Furthermore, selection at the individual level affected the frequencies of tactics at the population level, so that the frequency of the rare high RE tactic increased significantly at high densities. We hypothesize that these two selection mechanisms (density- and negative frequency-dependent selection) may promote genetic variability in cyclic mammal populations. Nevertheless, it remains to be determined whether the origin of genetic variance in life-history traits is causally related to density variation (e.g. population cycles).     

Alcohol dehydrogenase allele frequencies in Bactrocera oleae (Dipt., Tephritidae): effect of ethanol addition in larval diet

Abstract: The effect of ethanol in larval medium on Bactrocera oleae larvae was examined at four concentrations. Ethanol exerted a differential effect on the three alcohol dehydrogenase allele frequencies. While originally being at equilibrium under laboratory conditions, after three generations of larval development in a diet containing ethanol at 1% concentration, Adh -F allele frequency increased, that of Adh -I dropped significantly and the frequency of Adh -S remained unaltered. Adh -S allele seems to be adapted in nature where only minor quantities of alcohol are present in the insects' natural host, while Adh -I is best adapted in the alcohol-free laboratory culture medium. The frequency of Adh -F allele remains unaltered when feral populations are introduced in the laboratory.     

A fundamental relationship between genotype frequencies and fitnesses

The set of possible postselection genotype frequencies in an infinite, randomly mating population is found. Geometric mean heterozygote frequency divided by geometric mean homozygote frequency equals two times the geometric mean heterozygote fitness divided by geometric mean homozygote fitness. The ratio of genotype frequencies provides a measure of genetic variation that is independent of allele frequencies. When this ratio does not equal two, either selection or population structure is present. Within-population HapMap data show population-specific patterns, while pooled data show an excess of homozygotes.     

Changes in gene frequencies at the octanol dehydrogenase locus of Drosophila melanogaster imposed by environmental ethanol

Experiments have been performed to study the effect of selection at the Odh locus in Drosophila melanogaster populations using different alcohol concentrations in the medium. The data can be best interpreted by assuming frequency-dependent selection. When genotype frequencies are considered as independent variables and values of Wrightian fitness as dependent variables, it turns out that different functions describe the selection of the coexisting genotypes. A linear equation is used for the SF genotype and a hyperbolic function for the FF genotype. No function of good fit could be found for the SS genotype. Simulation experiments using these functions fit our data well.     

Differences in life history traits between alcohol dehydrogenase genotypes of Drosophila mercatorum: background and maternal genotype effects

The effects of genotype at the alcohol dehydrogenase (ADH) locus on developmental time and fecundity were studied using three strains of Drosophila mercatorum. All three strains had similar genetic backgrounds and two were homozygous for the same electrophoretic allele (F for fast). The third strain was homozygous for the slow allele (S). The life history traits of all possible progeny of these strains were studied. The results showed that for both mean developmental time and pattern of eclosion, strong maternal genotype and background effects were present. Fecundity among genotypes was not different, although there was an effect on the total number of offspring suggesting differences in egg-to-adult survivorship.     

Modes of Selection Maintaining an Inversion Polymorphism in DROSOPHILA PAULISTORUM

The possibility that fitness relationships associated with an inversion polymorphism in D. paulistorum were frequency dependent was investigated. Using allozymes of tetrazolium oxidase to mark inversions, the effects of genotype frequency, larval density, and culture conditions on fitness were assessed. The proportions of genotypes among egg-laying females were varied, thus changing the expected proportions of progeny produced in the absence of fecundity or viability selection. The genotypes of progeny were determined by electrophoresis and comparisons of the ratio of the numbers of the different genotypes produced to the expected ratio was used to evaluate fitness relationships. Fitness relationships were dependent on genotype frequency, larval density, and culture conditions. Selection was either absent, directional, frequency dependent (favoring rare types), or heterotic depending on density and culture conditions. It is implied that the adaptive value of genetic variants need not be apparent in all environments, or may change with changing conditions. There is evidence for different criteria for selection in the two sexes. These results add to the evidence supporting the importance of frequency-dependent selection. It is argued that for frequency dependence to be of general importance, selection must act on genes in groups, either as an inversion or as lengths of chromosome with integrity maintained by disequilibrium.     

Natural selection of UDP genotypes in an experimental population of Lolium perenne (Poaceae)

The mating system determines the range of genotypes on which selection will act. Ultimately, the population will contain only those genotypes that can survive in the current environment. This study was conducted to determine the extent to which selection affects genotype at four enzyme loci in Lolium perenne. Starch gel electrophoresis was used to determine genotype at the phosphoglucomutase, 6-phosphogluconate dehydrogenase, phosphoglucose isomerase, and UDP-glucose pyrophosphorylase loci. Genotypes of progeny arrays were used to infer maternal genotype in a field-grown population of plants. A mating system analysis was performed on maternal and progeny plants of the field population and also on a population of plants, from the same seed stock, grown in a greenhouse environment. Allelic frequencies differed among populations at the Udp locus. Results indicate that high mortality in the field strongly favored one allele. Assumptions of the mating system model are examined and used to interpret the observed differentiation among populations.     

The dynamics of cohort selection

The dynamics of cohort selection in age-structured populations is explored in a series of computer simulations using a single-locus two-allele model. Cohort selection is a directional process which is a consequence of genetically determined inter-cohort variation in the magnitude of expression of interference behavior; its essential premise is that all genotypes of a given age are equally susceptible to interference and that no advantage accrues to the interfering genotype. In these simulations, allele frequency, population size, intensity of interference, and reproductive apportionment are all major determinants of evolutionary rates. The generality of this process is demonstrated and an analysis of the observed frequency-dependent effects is presented. Cohort selection is distinct from other kinds of selection and may be an important cause of the persistence of interference behavior (culminating in acts of spite) within animal populations.     

The Alcohol Dehydrogenase Polymorphism in Drosophila Melanogaster: Fitness Measurements and Predictions under Conditions with No Alcohol Stress

The relative fitnesses of the different Adh genotypes under normal laboratory conditions and in the absence of alcohol stress were estimated in Drosophila melanogaster according to Prout's method. The larval component (viability) did not reveal fitness differences between the genotypes but for the adult component significant differences were observed. The female adult component (fecundity) showed an overdominant pattern: both homozygous genotypes showed a relative fitness significantly lower than the heterozygous genotype. For the male adult component (virility) also differences were observed. The homozygous SS genotype showed a lower relative fitness than the other two genotypes. Predictions for gene frequency changes based on the estimated fitness values do show a reasonably good correspondence with frequency changes actually observed in a number of experimental cage populations and indicate a globally stable equilibrium around a frequency of the F allele of 0.40-0.70. The relevance of these fitness estimates, obtained under conditions with no alcohol stress, for the explanation of the Adh polymorphisms observed in nature is discussed.