FIXATION OF MUTATORS IN ASEXUAL POPULATIONS: THE ROLE OF GENETIC DRIFT AND EPISTASIS

We study the evolutionary dynamics of an asexual population of nonmutators and mutators on a class of epistatic fitness landscapes. We consider the situation in which all mutations are deleterious and mutators are produced from nonmutators continually at a constant rate. We find that in an infinitely large population, a minimum nonmutator‐to‐mutator conversion rate is required to fix the mutators but an arbitrarily small conversion rate results in the fixation of mutators in a finite population. We calculate analytical expressions for the mutator fraction at mutation‐selection balance and fixation time for mutators in a finite population when the difference between the mutation rate for mutator and nonmutator is smaller (regime I) and larger (regime II) than the selection coefficient. Our main result is that in regime I, the mutator fraction and the fixation time are independent of epistasis but in regime II, mutators are rarer and take longer to fix when the decrease in fitness with the number of deleterious mutations occurs at an accelerating rate (synergistic epistasis) than at a diminishing rate (antagonistic epistasis). Our analytical results are compared with numerics and their implications are discussed.     

RATES OF FITNESS DECLINE AND REBOUND SUGGEST PERVASIVE EPISTASIS

Unraveling the factors that determine the rate of adaptation is a major question in evolutionary biology. One key parameter is the effect of a new mutation on fitness, which invariably depends on the environment and genetic background. The fate of a mutation also depends on population size, which determines the amount of drift it will experience. Here, we manipulate both population size and genotype composition and follow adaptation of 23 distinct Escherichia coli genotypes. These have previously accumulated mutations under intense genetic drift and encompass a substantial fitness variation. A simple rule is uncovered: the net fitness change is negatively correlated with the fitness of the genotype in which new mutations appear—a signature of epistasis. We find that Fisher's geometrical model can account for the observed patterns of fitness change and infer the parameters of this model that best fit the data, using Approximate Bayesian Computation. We estimate a genomic mutation rate of 0.01 per generation for fitness altering mutations, albeit with a large confidence interval, a mean fitness effect of mutations of ?0.01, and an effective number of traits nine in mutS^? E. coli. This framework can be extended to confront a broader range of models with data and test different classes of fitness landscape models.     

GENETIC ARCHITECTURE OF METABOLIC RATE: ENVIRONMENT SPECIFIC EPISTASIS BETWEEN MITOCHONDRIAL AND NUCLEAR GENES IN AN INSECT

The extent to which mitochondrial DNA (mtDNA) variation is involved in adaptive evolutionary change is currently being reevaluated. In particular, emerging evidence suggests that mtDNA genes coevolve with the nuclear genes with which they interact to form the energy producing enzyme complexes in the mitochondria. This suggests that intergenomic epistasis between mitochondrial and nuclear genes may affect whole‐organism metabolic phenotypes. Here, we use crossed combinations of mitochondrial and nuclear lineages of the seed beetle Callosobruchus maculatus and assay metabolic rate under two different temperature regimes. Metabolic rate was affected by an interaction between the mitochondrial and nuclear lineages and the temperature regime. Sequence data suggests that mitochondrial genetic variation has a role in determining the outcome of this interaction. Our genetic dissection of metabolic rate reveals a high level of complexity, encompassing genetic interactions over two genomes, and genotype × genotype × environment interactions. The evolutionary implications of these results are twofold. First, because metabolic rate is at the root of life histories, our results provide insights into the complexity of life‐history evolution in general, and thermal adaptation in particular. Second, our results suggest a mechanism that could contribute to the maintenance of nonneutral mtDNA polymorphism.     

EPISTASIS AND THE INCREASE IN ADDITIVE GENETIC VARIANCE: IMPLICATIONS FOR PHASE 1 OF WRIGHT'S SHIFTING-BALANCE PROCESS

Central to Wright's shifting-balance theory is the idea that genetic drift and selection in systems with gene interaction can lead to the formation of “adaptive gene complexes.” The theory of genetic drift has been well developed over the last 60 years; however, nearly all of this theory is based on the assumption that only additive gene effects are acting. Wright's theory was developed recognizing that there was a “universality of interaction effects,” which implies that additive theory may not be adequate to describe the process of differentiation that Wright was considering. The concept of an adaptive gene complex implies that an allele that is favored by individual selection in one deme may be removed by selection in another deme. In quantitative genetic terms, the average effects of an allele relative to other alleles changes from deme to deme. The model presented here examines the variance in local breeding values (LBVs) of a single individual and the covariance in the LBVs of a pair of individuals mated in the same deme relative to when they are mated in different demes. Local breeding value is a measure of the average effects of the alleles that make up that individual in a particular deme. I show that when there are only additive effects the covariance between the LBVs of individuals equals the variance in the LBV of an individual. As the amount of epistasis in the ancestral population increases, the variance in the LBV of an individual increases and the covariance between the LBVs of a pair of individuals decreases. The divergence in these two values is a measure of the extent to which the LBV of an individual varies independently of the LBVs of other individuals. When this value is large, it means that the relative ordering of the average effects of alleles will change from deme to deme. These results confirm an important component of Wright's shifting-balance theory: When there is gene interaction, genetic drift can lead to the reordering of the average effects of alleles and when coupled with selection this will lead to the formation of the adaptive gene complexes.     

MUTATION AND EXTINCTION: THE ROLE OF VARIABLE MUTATIONAL EFFECTS,SYNERGISTIC EPISTASIS,BENEFICIAL MUTATIONS,AND DEGREE OF OUTCROSSING

Recent theoretical studies have illustrated the potential role of spontaneous deleterious mutation as a cause of extinction in small populations. However, these studies have not addressed several genetic issues, which can in principle have a substantial influence on the risk of extinction. These include the presence of synergistic epistasis, which can reduce the rate of mutation accumulation by progressively magnifying the selective effects of mutations, and the occurrence of beneficial mutations, which can offset the effects of previous deleterious mutations. In stochastic simulations of small populations (effective sizes on the order of 100 or less), we show that both synergistic epistasis and the rate of beneficial mutation must be unrealistically high to substantially reduce the risk of extinction due to random fixation of deleterious mutations. However, in analytical calculations based on diffusion theory, we show that in large, outcrossing populations (effective sizes greater than a few hundred), very low levels of beneficial mutation are sufficient to prevent mutational decay. Further simulation results indicate that in populations small enough to be highly vulnerable to mutational decay, variance in deleterious mutational effects reduces the risk of extinction, assuming that the mean deleterious mutational effect is on the order of a few percent or less. We also examine the magnitude of outcrossing that is necessary to liberate a predominantly selfing population from the threat of long-term mutational deterioration. The critical amount of outcrossing appears to be greater than is common in near-obligately selfing plant species, supporting the contention that such species are generally doomed to extinction via random drift of new mutations. Our results support the hypothesis that a long-term effective population size in the neighborhood of a few hundred individuals defines an approximate threshold, below which outcrossing populations are vulnerable to extinction via fixation of deleterious mutations, and above which immunity is acquired.     

A NEW MEASURE OF GENETIC IDENTITY BETWEEN POPULATIONS OF SEXUAL AND ASEXUAL SPECIES

We define a new genetic identity measure that is especially well suited for asexual polyploid species as it circumvents errors in the estimation of gene frequencies. It also can be applied to sexuals allowing the study of phylogenetic relationships in species complexes consisting of sexuals and asexuals of different ploidy levels. The measure groups genotypes into classes dependent on homozygosity vs heterozygosity and the number of ancestral allele types vs the number of presumed new mutations. Its value is related to evolutionary time since divergence. The application of the method is illustrated by using electrophoretic data on the species group Solenobia triquetrella (Lepidoptera: Psychidae). A high similarity of estimated relationships among the proposed as well as other genetic identity measures is shown in the case of diploid sexual and asexual races of this species group. The phylogenetic relationships within the group are reanalyzed and monophyletic vs polyphyletic origin of parthenogenesis in this species complex is discussed. The genetic identity values found by the proposed procedure are explained by a polyphyletic origin of parthenogenesis, though a monophyletic origin of parthenogenesis in a broader sense cannot be excluded. The explanation of the phylogenetic relationships is based on the assumption of hybridization between related species and the extinction of one ancestral species. Furthermore, the genetic diversity is compared among sexual and parthenogenetic races of the species.     

SENSITIVITY OF THE DISTRIBUTION OF MUTATIONAL FITNESS EFFECTS TO ENVIRONMENT,GENETIC BACKGROUND,AND ADAPTEDNESS: A CASE STUDY WITH DROSOPHILA

Heterogeneity in the fitness effects of individual mutations has been found across different environmental and genetic contexts. Going beyond effects on individual mutations, how is the distribution of selective effects, f(s), altered by changes in genetic and environmental context? In this study, we examined changes in the major features of f(s) by estimating viability selection on 36 individual mutations in Drosophila melanogaster across two different environments in two different genetic backgrounds that were either adapted or nonadapted to the two test environments. Both environment and genetic background affected selection on individual mutations. However, the overall distribution f(s) appeared robust to changes in genetic background but both the mean, E(s), and the variance, V(s) were dependent on the environment. Between these two properties, V(s) was more sensitive to environmental change. Contrary to predictions of fitness landscape theory, the match between genetic background and assay environment (i.e., adaptedness) had little effect on f(s).     

A POPULATION GENETIC THEORY OF CANALIZATION

Canalization is the suppression of phenotypic variation. Depending on the causes of phenotypic variation, one speaks either of genetic or environmental canalization. Genetic canalization describes insensitivity of a character to mutations, and the insensitivity to environmental factors is called environmental canalization. Genetic canalization is of interest because it influences the availability of heritable phenotypic variation to natural selection, and is thus potentially important in determining the pattern of phenotypic evolution. In this paper a number of population genetic models are considered of a quantitative character under stabilizing selection. The main purpose of this study is to define the population genetic conditions and constraints for the evolution of canalization. Environmental canalization is modeled as genotype specific environmental variance. It is shown that stabilizing selection favors genes that decrease environmental variance of quantitative characters. However, the theoretical limit of zero environmental variance has never been observed. Of the many ways to explain this fact, two are addressed by our model. It is shown that a “canalization limit” is reached if canalizing effects of mutations are correlated with direct effects on the same character. This canalization limit is predicted to be independent of the strength of stabilizing selection, which is inconsistent with recent experimental data (Sterns et al. 1995). The second model assumes that the canalizing genes have deleterious pleiotropic effects. If these deleterious effects are of the same magnitude as all the other mutations affecting fitness very strong stabilizing selection is required to allow the evolution of environmental canalization. Genetic canalization is modeled as an influence on the average effect of mutations at a locus of other genes. It is found that the selection for genetic canalization critically depends on the amount of genetic variation present in the population. The more genetic variation, the stronger the selection for canalizing effects. All factors that increase genetic variation favor the evolution of genetic canalization (large population size, high mutation rate, large number of genes). If genetic variation is maintained by mutation-selection balance, strong stabilizing selection can inhibit the evolution of genetic canalization. Strong stabilizing selection eliminates genetic variation to a level where selection for canalization does not work anymore. It is predicted that the most important characters (in terms of fitness) are not necessarily the most canalized ones, if they are under very strong stabilizing selection (k > 0.2V_e). The rate of decrease of mutational variance V_m is found to be less than 10% of the initial V_m. From this result it is concluded that characters with typical mutational variances of about 10^–3 V_e are in a metastable state where further evolution of genetic canalization is too slow to be of importance at a microevolutionary time scale. The implications for the explanation of macroevolutionary patterns are discussed.     

EPISTASIS AND THE EVOLUTION OF ADDITIVE GENETIC VARIANCE IN POPULATIONS THAT PASS THROUGH A BOTTLENECK

Traditional models of genetic drift predict a linear decrease in additive genetic variance for populations passing through a bottleneck. This perceived lack of heritable variance limits the scope of founder-effect models of speciation. We produced 55 replicate bottleneck populations maintained at two male-female pairs through four generations of inbreeding (average F = 0.39). These populations were formed from an F₂ intercross of the LG/J and SM/J inbred mouse strains. Two contemporaneous control strains maintained with more than 60 mating pairs per generation were formed from this same source population. The average level of within-strain additive genetic variance for adult body weight was compared between the control and experimental lines. Additive genetic variance for adult body weight within experimental bottleneck strains was significantly higher than expected under an additive genetic model This enhancement of additive genetic variance under inbreeding is likely to be due to epistasis, which retards or reverses the loss of additive genetic variance under inbreeding for adult body weight in this population. Therefore, founder-effect speciation processes may not be constrained by a loss of heritable variance due to population bottlenecks.     

FITNESS SENSITIVITY AND THE CANALIZATION OF LIFE-HISTORY TRAITS

Canalization is an abstract term that describes unknown developmental mechanisms that reduce phenotypic variation. A trait can be canalized against environmental perturbations (e.g., changes in temperature or nutrient quality), or genetic perturbations (e.g., mutations or recombination); this paper is about genetic canalization. Stabilizing selection should improve the canalization of traits, and the degree of canalization should be positively correlated with the traits' impact on fitness. Experiments testing this idea should measure the canalization of a series of traits whose impact on fitness is known or can be inferred, exclude differences among traits in the number of loci and alleles segregating as an explanation for the pattern of variability found, and distinguish between canalization against genetic and environmental variation. These conditions were met by three experiments within which the variation of fitness components among Drosophila melanogaster lines was measured and among which the genetic contribution to the variation among lines was clearly different. The canalization of the traits increased with their impact on fitness and did not depend on the degree of genetic differences among lines. That the flies used had been transformed by a P-element insert suggests that canalization was also effective against novel genetic variation. The results reported here cannot be explained by the classical hypothesis of reduction in the number of loci segregating for traits with greater impact on fitness and confirm that traits with greater impact on fitness are more strongly canalized. This pattern of canalization reveals an underappreciated role for development in microevolution. There is differential genetic canalization of fitness components in D. melanogaster.     

EPISTASIS AS A SOURCE OF INCREASED ADDITIVE GENETIC VARIANCE AT POPULATION BOTTLENECKS

The role of epistasis in evolution and speciation has remained controversial. We use a new parameterization of physiological epistasis to examine the effects of epistasis on levels of additive genetic variance during a population bottleneck. We found that all forms of epistasis increase average additive genetic variance in finite populations derived from initial populations with intermediate allele frequencies. Average additive variance continues to increase over many generations, especially at larger population sizes (N = 32 to 64). Additive-by-additive epistasis is the most potent source of additive genetic variance in this situation, whereas dominance-by-dominance epistasis contributes smaller amounts of additive genetic variance. With additive-by-dominance epistasis, additive genetic variance decreases at a relatively high rate immediately after a population bottleneck, rebounding to higher levels after several generations. Empirical examples of epistasis for murine adult body weight based on measured genotypes are provided illustrating the varying effects of epistasis on additive genetic variance during population bottlenecks.     

THE EVOLUTION OF CANALIZATION AND THE BREAKING OF VON BAER'S LAWS: MODELING THE EVOLUTION OF DEVELOPMENT WITH EPISTASIS

Evolution can change the developmental processes underlying a character without changing the average expression of the character itself. This sort of change must occur in both the evolution of canalization, in which a character becomes increasingly buffered against genetic or developmental variation, and in the phenomenon of closely related species that show similar adult phenotypes but different underlying developmental patterns. To study such phenomena, I develop a model that follows evolution on a surface representing adult phenotype as a function of underlying developmental characters. A contour on such a “phenotype landscape” is a set of states of developmental characters that produce the same adult phenotype. Epistasis induces curvature of this surface, and degree of canalization is represented by the slope along a contour. I first discuss the geometric properties of phenotype landscapes, relating epistasis to canalization. I then impose a fitness function on the phenotype and model evolution of developmental characters as a function of the fitness function and the local geometry of the surface. This model shows how canalization evolves as a population approaches an optimum phenotype. It further shows that under some circumstances, “decanalization” can occur, in which the expression of adult phenotype becomes increasingly sensitive to developmental variation. This process can cause very similar populations to diverge from one another developmentally even when their adult phenotypes experience identical selection regimes.     

EVOLUTION OF GENETIC INTEGRATION BETWEEN DISPERSAL AND COLONIZATION ABILITY IN A BIRD

Discrete behavioral strategies comprise a suite of traits closely integrated in their expression with consistent natural selection for such coexpression leading to developmental and genetic integration of their components. However, behavioral traits are often also selected to respond rapidly to changing environments, which should both favor their context-dependent expression and inhibit evolution of genetic integration with other, less flexible traits. Here we use a multigeneration pedigree and long-term data on lifetime fitness to test whether behaviors comprising distinct dispersal strategies of western bluebirds—a species in which the propensity to disperse is functionally integrated with aggressive behavior—are genetically correlated. We further investigated whether selection favors flexibility in the expression of aggression in relation to current social context. We found a significant genetic correlation between aggression and dispersal that is concordant with consistent selection for coexpression of these behaviors. To a limited extent, individuals modified their aggression to match their mate; however, we found no fitness consequences on such adjustments. These results introduce a novel way of viewing behavioral strategies, where flexibility of behavior, while often aiding an organism's fit in its current environment, may be limited and thereby enable integration with less flexible traits.     

MITOCHONDRIAL–NUCLEAR EPISTASIS AFFECTS FITNESS WITHIN SPECIES BUT DOES NOT CONTRIBUTE TO FIXED INCOMPATIBILITIES BETWEEN SPECIES OF DROSOPHILA

Efficient mitochondrial function requires physical interactions between the proteins encoded by the mitochondrial and nuclear genomes. Coevolution between these genomes may result in the accumulation of incompatibilities between divergent lineages. We test whether mitochondrial–nuclear incompatibilities have accumulated within the Drosophila melanogaster species subgroup by combining divergent mitochondrial and nuclear lineages and quantifying the effects on relative fitness. Precise placement of nine mtDNAs from D. melanogaster, D. simulans, and D. mauritiana into two D. melanogaster nuclear genetic backgrounds reveals significant mitochondrial–nuclear epistasis affecting fitness in females. Combining the mitochondrial genomes with three different D. melanogaster X chromosomes reveals significant epistasis for male fitness between X‐linked and mitochondrial variation. However, we find no evidence that the more than 500 fixed differences between the mitochondrial genomes of D. melanogaster and the D. simulans species complex are incompatible with the D. melanogaster nuclear genome. Rather, the interactions of largest effect occur between mitochondrial and nuclear polymorphisms that segregate within species of the D. melanogaster species subgroup. We propose that a low mitochondrial substitution rate, resulting from a low mutation rate and/or efficient purifying selection, precludes the accumulation of mitochondrial–nuclear incompatibilities among these Drosophila species.     

MUTATIONAL CONTRIBUTIONS TO GENETIC VARIANCE-COVARIANCE MATRICES: AN EXPERIMENTAL APPROACH USING INDUCED MUTATIONS IN ARABIDOPSIS THALIANA

Genetic potential for evolutionary change and covariational constraints are typically summarized as the genetic variance-covariance matrix G , and there is currently debate over the extent to which G remains effectively constant during the course of adaptive evolution. However, G provides only a temporally restricted view of constraints that ignores possible biases in how new mutations affect multivariate phenotypes. We used chemical mutagenesis to study the effect of mutations as summarized by the mutational covariance matrix, M , in Arabidopsis thaliana. By introducing mutations into three isogenic strains of A. thaliana, we were able to quantify M directly as the genetic variance-covariance matrix of mutagenized lines. Induced mutations generally did not alter the means of the six morphology and life-history traits we measured, but they did affect the levels of available genetic variation and the covariances among traits. However, these effects were not consistent among the three isogenic lines; that is, there were significant differences among the lines in both the number of mutations produced by ethyl-methane-sulfonate treatment and the M matrices they induced. The evolutionary implications of the dependence of M on the number of mutations, the particular genetic background, and the mutagenic sampling of loci in the genome are discussed in light of commonly applied models of multivariate evolution and the potential for the genetic architecture itself to change in ways that facilitate the coordinated evolution of complex phenotypes.     

PLASTICITY, CANALIZATION, AND DEVELOPMENTAL STABILITY OF THE DROSOPHILA WING: JOINT EFFECTS OF MUTATIONS AND DEVELOPMENTAL TEMPERATURE

The phenotypic effects of genetic and environmental manipulations have been rarely investigated simultaneously. In addition to phenotypic plasticity, their effect on the amount and directions of genetic and phenotypic variation is of particular evolutionary importance because these constitute the material for natural selection. Here, we used heterozygous insertional mutations of 16 genes involved in the formation of the Drosophila wing. The flies were raised at two developmental temperatures (18°C and 28°C). Landmark-based geometric morphometrics was used to analyze the variation of the wing size and shape at different hierarchical levels: among genotypes and temperatures; among individuals within group; and fluctuating asymmetry (FA). Our results show that (1) the phenotypic effects of the mutations depend on temperature; (2) reciprocally, most mutations affect wing plasticity; (3) both temperature and mutations modify the levels of FA and of among individuals variation within lines. Remarkably, the patterns of shape FA seem unaffected by temperature whereas those associated with individual variation are systematically altered. By modifying the direction of available phenotypic variation, temperature might thus directly affect the potential for further evolution. It suggests as well that the developmental processes responsible for developmental stability and environmental canalization might be partially distinct.     

EVOLUTIONARY ADAPTATION TO TEMPERATURE. VII. EXTENSION OF THE UPPER THERMAL LIMIT OF ESCHERICHIA COLI

What factors influence the ability of populations to adapt to extreme environments that lie outside their current tolerance limits? We investigated this question by exposing experimental populations of the bacterium Escherichia coli to lethally high temperatures. We asked: (1) whether we could obtain thermotolerant mutants with an extended upper thermal limit by this selective screen; (2) whether the propensity to obtain thermotolerant mutants depended on the prior selective history of the progenitor genotypes; and (3) how the fitness properties of these mutants compared to those of their progenitors within the ancestral thermal niche. Specifically, we subjected 15 independent populations founded from each of six progenitors to 44°C; all of the progenitors had upper thermal limits between about 40°C and 42°C. Two of the progenitors were from populations that had previously adapted to 32°C, two were from populations adapted to 37°C, and two were from populations adapted to 41–42°C. All 90 populations were screened for mutants that could survive and grow at 44°C. We obtained three thermotolerant mutants, all derived from progenitors previously adapted to 41–42°C. In an earlier study, we serendipitously found one other thermotolerant mutant derived from a population that had previously adapted to 32°C. Thus, prior selection at an elevated but nonlethal temperature may predispose organisms to evolve more extreme thermotolerance, but this is not an absolute requirement. It is evidently possible to obtain mutants that tolerate more extreme temperatures, so why did they not become prevalent during prior selection at 41–42°C, near the upper limit of the thermal niche? To address this question, we measured the fitness of the thermotolerant mutants at high temperatures just within the ancestral niche. None of the four thermotolerant mutants had an advantage relative to their progenitor even very near the upper limit of the thermal niche; in fact, all of the mutants showed a noticeable loss of fitness around 41°C. Thus, the genetic adaptations that improve competitive fitness at high but nonlethal temperatures are distinct from those that permit tolerance of otherwise lethal temperatures.     

PROTOPLAST FUSION AND GENETIC COMPLEMENTATION OF PIGMENT MUTATIONS IN THE RED MICROALGA PORPHYRZDZUM SP.1

Protoplasts from two green pigment mutants of Porphyridium sp. (UTEX 637) containing a low phycoerythrin level were fused by exposure to polyethylene glycol (MW 6000) combined with a short heat shock (45° C, 5 min). Following regeneration on agar plates, red colonies arose in which complementation of the phycoerythrin deficiency had occurred. The complementation frequency was estimated to be 0.2%. Eight progeny showing red pigmentation were isolated and purified by consecutive transfers on agar plates. Characterization of the fusion progeny revealed that their phycobiliprotein and chlorophyll contents per cell were higher than those of their parental mutant strains and, in most strains, similar to that of the wild type. The fusion products proved to be stable over many growth cycles. The DNA content of the wild type and of the parental mutant strains was about 0.05 pg-cell^?1. Fusion progeny strains showed a variable DNA content: a few fusants contained the same amount of DNA as the wild type and the parental strains, while others had about 50% more DNA per cell. The DNA content of one of the progeny strains (CF1c) was double that of the wild type (0.1 pg. cell^?1). Cells of this fusion progeny contained one nucleus per cell, which suggests that nuclear fusion and the formation of a stable diploid followed cell fusion. Analysis of phycobilisome components of CF1c revealed complementation of linker polypeptides associated with phycoerythrin (γ subunits). CF1c contained, like the wild-type strain, four linker polypeptides; all of these were absent in one parental strain and one was absent in the second. To the best of our knowledge, this is the first report of protoplast fusion, formation of somatic hybrids, and the apparent completion of a parasexual cycle in a red microalga.