Dietary Effects on Fitness Components at the PGM-1 Locus of TRIBOLIUM CASTANEUM

The pattern of genetic differentiation among experimental populations of the flour beetle Tribolium castaneum suggested the hypothesis that relative fitness of three genotypes at the PGM-1 locus (or other linked loci) depends directly on diet. This hypothesis was tested by measuring several fitness components (developmental time, survival, fecundity, rate of egg cannibalism) on groups of individuals differing at the PGM-1 locus that were reared on three types of flour (wheat, corn and a mixture of wheat, corn, barley and rye). Flour type had large effects on all traits except larval survival to 3 weeks of age. Relative fitnesses of the three genotypes differed significantly for fecundity. Diet was found to significantly influence the relative developmental times of the three genotypes.     

An Experimental Test of the Relationship between Genetic Variation and Environmental Variation in Tribolium Flour Beetles

The hypothesis that a component of genetic variation for polygenic fitness traits is maintained by environmental heterogeneity was tested using an experimental system involving two species of flour beetles, Tribolium castaneum and T. confusum. Replicated populations of each species from a number of environmental treatments were analyzed for various fitness components following almost 60 generations of natural selection. Environmental differences consisted of flours of cereals commonly invaded by natural populations of these insects.—Tests for adaptation to environments were based on experiments in which populations were reared factorially on each flour, such that population treatment x flour interactions could be detected. Measurements were made of survival, growth rate, larval weight, pupal weight, developmental time, fecundity of individuals at low density and fecundity and cannibalism at high density in both fresh and conditioned media.—Flour differences were found to have significant effects on most traits. Evidence for significant genetic variation and significant genotype x environment interaction was also found. However, no evidence could be found to support the hypothesis that genetic variation was maintained by environmental heterogeneity in food resources. The absence of adaptation to the experimental treatments despite the presence of genetic variation in fitness components suggests that pleiotropy may assume an important role in determining net fitness values of polygenes.     

Fitness of Allozyme Variants in DROSOPHILA PSEUDOOBSCURA. I. Selection at the PGM-1 and ME-2 Loci

We have studied in Drosophila pseudoobscura the effect of allozyme variation on seven fitness components: female fecundity, egg hatchability, egg-to-adult survival under near-optimal and under competitive conditions, rate of development under near-optimal and under competitive conditions, and mating capacity of males. Three genotypes at each of two loci, Pgm-1 and Me-2, have been studied in various combinations. These two loci are highly polymorphic in natural populations of D. pseudoobscura. Statistically significant differences involving one or more genotypes exist for all components of fitness. No single genotype is best for all fitness components; rather the relative fitnesses of genotypes are reversed when different parameters are considered, or when they are studied in different environmental conditions. Also, the average egg-to-adult survival and rate of development are better when different genotypes are reared together than when they occur in pure culture. Four different modes of selection have been uncovered by our experiments. These forms of selection may account for the persistence of the two allozyme polymorphisms in nature, and for previously observed seasonal fluctuations of the allelic frequencies in natural populations.     

RARER NEED NOT BE BETTER IF COMMONER IS WORSE: FREQUENCY-DEPENDENT SELECTION FOR DEVELOPMENTAL TIME AT THE ALCOHOL DEHYDROGENASE LOCUS OF THE OLIVE FRUIT FLY,BACTROCERA OLEAE

Whereas the importance of frequency-dependent selection in life-history traits, behavioral characters and source allocation patterns is widely accepted, its role in governing biochemical and molecular polymorphisms remains poorly understood. Here we demonstrate a case of allozyme frequency-dependent selection. When olive fruit flies (Bactrocera oleae) are reared on an artificial larval medium, an allele at the alcohol dehydrogenase locus that is present in very low frequency in natural populations increases to about one-third in less than five generations. We show here that the time from the hatching of the egg to the eclosion of the adult is affected by the genotype composition of the larval population that grows in the same cup of food. Cultures consisting of one genotype only have the longest developmental time, and two-allele cultures in which the two homozygotes and the heterozygote occur in a 1:1:2 ratio show the shortest developmental time. Cultures with intermediate genotypic compositions show intermediate levels of developmental time. The results can be explained by assuming that the developmental time of a genotype depends on the frequency array of all genotypes in the larval population and is not merely a function of its own frequency. It is even possible that the developmental time of a genotype becomes longer as the genotype becomes rarer, yet the genotype will be favored because the developmental times of the competing genotypes become even longer owing to the associated increase of their frequencies. Given that developmental time is inversely related to fitness, this generates a frequency-dependent selection, with developmental times changing progressively until the population arrives at an equilibrium. One optimum population composition that provides a satisfactory fit to allele frequency changes in our experimental populations is when the two alleles occur in equal frequencies and genotypes are in Hardy-Weinberg proportions. We argue that this type of selection is consistent with the role of alcohol dehydrogenase as a detoxifying enzyme in a medium that undergoes continuous chemical changes during its use by the feeding larvae.     

LIFE-CYCLE COMPONENTS OF SELECTION IN ERIGERON ANNUUS: II. GENETIC VARIATION

Genetic variation for seedling and adult fitness components was measured under natural conditions to determine the relative importance of the seedling stage for lifetime fitness in Erigeron annuus. Variation in lifetime reproductive success can result from both the persistent effects of genetic variation expressed among seedlings and from variation in adult fitness components. Analysis of covariance was used to separate the stage specific from the cumulative effects of genetic variance expressed earlier in the life cycle. E. annuus produces seeds through apomixis, which allowed measurement of the fitness of replicate genotypes from germination through the entire life cycle. There were significant differences among genotypes for date of emergence, seedling size, survivorship and fecundity, but heritabilities were low, indicating slow response to selection. For all characters, environmental components of variance were one to two orders of magnitude larger than genetic variance components, resulting in broad sense heritabilities less than 0.1. For seedling size and fecundity, all of the genetic variance was in the form of genotype-environment interactions, often with large negative genetic correlations across environments. In contrast, genotypes differed in mean survivorship through one year, but there were no genotype-environment interactions for viability. Genetic differences in viability were primarily expressed as differences in overwinter survivorship. Genotype × environment interactions among sites and blocks were generated early in the life cycle while the genotype × environment interactions in response to competitive environment (open, annual cover, perennial cover) first appeared in adult fecundity. Genetic variation in lifetime fitness was not significant, despite a fourfold difference in mean fitness among genotypes.     

Genetic structure,allozyme-habitat associations and reproductive fitness in Gypsophila fastigiata (Caryophyllaceae)

Relationships between allozyme differentiation, habitat variation and individual reproductive success were examined in local populations of a perennial herb, Gypsophila fastigiata, on the Baltic island of Öland (Sweden). Relatively little (c. 2%) of the total allozyme diversity in this largely outcrossing species is explained by differentiation between sites tens of kilometres apart. The low level of geographic differentiation suggests that gene flow between sites is, or has recently been, extensive. Yet the component of allozyme diversity due to differentiation between plots (only tens of meters apart) within sites is 3 times larger than the between-site component of diversity. Allozyme variation, especially at the Pgi-2 locus, is significantly associated with habitat variation within sites. Different allele x habitat combinations for the Pgi-2 locus are associated with differences in individual reproductive fitness. Differential selection in different local habitats may thus contribute to the fine-scale structuring of genetic diversity within sites.     

Genetic differentiation between alpine and lowland populations of a butterfly is related to PGI enzyme genotype

Understanding the ecological process of population differentiation and identifying the molecular changes that contribute to adaptation is a central aspect of evolutionary biology. In this study we analyzed the geographic variation in allozyme allele frequencies (based on 15 enzyme systems representing 18 loci) across 18 populations of the butterfly Lycaena tityrus from different altitudes. Population genetic analyses showed that within population genetic diversity (e.g. mean number of alleles per loci: 1.8; expected heterozygosity: 12%) was within the typical value range for the Lepidoptera. The populations of L. tityrus investigated showed a remarkable genetic differentiation (F_ST: 0.065), being clearly divided into an alpine (high-altitude) and a non-alpine (low-altitude) cluster. This differentiation was almost entirely caused by variation at a single enzyme locus, PGI. Although the involvement of historical events cannot be ruled out, several lines of evidence suggest that the specific pattern of allozyme (and in this case particularly PGI) variation found is caused by thermal selection. For example, genetic variation was highly locus-specific rather than relatively uniform, as should be expected for effects of natural selection. Further, the PGI 2–2 genotype dominating in alpine (in contrast to low-altitude) populations is known to exhibit increased cold stress resistance and relatively long development times typical of alpine populations. Thus, PGI (or possibly a closely linked gene) is an obvious target for thermal selection in L. tityrus . This study exemplifies how allozyme analyses can be used to detect candidate loci likely to be under selection.     

Genetic heterogeneity among intertidal habitats in the flat periwinkle, Littorina obtusata

Comparisons among patterns exhibited by functionally distinct genetic markers have been widely used to infer the impacts of demography and selection in structuring genetic variation in natural populations. However, such multilocus comparisons remain an indirect evaluation of selection at particular candidate loci; ideally, the identification of a candidate gene by comparative genetic methodologies should be complemented by functional analyses and experimental manipulations of genotypes in the laboratory or field. We examined genotype frequency variation among replicated intertidal habitats at two spatial scales in the grazing snail Littorina obtusata. Both of the candidate allozyme markers varied predictably with environment, and these patterns were consistent at both spatial scales. Three of four reference loci were spatially homogeneous, but one microsatellite exhibited significant structure at both geographical and mesoscales. To initiate a direct examination of whether the observed genotype frequency variation at one of the candidate markers, mannose-6-phosphate isomerase (MPI), was impacted by differential survivorship of genotypes, we conducted a series of laboratory-based thermal stress assays using snails from two geographically disparate source populations. When snails were exposed to bouts of thermal/desiccation stress, patterns of mortality were nonrandom with respect to MPI genotype. Furthermore, patterns of mortality in the laboratory manipulation coincided with the observed distribution of genotypes in the field. The data suggest the operation of selection at the Mpi or a linked locus, but functional studies and further experimentation are required to establish the relationship between MPI genotype and fitness across heterogeneous intertidal environments.     

The effect of superoxide dismutase alleles on aging inDrosophila

The effects of superoxide dismutase on aging were tested using two differt experimental approaches. In the first, replicated populations with postponed aging were compared with their controls for frequencies of electrophoretic alleles at the SOD locus. Populations with postponed aging had consistently greater frequencies of the allele coding for more active SOD protein. This allele was not part of a segregating inversion polymorphism. The second experimental approach was the extraction ofSOD alleles from different natural populations followed by the construction of differentSOD genotypes on hybrid genetic backgrounds. This procedure did not uncover any statistical effect ofSOD genotype on hybrid genetic backgrounds. This effects on longevity and fecundity due to the family from which a particularSOD genotype was derived. To detect the effects ofSOD genotypes on longevity with high probability would require a ten-fold increase in the number of families used.     

Analysis of F ST outliers at allozyme loci in Pacific salmon: implications for natural selection

Natural selection has been invoked to explain the observed geographic distribution of allozyme allele frequencies for a number of teleost species. The effects of selection on allozyme loci in three species of Pacific salmon were tested. A simulation-based approach to estimate the null distribution of population differentiation (F _ST) and test for F _ST outliers was used. This approach showed that a majority of allozyme loci conform to neutral expectations predicted by the simulation model, with relatively few F _ST outliers found. No consistent F _ST outlier loci were found across species. Analysis of population sub-groups based on geography and genetic identity reduced the number of outlier loci for some species, indicating that large geographic groups may include genetically divergent populations and/or that there is geographic heterogeneity in selection pressure upon allozyme loci. Two outlier allozyme loci found in this analysis, lactate dehydrogenase-B and malic enzyme, have been shown to be influenced by selection in other teleost species. This approach is also useful in identifying allozyme loci (or other genetic markers) that meet assumptions for population genetic study.     

Electrophoretic variation at flight-related enzyme loci and its possible association with flight capacity inEpiphyas postvittana

Variations at three flight-related enzyme loci, -glycerophosphate dehydrogenase (-Gpdh), glucose-6-phosphate dehydrogenase (G6pd), and phosphoglucomutase (Pgm), ofEpiphyas postvittana (Walker) moths were investigated using starch gel electrophoresis. Among the three enzyme loci, -Gpdh andG6pd were found to be monomorphic, butPgm was polymorphic, with a total of seven different genotypes and five alleles identified in this study. Comparisons of allozyme variability at thePgm locus showed significant differentiation among five natural populations sampled from geographically distinct localities in New Zealand and Australia and between laboratory populations differentiated by artificial selection on flight capacity. ThePgm polymorphism was shown to be associated with the variation of flight capacity, but the role of the enzyme locus in the evolution of flight behavior is to be demonstrated in this species.This work was supported by the research scholarship of the University of New England.     

Genetic variability in Plantago species in relation to their ecology

Summary Genetic variation in leaf and inflorescence morphology and in generative development within the species Plantago major has been analysed by means of crosses between members of two different subspecies. The variable characters chosen are supposed to be important for determining the ecological differences between the subspecies and other ecotypes. The analyses of F₂'s indicated that a substantial number of loci controlling the above mentioned characters are situated near the Pgm-1 locus, forming a gene complex. This gene complex can exist in at least three different forms in ssp. pleiosperma, ssp. major lawn type and ssp. major roadside type, respectively. In addition, some important factors for ecotypic differentiation are situated in the neighbourhood of the Got-1 locus and in a linkage group containing three other allozyme loci. These linkages between allozyme loci and fitness-affecting loci can explain the restriction of some enzyme alleles to a particular subspecies.Grassland Species Research Group Publication No. 50     

Enzyme Polymorphism and Cyclic Parthenogenesis in DAPHNIA MAGNA. I. Selection and Clonal Diversity

Genotype frequencies and fecundities were recorded over a period of two years for three polymorphic enzyme loci (Est, Mdh and Got) in a parthenogenetic natural population of Daphnia magna Straus (Crustacea: Cladocera). There was a large excess of heterozygotes at each locus, and some nonrandom association between loci, although 29 different three-locus genotypes were detected. There were small but significant changes in genotype frequencies that did not follow any clear seasonal cycles or overall trends, and the genotypes often differed significantly in fecundity, although the direction of the difference was not constant. These fitness differences were probably not attributable to the specific loci studied.--Models of balancing selection are of two types: segregation-balanced (e.g., heterosis) and competition-balanced (e.g., frequency dependence). Only the latter type can stabilize diversity in a clonal population. The observed selection was not heterotic, but it is not certain that it was stabilizing either. Clonal competition did not lead to victory by a single, fittest clone; genotypic diversity remained high.     

Effects of founding genetic variation on adaptation to a novel resource

Population genetic theory predicts that adaptation in novel environments is enhanced by genetic variation for fitness. However, theory also predicts that under strong selection, demographic stochasticity can drive populations to extinction before they can adapt. We exposed wheat-adapted populations of the flour beetle (Tribolium castaneum) to a novel suboptimal corn resource, to test the effects of founding genetic variation on population decline and subsequent extinction or adaptation. As previously reported, genetically diverse populations were less likely to go extinct. Here, we show that among surviving populations, genetically diverse groups recovered faster after the initial population decline. Within two years, surviving populations significantly increased their fitness on corn via increased fecundity, increased egg survival, faster larval development, and higher rate of egg cannibalism. However, founding genetic variation only enhanced the increase in fecundity, despite existing genetic variation-and apparent lack of trade-offs-for egg survival and larval development time. Thus, during adaptation to novel habitats the positive impact of genetic variation may be restricted to only a few traits, although change in many life-history traits may be necessary to avoid extinction. Despite severe initial maladaptation and low population size, genetic diversity can thus overcome the predicted high extinction risk in new habitats.     

A Monte Carlo simulation model for studying evolution in age-structured populations

This model provides for any number of genotypes defined by age-specific survival and fecundity rates in a population with completely overlapping generations and growing under the control of density-governing functions affecting survival or fecundity. It is tested in situations involving two alleles at one locus. Nonselection populations at Hardy–Weinberg equilibrium obey the ecogenetic law; i.e., each genotype follows Lotka's law regarding rate of increase and stable age distribution as if it were an independent true-breeding population. Populations experiencing age- and density-independent selection approximate this situation, and the changes in gene frequency are predicted by relative fitnesses bases on λ, the finite rate of increase of the genotypes. Polymorphic gene equilibria occurring at steady-state population sizes are determined by fitnesses based on R, the net reproductive rate. In examples involving differences in generation time produced by age-dependent differences in fecundity, the allele associated with longer generation time may be favored or opposed by selection, depending on whether the density-governing factor controlling population size affects survival or fecundity. If such genotypes have similar R's, a genetic equilibrium may be established if the population is governed by a density function acting upon fecundity. Received: August 23, 1999 / Accepted: July 13, 2000     

Fitness differences associated with Pgi SNP genotypes in the Glanville fritillary butterfly (Melitaea cinxia)

Allozyme variation at the phosphoglucose isomerase (PGI) locus in the Glanville fritillary butterfly (Melitaea cinxia) is associated with variation in flight metabolic rate, dispersal rate, fecundity and local population growth rate. To map allozyme to DNA variation and to survey putative functional variation in genomic DNA, we cloned the coding sequence of Pgi and identified nonsynonymous variable sites that determine the most common allozyme alleles. We show that these single‐nucleotide polymorphisms (SNPs) exhibit significant excess of heterozygotes in field‐collected population samples as well as in laboratory crosses. This is in contrast to previous results for the same species in which other allozymes and SNPs were in Hardy–Weinberg equilibrium or exhibited an excess of homozygotes. Our results suggest that viability selection favours Pgi heterozygotes. Although this is consistent with direct overdominance at Pgi, we cannot exclude the possibility that heterozygote advantage is caused by the presence of one or more deleterious alleles at linked loci.     

A comparative study of cannibalism and predation in seven species of flour beetle

Abstract 1. The present study quantified egg and pupal cannibalism, and interspecific predation on eggs and pupae, by larvae and adults of seven species of flour beetle (Tribolium spp.) under laboratory conditions: T. anaphe, T. brevicornis, T. castaneum, T. confusum, T. destructor, T. freemani, and T. madens. 2. Variation among species in cannibalism and predation propensities did not reflect taxonomic affinities within the genus, indicating that these behaviours were shaped by ecology at species level. 3. Within species, larvae and adults displayed different propensities for cannibalism and predation, leading to the conclusion that these behaviours evolve independently in the two life stages. 4. All species behaved as intraguild predators to some degree, especially in the adult stage. 5. Three general patterns of cannibalism and predation were described by principal component mapping and cluster analysis. 6. The first group comprised three cosmopolitan pest species that were more voracious as adults than as larvae: T. castaneum, T. confusum, and T. destructor. It is proposed that stored product environments select for high adult voracity because the costs associated with emigration from such rare, but resource‐rich, habitats intensifies interference competition among adults. 7. The second group consisted of species that inhabit natural environments and that were more voracious as larvae: T. anaphe, T. freemani, and T. madens. Habitats for these species are probably numerous, but generally poor in quality, a situation that intensifies larval competition, while favouring earlier adult emigration. 8. The largest species, T. brevicornis, demonstrated inconsistent voracity between life stages and was the only species with chemically defended pupae. 9. It is proposed that consumption of eggs provides primarily nutritional benefits, whereas consumption of pupae has a more important role in interference competition.     

Thermal‐specific patterns of longevity and fecundity in a set of heat‐sensitive and heat‐resistant genotypes of Drosophila melanogaster

Fitness‐related traits are often affected by temperature. Heat‐resistant genotypes could influence the dependence of fitness traits on temperature, which should be important in adaptation to directional changes in temperature including global warming. Here, we tested temperature‐dependent variation in longevity and fecundity between Drosophila melanogaster Meigen (Diptera: Drosophilidae) genotypes that differ in heat‐resistance QTL. Longevity and fecundity were affected by heat‐resistance genotypes at constant moderate and high temperature. However, these differences between heat‐resistant and heat‐sensitive genotypes disappeared in a cyclic thermal regime. Analysis with the logistic mortality function indicated that mortality patterns are dependent on temperature and genotype. The results suggest that genotype*temperature interactions are substantial for senescence‐related traits. In particular, fluctuating temperatures can drastically reduce any differences in life‐history traits between heat‐resistance genotypes, even if such genotypes differentially affect the traits at constant temperatures.     

MATE AVAILABILITY AND FECUNDITY SELECTION IN MULTI-ALLELIC SELF-INCOMPATIBILITY SYSTEMS IN PLANTS

We investigate mate availability in different models of multiallelic self-incompatibility systems in mutation-selection-drift balance in finite populations. Substantial differences among self-incompatibility systems occur in average mate availability, and in variances of mate availability among individual plants. These differences are most pronounced in small populations in which low mate availability may reduce seed set in some types of sporophytic self-incompatibility. In cases where the pollination system causes a restriction in the number of pollen genotypes available to an individual plant, the fecundity of that plant depends on the availability of compatible pollen, which is determined by its genotype at the incompatibility locus. This leads to an additional component of selection acting on self-incompatibility systems, which we term “fecundity selection.” Fecundity selection increases the number of alleles maintained in finite populations and increases mate availability in small populations. The strength of fecundity selection is dependent on the type of self-incompatibility. In some cases, fecundity selection markedly alters the equilibrium dynamics of self-incompatibility alleles. We discuss the population genetic consequences of mate availability and fecundity selection in the contexts of conservation management of self-incompatible plant species and experimental investigations on self-incompatibility in natural populations.     

The role of stress proteins in responses of a montane willow leaf beetle to environmental temperature variation

The heat shock response is a critical mechanism by which organisms buffer effects of variable and unpredictable environmental temperatures. Upregulation of heat shock proteins (Hsps) increases survival after exposure to stressful conditions in nature, although benefits of Hsp expression are often balanced by costs to growth and reproductive success. Hsp-assisted folding of variant polypeptides may prevent development of unfit phenotypes; thus, some differences in Hsp expression among natural populations of ectotherms may be due to interactions between enzyme variants (allozymes) and Hsps. In the Sierra willow leaf beetle Chrysomela aeneicollis, which lives in highly variable thermal habitats at the southern edge of their range in the Eastern Sierra Nevada, California, allele frequencies at the enzyme locus phosphoglucose isomerase (PGI) vary across a climatic latitudinal gradient. PGI allozymes differ in kinetic properties, and expression of a 70 kDa Hsp differs between populations, along elevation gradients, and among PGI genotypes. Differences in Hsp 70 expression among PGI genotypes correspond to differences in thermal tolerance and traits important for reproductive success, such as running speed, survival and fecundity. Thus, differential Hsp expression among genotypes may allow functionally important genetic variation to persist, allowing populations to respond effectively to environmental change.