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.     

Direct and correlated responses to selection for larval ethanol tolerance in Drosophila melanogaster

Ethanol is an important larval resource and toxin for natural Drosophila melanogaster populations, and ethanol tolerance is genetically variable within and among populations. If ethanol‐tolerant genotypes have relatively low fitness in the absence of ethanol, as suggested by the results of an earlier study, genetic variation for ethanol tolerance could be maintained by variation in ethanol levels among breeding sites. I selected for ethanol tolerance in large laboratory populations by maintaining flies on ethanol‐supplemented media. After 90 generations, the populations were compared with control populations in egg‐to‐adult survival and development rate on ethanol‐supplemented and unsupplemented food. When compared on ethanol‐supplemented food, the ethanol‐selected populations had higher survival and faster development than the control populations, but on unsupplemented food, the populations did not differ in either trait. These results give no evidence for a ‘trade‐off’ between the ability to survive and develop rapidly in the presence of ethanol and the ability to do so in its absence. The effect of physiological induction of ethanol tolerance by exposing eggs to ethanol was also investigated; exposing eggs to ethanol strongly increased subsequent larval survival on ethanol‐supplemented food, but did not affect survival on regular food, and slowed development on both ethanol‐supplemented and regular food, partly by delaying egg hatch.     

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.     

Selection for mating propensity in irradiated populations of the cotton boll weevil

Summary Twelve generations of family selection for 10-day post-irradiation male mating propensity resulted in significant divergence between the selected and unselected control populations. Much of this divergence was the result of a decline in the control population which was believed to have been a function of both inbreeding and environmental effects. Significant correlated responses as measured by differences in the two populations for linear time trends of performance on generations of selection were observed for 10-day post-irradiation survival, percent survivors mating at 10 days, and 7-day egg production of unirradiated females.     

High temperature intensifies negative density dependence of fitness in red flour beetles

Competition for food, space, or other depletable resources has strong impacts on the fitness of organisms and can lead to a pattern known as negative density dependence, where fitness decreases as population density increases. Yet, many resources that have strong impacts on fitness are nondepletable (e.g., moisture or temperature). How do these nondepletable resources interact with depletable resources to modify negative density dependence? We tested the hypothesis that negative density dependence is modulated by temperature in red flour beetles and tested the prediction that the strength of negative density dependence should decrease as temperature decreases. We measured the number of eggs laid, offspring development time, and the number of offspring that reached maturity at three temperatures and two food treatment combinations as we simultaneously manipulated adult population density. We demonstrated that low temperatures weaken negative density dependence in the number of eggs laid; this pattern was most evident when food was abundant. Density had no effect on development time, but low temperatures increased development time. The percent of eggs that emerged as adults decreased with both density and temperature and increased with food. Temperature, an abiotic driver, can thus modulate density-dependent processes in ectotherms. Therefore, models of population growth for ectotherms should incorporate the effects of temperature.     

Does fitness erode in the absence of selection? An experimental test with Tribolium

In the absence of natural selection, average fitness in the population is expected to decline due to the accumulation of deleterious mutations. Replicate populations of flour beetles (Tribolium confusum) were maintained for 22 generations in the virtual absence of selection (random mating, favorable environment, excess of food, and mortality of only 3%). Larva-to-adult survivorship rates were similar in the stock population and selection-free populations. In contrast, starvation resistance of adult beetles from selection-free populations was significantly reduced (by more than 2% per generation). When tested in the favorable environment, beetles in one selection-free population had significantly slower development and smaller sizes of females than beetles from the stock population. Since such changes in these fitness components are usually maladaptive, they indicate possible erosion of fitness under relaxed selection at the rate of 0.1-0.2% per generation. No fitness erosion was detectable in the second selection-free population.     

Phenotypic variation in invasive and biocontrol populations of the harlequin ladybird, Harmonia axyridis

Despite numerous releases for biological control purposes during more than 20 years in Europe, Harmonia axyridis failed to become established until the beginning of the 21st century. Its status as invasive alien species is now widely recognised. Theory suggests that invasive populations should evolve toward greater phenotypic plasticity because they encounter differing environments during the invasion process. On the contrary, populations used for biological control have been maintained under artificial rearing conditions for many generations; they are hence expected to become specialised on a narrow range of environments and show lower phenotypic plasticity. Here we compared phenotypic traits and the extent of adaptive phenotypic plasticity in two invasive populations and two populations commercialized for biological control by (i) measuring six phenotypic traits related to fitness (eggs hatching rate, larval survival rate, development time, sex ratio, fecundity over 6 weeks and survival time of starving adults) at three temperatures (18, 24 and 30°C), (ii) recording the survival rate and quiescence aggregation behaviour when exposed to low temperatures (5, 10 and 15°C), and (iii) studying the cannibalistic behaviour of populations in the absence of food. Invasive and biocontrol populations displayed significantly different responses to temperature variation for a composite fitness index computed from the traits measured at 18, 24 and 30°C, but not for any of those traits considered independently. The plasticity measured on the same fitness index was higher in the two invasive populations, but this difference was not statistically significant. On the other hand, invasive populations displayed significantly higher survival and higher phenotypic plasticity when entering into quiescence at low temperatures. In addition, one invasive population displayed a singular cannibalistic behaviour. Our results hence only partly support the expectation of increased adaptive phenotypic plasticity of European invasive populations of H. axyridis, and stress the importance of the choice of the environmental parameters to be manipulated for assessing phenotypic plasticity variation among populations.     

Food deprivation-dependent development and fecundity in Ophraella communa

Ophraella communa LeSage is native to North America and a biological control agent of the invasive weed Ambrosia artemisiifolia L. Since A. artemisiifolia plants grow old and die after September annually, O. communa suffers from food shortage. To understand the effect of food shortage or deprivation on population fitness of O. communa, the development and fecundity and hatchability of its progeny eggs were observed when larvae were offered A. artemisiifolia plants for either 3, 6, 12 or 24 hours daily. The results showed that larval food deprivation significantly influenced survival and developmental durations of larvae and pupae. Survival rates and developmental durations of larvae and pupae decreased and were prolonged significantly with decreasing time of larval daily food intake. Longevity and fecundity of adults shortened and decreased significantly with decreasing time of larval daily food intake. In addition, the hatch rates of progeny eggs decreased significantly with decreasing time of larval daily food intake. The present study suggests that food shortage is one of the most critical factors that suppresses O. communa populations. This can explain why field populations of the beetle decrease significantly with ageing and death of A. artemisiifolia plants after late September.     

LIFE-HISTORY CONSEQUENCES OF ADAPTATION TO LARVAL NUTRITIONAL STRESS IN DROSOPHILA

Many animal species face periods of chronic nutritional stress during which the individuals must continue to develop, grow, and/or reproduce despite low quantity or quality of food. Here, we use experimental evolution to study adaptation to such chronic nutritional stress in six replicate Drosophila melanogaster populations selected for the ability to survive and develop within a limited time on a very poor larval food. In unselected control populations, this poor food resulted in 20% lower egg‐to‐adult viability, 70% longer egg‐to‐adult development, and 50% lower adult body weight (compared to the standard food on which the flies were normally maintained). The evolutionary changes associated with adaptation to the poor food were assayed by comparing the selected and control lines in a common environment for different traits after 29–64 generations of selection. The selected populations evolved improved egg‐to‐adult viability and faster development on poor food. Even though the adult dry weight of selected flies when raised on the poor food was lower than that of controls, their average larval growth rate was higher. No differences in proportional pupal lipid content were observed. When raised on the standard food, the selected flies showed the same egg‐to‐adult viability and the same resistance to larval heat and cold shock as the controls and a slightly shorter developmental time. However, despite only 4% shorter development time, the adults of selected populations raised on the standard food were 13% smaller and showed 20% lower early‐life fecundity than the controls, with no differences in life span. The selected flies also turned out less tolerant to adult malnutrition. Thus, fruit flies have the genetic potential to adapt to poor larval food, with no detectable loss of larval performance on the standard food. However, adaptation to larval nutritional stress is associated with trade‐offs with adult fitness components, including adult tolerance to nutritional stress.     

EXPERIMENTAL EVOLUTION OF ACCELERATED DEVELOPMENT IN DROSOPHILA. 1. DEVELOPMENTAL SPEED AND LARVAL SURVIVAL

Developmental time is a trait of great relevance to fitness in all organisms. In holometabolous species that occupy ephemeral habitat, like Drosophila melanogaster, the impact of developmental time upon fitness is further exaggerated. We explored the trade-offs surrounding developmental time by selecting 10 independent populations from two distantly related selection treatments (CB_1-5 and CO_1-5) for faster development. After 125 generations, the resulting accelerated populations (ACB_1-5 and ACO_1-5) displayed net selection responses for development time of -33.4 hours (or 15%) for ACB and -38.6 hours (or 17%) for ACO. Since most of the change in egg-to-adult developmental time was accounted for by changes in larval duration, the “accelerated” larvae were estimated to develop 25-30% faster than their control/ancestor populations. The responses of ACB and ACO lines were remarkably parallel, despite being founded from populations evolved independently for more than 300 generations. On average, these “A” populations developed from egg to adult in less than eight days and produced fertile eggs less than 24 hours after emerging. Accelerated populations showed no change in larval feeding rate, but a reduction in pupation height, the latter being a trait relating to larval energetic expenditure in wandering prior to pupation. This experiment demonstrates the existence of a negative evolutionary correlation between preadult developmental time and viability, as accelerated populations experienced a severe cost in preadult survivorship. In the final assay generation, viability of accelerated treatments had declined by more than 10%, on average. A diallel cross demonstrated that the loss of viability in the ACO lines was not due to inbreeding depression. These results suggest the existence of a rapid development syndrome, in which the fitness benefits of fast development are balanced by fitness costs resulting from reduced preadult survivorship, marginal larval storage of metabolites, and reduced adult size.     

小菜蛾对Bt毒素Cry1Ac和Bt制剂抗性的选育及其抗性种群的生物学适应性

用Bt制剂和Bt毒素Cry1Ac分别对源自深圳田间的小菜蛾Plutella xylostella在室内进行抗性种群选育,获得相对抗性倍数分别为24.36、38.16倍的抗性种群DBM.1Ac-R30和DBM.Bt-R46。对这2个抗性种群及其敏感种群（DBM.Bt-S）的生长发育、存活及繁殖特征进行了详细地观察与比较,并以甘蓝为饲喂材料构建了2个抗性实验种群的生命表。结果表明,DBM.1Ac-R30和DBM.Bt-R46种群较DBM.Bt-S种群的产卵量和孵化率下降,幼虫发育历期延长,雌雄比显著降低,雌成虫数量、寿命减少。DBM.1Ac-R30和DBM.Bt-R46种群相对于DBM.Bt-S种群的相对适合度分别为0.75和0.65,抗性种群在繁殖能力上存在明显的生存劣势。     

Assessment of fitness costs of resistance against the parasitoid Leptopilina victoriae in Drosophila bipectinata

How insects evolve resistance or counter-resistance against antagonists is a basic issue in the study of host-parasitoid coevolution. One of the factors that affect their coevolution is fitness costs of resistance and counter-resistance. Here, we assess fitness costs of resistance against the parasitoid Leptopilina victoriae in Drosophila bipectinata on the basis of selection experiments. We made a base population by mixing three geographic fly populations that differed in resistance. The established base population was divided into four populations, two for selection of resistance against a L. victoriae population and two for control. Resistance increased rapidly in response to selection and reached a very high level within four generations in the selected populations, while resistance of the control populations remained low during 20 generations. High resistance of the selected populations was maintained at least for 10 generations after selection was stopped. Both selected populations had lower female longevity than the control populations, and at least one of the selected populations had shorter thorax length, lower female desiccation tolerance and adult heat tolerance than both or either of the control populations. On the other hand, the selected populations had higher male starvation tolerance and longevity than the control populations. There were no significant differences in resistance against another population of L. victoriae and two other parasitoid species between the selected and control populations. These results suggest that the resistance against the L. victoriae population in D. bipectinata may incur some but not so high costs and act parasitoid-species- and/or parasitoid-population-specifically.     

Costs and stability of cabbage looper resistance to a nucleopolyhedrovirus

The goal of this study was to examine the possible costs and the stability of the resistance of cabbage loopers (Trichoplusia ni) to the single (S) nucleocapsid nucleopolyhedrovirus of T. ni (TnSNPV). Resistance to the virus did not appear to incur any measurable fitness costs under laboratory conditions. When reared in the absence of the virus, there was no difference in the number of eggs produced or egg hatch of control and selected individuals. There even was a tendency for selected cabbage loopers to develop faster and to produce heavier pupae. The difference in the pupal weight and developmental time of control and selected T. ni did not covary with the number of generations of selection. Furthermore, the offspring of hybrid crosses (control × selected moths) were as fit as those of pure pairings (control × control or selected × selected adults). Finally, the resistance of cabbage loopers to TnSNPV did not decline when exposure to the virus ceased for nine generations.     

COSTS AND BENEFITS OF MOSQUITO REFRACTORINESS TO MALARIA PARASITES: IMPLICATIONS FOR GENETIC VARIABILITY OF MOSQUITOES AND GENETIC CONTROL OF MALARIA

The problem of fitness costs associated with host resistance to parasitism is related to the evolution of parasite virulence, population genetic diversity and the dynamics of host-parasite relationships, and proposed strategies for disease control through the genetic manipulation of mosquito vectors. Two Aedes aegypti populations, refractory and susceptible to Plasmodium gallinaceum, were previously selected from the Moyo-In-Dry strain (MOYO) through inbreeding (F = 0.5). Reproductive success and survivorship of the two populations were compared, and the influence of the parasite on mosquito fitness also was evaluated. Fitness components studied include fecundity, adult survivorship and egg-to-adult developmental time, blood-meal size, and adult body size. The refractory population has a significantly shorter egg-to-adult developmental time and a smaller body size, takes a smaller blood meal, and subsequently lays fewer eggs than the susceptible population. The mean longevity of the refractory population is significantly shorter than the susceptible population. Exposure to the parasite exhibited little effect on the survivorship and fecundity of either population. Several factors may contribute to the lower fitness of the refractory population, including founder effect, inbreeding depression, the effect of other uncharacterized genes linked to genes conferring refractoriness, and pleiotropic effects associated with these genes. The results are discussed in relation to the genetic diversity of natural mosquito populations and their implications for the genetic control of malaria.     

THE GENETICS OF CENTRAL AND MARGINAL POPULATIONS OF DROSOPHILA SERRATA. II. HYBRID BREAKDOWN IN FITNESS COMPONENTS AS A CORRELATED RESPONSE TO SELECTION FOR DESICCATION RESISTANCE

Multiple-peak epistasis is one of the four premises that underlie Wright's shifting-balance theory of evolution. A selection experiment was conducted in an attempt to push different geographic populations to different fitness peaks as a correlated response to selection for an additively controlled character (desiccation resistance). Four populations of Drosophila serrata, sampled from central and marginal areas of its distribution along a 3000-km stretch of Australia's east coast, underwent selection for desiccation resistance for 14 generations. After selection had ceased, control lines from each of the populations were crossed to determine the amount of hybrid breakdown that existed before selection and selected lines were crossed to determine the amount of hybrid breakdown after selection. Hybrid breakdown was measured in three fitness traits: developmental time, viability, and fecundity. When the individual crosses were examined, virtually no evidence was found for hybrid breakdown between these populations. However, the level of hybrid breakdown in development time in the control lines increased as the distance between the populations in the field increased. This relationship was lost in the selected lines. Therefore, selection for desiccation resistance influenced the level of hybrid breakdown in a fitness trait, although selection may need to be maintained for longer than 14 generations if a new relationship between hybrid breakdown and distance is to be formed.     

Bacteriological survey of sixty health foods

A bacteriological survey was performed on 1,960 food samples encompassing 60 types of health foods available in the Baltimore-Washington, D.C., metropolitan area. No consistent bacteriological distinction (aerobic plate counts, total coliform and fecal coliform most probable numbers) was observed between foods labeled as organic (raised on soil with compost or nonchemical fertilizer and without application of pesticides, fungicides, and herbicides) and their counterpart food types bearing no such label. Types and numbers of samples containing Salmonella were: sunflower seeds, 4; soy flour, 3; soy protein powder, 2; soy milk powder, 1; dried active yeast, 1; brewers' years, 1; rye flour, 1; brown rice, 1; and alfalfa seeds,1. The occurrence of this pathogen in three types of soybean products should warrant further investigation of soybean derivatives as potentially significant sources of Salmonella.     

Bacteriological survey of sixty health foods.

A bacteriological survey was performed on 1,960 food samples encompassing 60 types of health foods available in the Baltimore-Washington, D.C., metropolitan area. No consistent bacteriological distinction (aerobic plate counts, total coliform and fecal coliform most probable numbers) was observed between foods labeled as organic (raised on soil with compost or nonchemical fertilizer and without application of pesticides, fungicides, and herbicides) and their counterpart food types bearing no such label. Types and numbers of samples containing Salmonella were: sunflower seeds, 4; soy flour, 3; soy protein powder, 2; soy milk powder, 1; dried active yeast, 1; brewers' years, 1; rye flour, 1; brown rice, 1; and alfalfa seeds,1. The occurrence of this pathogen in three types of soybean products should warrant further investigation of soybean derivatives as potentially significant sources of Salmonella.     

Patterns and dynamics of rapid local adaptation and sex in varying habitat types in rotifers

Local adaptation is an important principle in a world of environmental change and might be critical for species persistence. We tested the hypothesis that replicated populations can attain rapid local adaptation under two varying laboratory environments. Clonal subpopulations of the cyclically parthenogenetic rotifer Brachionus calyciflorus were allowed to adapt to two varying harsh and a benign environment: a high‐salt, a food‐limited environment and untreated culture medium (no salt addition, high food). In contrast to most previous studies, we re‐adjusted rotifer density to a fixed value (two individuals per ml) every 3–4 days of unrestricted population growth, instead of exchanging a fixed proportion of the culture medium. Thus our dilution regime specifically selected for high population growth during the entire experiment and it allowed us to continuously track changes in fitness (i.e., maximum population growth under the prevailing conditions) in each population. After 56 days (43 asexual and eight sexual generations) of selection, the populations in the harsh environments showed a significant increase in fitness over time relative to the beginning compared to the population in untreated culture medium. Furthermore, the high‐salt population exhibited a significantly elevated ratio of sexual offspring from the start of the experiment, which suggested that this environment either triggered higher rates of sex or that the untreated medium and the food‐limited environment suppressed sex. In a following assay of local adaptation we measured population fitness under “local” versus “foreign” conditions (populations adapted to this environment compared to those of the other environment) for both harsh habitats. We found significantly higher fitness values for the local populations (on average, a 38% higher fitness) compared to the foreign populations. Overall, local adaptation was formed rapidly and it seemed to be more pronounced in the high‐salt treatment.     

嗜卷书虱的实验生态研究

在8种温度(17.5～35℃)及5种相对湿度(50%、60%、70%、80%和90%)条件下,以全麦粉、酵母粉、脱脂奶粉(10∶1∶1)为食料,系统研究了嗜卷书虱Liposcelis bostrychophila Badonnel实验种群生态学特性。获得了该虫在不同温、湿度条件下生长发育、生存和繁殖的一系列特性和参数,包括各虫态发育历期、存活率、发育起点温度、有效积温和平均繁殖力等,并建立了发育速率、存活率及繁殖力的理论模型。     

Large Genetic Change at Small Fitness Cost in Large Populations of Drosophila Melanogaster Selected for Wind Tunnel Flight: Rethinking Fitness Surfaces

The fitness effects of extreme genetic change by selection were studied in large populations subjected to prolonged, intense selection. Two replicate populations of Drosophila melanogaster, with estimated effective sizes 500 <= N(e) <= 1000, were selected for increased performance in a wind tunnel, selecting on average the fastest 4.5% of flies. The mean apparent flying speed of both lines increased from ~2 to 170 cm/sec and continued to respond at diminishing rates, without reaching a plateau, for 100 generations. Competitive fitness tests in generations 50 and 85 showed minimal or no fitness loss in selected lines compared to controls. Sublines relaxed in generations 65 and 85 showed minimal or no regression in apparent flying speed. Hybrid lines, from a cross of selected X control lines in generation 75, responded to reselection saltationally, showing that the chromosomes of the selected lines had been assembled from alleles at many loci, from many different chromosomes in the base population. Thus, major genetic change was achieved, but without the costs usually associated with strong directional selection. Large population size has been interpreted, in opposing models, as either a brake or an accelerator in its effects on long-term change by selection. These results favor the second model, and challenge the concept of rugged fitness surfaces underlying the first model.