EVOLUTION OF THE GENETIC ARCHITECTURE UNDERLYING FITNESS IN THE PITCHER-PLANT MOSQUITO,WYEOMYIA SMITHII

We examined the genetic basis for evolutionary divergence among geographic populations of the pitcher-plant mosquito, Wyeomyia smithii, using protein electrophoresis and line-cross analysis. Line-cross experiments were performed under both low density, near-optimal conditions, and at high, limiting larval densities sufficient to reduce fitness (r_c) in parental populations by approximately 50%. We found high levels of electrophoretic divergence between ancestral and derived populations, but low levels of divergence between two ancestral populations and between two derived populations. Assessed under near-optimal conditions, the genetic divergence of fitness (r_c) between ancestral and derived populations, but not between two derived populations or between two ancestral populations, has involved both allelic (dominance) and genic (epistatic) interactions. The role of dominance and epistasis in the divergence of r_c among populations affects its component traits in a pattern that is unique to each cross. Patterns of genetic differentiation among populations of W. smithii provide evidence for a topographically complex “adaptive landscape” as envisioned by Wright in his “shifting balance” theory of evolution. Although we cannot definitively rule out the role of deterministic evolution in the divergence of populations on this landscape, ecological inference and genetic data are more consistent with a stochastic than a deterministic process. At high, limiting larval density, hybrid vigor is enhanced and the influence of epistasis disappears. Thus, under stressful conditions, the advantages to fitness due to hybrid heterozygosity can outweigh the deleterious effects of fragmented gene complexes. These results have important implications for the management of inbred populations. Outbreeding depression assessed in experimental crosses under benign lab, zoo, or farm conditions may not accurately reveal the increased advantages of heterozygosity in suboptimal or marginal conditions likely to be found in nature.     

Genetic coordination of demography and phenology in the pitcher-plant mosquito,Wyeomyia smithii

Demographic and phenological traits compose the basic elements of an insect's life-history strategy in a seasonal environment. An insect's long-term fitness depends on its ability to exploit favorable conditions, to avoid unfavorable conditions, and to convert from one life style to the other. For the pitcher-plant mosquito, Wyeomyia smithii, we show that genetic variation exists in both development time (a demographic trait) and critical photoperiod (a phenological trait) in six populations spanning much of the species' geographical range. During the northward range expansion of W. smithii in North America, these traits have evolved independently under strong directional and stabilizing selection. The correlated response in critical photoperiod to divergent selection on development time reveals significantly positive genetic correlations in five populations and a negative correlation in one population. The positive correlations form a genetically coordinated phenotype: faster developing individuals use a shorter photoperiodic switch point and are able to exploit the late favorable season; slower developing individuals use a longer photoperiodic switch point and are able to avoid extending development into the unfavorable season. This genetic coordination of demography and phenology has not, however, prevented their independent evolution. We propose that evolutionary flexibility in W. smithii may arise in part from the reorganization of their genetic architecture following repeated founder events during their northward invasion of North America.     

Population Dynamics of Evolutionary Change: Demographic Parameters as Indicators of Fitness

I evaluated demographic parameters as indicators of fitness by calculating the net reproductive rate (R₀), exponential rate of change (r), lifetime reproductive success (LRS), and Malthusian parameter (m) for nine genotypes and four phenotypes (two alleles at each of two independent loci) of an age-structured population. The given starting conditions included age-specific survival rates of males and females and age-specific fecundity of females for each genotype (to simplify the problem I presumed no differences in survivorship or fecundity of genotypes with the same phenotype) and the same age structure for each genotype. The prevailing genotype had the greatestm, but it did not have the greatestr,R₀, or LRS, or even the greatest survivorship of either juveniles or adults, or the greatest fecundity. This result indicates thatmis the only correct measure of fitness (i.e., as a predictor of which genotype should prevail from among a group of genotypes) and that comparisons ofr,R₀, LRS, juvenile or adult survival rates, or fecundity may be misleading indicators of which genotype should prevail (i.e., be most “fit”) over time (i.e., be selected for).     

Impact of inter‐ and intra‐specific competition among larvae on larval,adult, and life‐table traits of Aedes aegypti and Aedes albopictus females

1. Few studies have taken a comprehensive approach of measuring the impact of inter‐ and intra‐specific larval competition on adult mosquito traits. In this study, the impact of competition among Aedes aegypti (L.) and A. albopictus (Skuse) was quantified over the entire life of a cohort. 2. Competitive treatments affected hatch‐to‐adult survivorship and the development time to adulthood of females for both species but affected the median wing length of females only for A. albopictus. Competitive treatments had no significant effect on the median adult female longevity nor were there any effects on other individual traits related to blood feeding and reproductive success. 3. Analysis of life table traits revealed no effect of competitive treatment on the net reproductive rate (R₀) but there were significant effects on the cohort generation time (T_c) and the cohort rate of increase (r) for both species. 4. Inter‐ and intra‐specific competition among Aedes larvae may produce individual and population‐level effects that are manifest in adults; however, benign conditions may enable resulting adults to compensate for some impacts of competition, particularly those affecting blood‐feeding success, fecundity, and the net reproductive rate, R₀. The effect of competition, therefore, affects primarily larva‐to‐adult survivorship and the larval development time, which in turn impacts the cohort generation time, T_c, and ultimately the cohort rate of increase, r. 5. The lack of effects of the larval rearing environment on adult longevity suggests that effects on vectorial capacity owing to longevity may be limited if adults have easy access to sugar and bloodmeals.     

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 capacity for increase at a low temperature of some Australian populations of the granary weevil,Sitophilus granarius (L.)

The capacity for increase (r_c) of one laboratory and seven field populations of young adult S. granarius from different sites in Australia was determined over thirty-two weeks at 15°C in wheat of 14% moisture content. The mean value of r_c was 0.0704 ±0.0016 and the populations differed significantly with respect to this parameter. Variation in the net reproductive rate (R₀), which averaged 25.4± 1.29, had a greater effect on the value of I_c than did variation in the cohort generation time (T_c), which averaged 45.7±0.37 weeks. The populations did not differ significantly in terms of adult survivorship and 94% of females were still alive at thirty-two weeks. The maximum rate of oviposition, about ten eggs per female per week, was achieved in the eighteen-twenty week age-interval. About 41% of the immature stages survived to adulthood. Estimates of r_c over a twenty-four week period were only slightly lower than those over thirty-two weeks. The capacity for increase at 15°C of a given population was shown to be correlated with its fertility at 29°C, an optimal temperature, and with its body weight rather than with its cold tolerance, as indicated by its chill-coma temperature, or its previous temperature-history. The temperature experienced by the immature stages had a marked influence on r_c in that weevils reared at 15°C and 27°C had respective values of 0.0654 and 0.0786 when subsequently held at 15°C. The differences in the survivorship and fertility of S. granarius and S. oryzae (L.) when both species were reared at 15°C are considered.     

Propagule interactions and the evolution of virulence

The evolution of parasite virulence is thought to involve a trade‐off between parasite reproductive rate and the effect of increasing the number of propagules on host survivorship. Such a trade‐off should lead to selection for an intermediate level of within‐host reproduction (λ). Here I consider the effects of parasite propagule number on selection affecting λ when (i) the effect of each propagule is independent of propagule number, and (ii) when the effect of each propagule changes as a function of propagule number. Virulence evolves in these models as a correlated response to selection on λ. If each propagule has the same effect (s) as all previous propagules, the survivorship of infected hosts is reduced by more than 60% at equilibrium, independent of the value of s. If, instead, each propagule has a more negative effect on host survivorship than previous propagules, host survivorship at equilibrium is expected to increase as the effect becomes more pronounced. These results are directly parallel to results derived for population mean fitness at mutation‐selection balance; and they suggest that high virulence should be associated with parasites for which the effect of adding propagules either remains constant or diminishes with propagule number.     

Studies on the color variation in larvae ofEphestia kühniella Zeller (Lepidoptera: Phycitidae) III. Fitness of different larval color strains

The population fitness in terms of the intrinsic rate of increase r_m was measured in eight pure (homogenic) strains of Ephestia kühniellaZeller with different larva color at an optimal temperature of 25°C, and in three strains at unfavorble temperatures of 15, 17, 28, and 30°C, to understand a mechanism of maintenance of a larval color variation found in wild populations. The survival rate, hatchability, and gross rate of reproduction were poorly correlated with the fitness but the mean generation time and net reproduction rate were correlated with the fitness significantly at 25°C. Intermediate color (pink) strain(s) grew faster, initiated reproduction earlier and had shorter longevities than other strain(s) under the range of 15 and 28°C; the fitness was highest in the intermediate larval color strains except at 30°C in which all strains had a negative rate of increase. The results were discussed with reference to the relationship between the larval color and fitness along with a maintenance mechanism of the variation.     

Predicting evolutionary responses to selection on polyandry in the wild: additive genetic covariances with female extra-pair reproduction

The evolutionary forces that underlie polyandry, including extra-pair reproduction (EPR) by socially monogamous females, remain unclear. Selection on EPR and resulting evolution have rarely been explicitly estimated or predicted in wild populations, and evolutionary predictions are vulnerable to bias due to environmental covariances and correlated selection through unmeasured traits. However, evolutionary responses to (correlated) selection on any trait can be directly predicted as additive genetic covariances (cov_A) with appropriate components of relative fitness. I used comprehensive life-history, paternity and pedigree data from song sparrows (Melospiza melodia) to estimate cov_A between a female''s liability to produce extra-pair offspring and two specific fitness components: relative annual reproductive success (ARS) and survival to recruitment. All three traits showed non-zero additive genetic variance. Estimates of cov_A were positive, predicting evolution towards increased EPR, but 95% credible intervals overlapped zero. There was therefore no conclusive prediction of evolutionary change in EPR due to (correlated) selection through female ARS or recruitment. Negative environmental covariance between EPR and ARS would have impeded evolutionary prediction from phenotypic selection differentials. These analyses demonstrate an explicit quantitative genetic approach to predicting evolutionary responses to components of (correlated) selection on EPR that should be unbiased by environmental covariances and unmeasured traits.     

Occurrence of an herbicide‐resistant plant trait in agricultural field margins

Agricultural environments allow study of evolutionary change in plants. An example of evolution within agroecological systems is the selection for resistance to the herbicide glyphosate within the weed, Conyza canadensis. Changes in survivorship and reproduction associated with the development of glyphosate resistance (GR) may impact fitness and influence the frequency of occurrence of the GR trait. We hypothesized that site characteristics and history would affect the occurrence of GR C. canadensis in field margins. We surveyed GR occurrence in field margins and asked whether there were correlations between GR occurrence and location, crop rotation, GR crop trait rotation, crop type, use of tillage, and the diversity of herbicides used. In a field experiment, we hypothesized that there would be no difference in fitness between GR and glyphosate‐susceptible (GS) plants. We asked whether there were differences in survivorship, phenology, reproduction, and herbivory between 2 GR and 2 GS populations of C. canadensis in agrestal and ruderal habitats. We found that geographic location was an important factor in the occurrence of GR C. canadensis in field margins. Although not consistently associated with either glyphosate resistance or glyphosate susceptibility, there were differences in phenology, survivorship, and herbivory among biotypes of C. canadensis. We found equal or greater fitness in GR biotypes, compared to GS biotypes, and GR plants were present in field margins. Field margins or ruderal habitats may provide refugia for GR C. canadensis, allowing reproduction and further selection to occur as seeds recolonize the agrestal habitat. Agricultural practices may select for ecological changes that feed back into the evolution of plants in ruderal habitats.     

Genetic Analysis of Natural Populations of DROSOPHILA MELANOGASTER in Japan. II. the Measurement of Fitness and Fitness Components in Homozygous Lines

Fifty lethal-free, sterility-free isogenic lines of Drosophila melanogaster that were randomly sampled from a natural population were tested for net fitness and other components of fitness by competition with D. hydei. Larval viability and developmental time were also measured using the balanced marker method. Distribution patterns of these fitness components were similar, but correlation between the fitness components varied depending on the combinations used. The highest correlations were obtained between net fitness and productivity (r_p = 0.6987, r_g = 0.9269). The correlation between net fitness and total larval viability was much lower (r_p = 0.1473 and r_g = 0.2171). These results indicate that measuring net fitness, not just a component of fitness, is necessary for the good understanding of the genetic structures of natural populations.     

A comparison of the capacity for increase at a low temperature of foreign and Australian populations of Sitophilus oryzae (L.) and S. granarius (L.)

The capacity for increase (r_c) of three populations of S. oryzae and three populations of S. granarius from Great Britain and Canada was determined over 15 and 24 weeks respectively at 15°C in wheat of 14% moisture content. The fertility of these populations at 27°C, their body weight and chill-coma threshold was also measured. With both S. oryzae and S. granarius, values of r_c for the foreign populations fell within the range previously observed when several Australian populations were reared under the same conditions. The cohort generation times (T_c) of the foreign S. oryzae populations were significantly shorter than those of the Australian populations because of the shorter immature development periods (D) of the former. Two of the foreign S. granarius populations had lower values for D than all but one of their Australian counterparts but their values of T_c were within the range previously observed for Australian weevils. When the Australian and foreign populations were considered together, the value of r_c was more strongly correlated with fertility at 27°, an optimal temperature, than with body weight in both S. oryzae and S. granarius. With S. oryzae, r_c was correlated with the chill-coma threshold of cold-acclimated weevils only. There were no correlations between r_c and chill-coma thresholds in S. granarius. It was concluded that the observed differences in r_c were related to differences in the vigour of the populations rather than to physiological differences in cold-tolerance. The likelihood of grain weevils becoming cold-tolerant as a result of grain aeration is discussed and the practical significance of differences in r_c considered.     

Hourglass and rhythmic components of photoperiodic time measurement in the pitcher plant mosquito, Wyeomyia smithii

The mosquito, Wyeomyia smithii, enters a larval dormancy or diapause that is initiated, maintained, and terminated by photoperiod. The median or critical photoperiod regulating diapause increases from 12 h of light per day along the Gulf of Mexico, USA (30° N), to over 15 h in southern Canada (49° N). Photoperiodic time measurement in W. smithii comprises both rhythmic and hourglass (interval timer) components. Using interrupted-night and resonance experiments, we show that both the rhythmic and hourglass components are prominent in the southern (ancestral) populations and that the influence of the rhythmic component declines with increasing latitude, while the hourglass component remains strong in northern (derived) populations. Previously, it has been shown that the genetic differences in critical photoperiod between northern populations and their southern ancestors involve not only the additive (independent) effects of genes, but also gene-gene interaction (epistasis). We therefore conclude that adaptive evolution of W. smithii has probably involved the progressive epistatic masking of the ancestral rhythmic component resulting in photoperiodic time measurement in northern populations accomplished principally through a day-interval timer. A comparison of W. smithii with previous studies indicates that the decline in critical photoperiod with increasing latitude represents an overall decrease in response to light rather than a shift in the timing of photosensitivity among arthropods in general. We propose that the underlying functional components of photoperiodic time measurement, as well as the overt photoperiodic response, are either homologous or are themselves responding directly to selection over latitudinal gradients in seasonality. Received: 18 May 1998 / Accepted: 14 September 1998     

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 Analysis of Natural Populations of DROSOPHILA MELANOGASTER in Japan. III. Genetic Variability of Inducing Factors of Amylase and Fitness

"Inducibility" of amylase in Drosophila melanogaster was defined and investigated in a natural population from Japan. Inducibility represents the effects of factors remote from the structural gene that control the amount of enzyme produced. Inducibility of an isogenic line is measured as the ratio of the enzyme's specific activity in two different inducing environments. There was considerable genetic variability with respect to inducibility of amylase in 44 isogenic lines derived from a natural population of D. melanogaster . Net fitness and its components in these isogenic lines were also measured. The results indicated that, although the inducibility of the enzyme was positively correlated with the net fitness (r_g = 0.63 ± 0.2), the enzyme activities in the normal medium were not (r_g = 0.12 ± 0.37). The analysis of the data shows that the differences in inducing factors are mainly responsible for the differences in the fitness of lines and are the genetic materials for the adaptive evolution of organisms.     

The effect of pH onBrachionus calyciflorus Pallas (Rotifera)

Cohorts of the rotiferBrachionus calyciflorus were grown in media with pH of between 2.5 and 11.5 in increments of 1 unit. pH was shown to exert a major influence on the growth rate and reproductive capacity of theB. calyciflorus. At pH 2.5 theB. calyciflorus showed a negative capacity for population increase (r _c). Ther _c values of the cohorts reared at pH 3.5 and 4.5 were positive and these cohorts showed high net reproductive rates (R₀). Ther _c's of cohorts reared at pH of 5.5 and 6.5 were negative, while those of cohorts reared at pH 7.5 to 10.5 were positive with a peak in theR ₀ and cohort generation time (T _c) at pH 8.5 and 9.5. Total mortality occurred in less that 24 h at pH 11.5.Successive generations of the progeny from the cohort held at pH 3.5 were reared under the same conditions. Although ther _c of the F₁ and F₂ cohorts increased, the organism was unable to maintain the increased population growth rate, and the F₄ generation showed anr _c of almost 0.     

INBREEDING EFFECTS IN A GYNODIOECIOUS POPULATION OF THE COLONIZING SPECIES TRIFOLIUM HIRTUM ALL.

The demographic consequences of one and two generations of selfing were examined in a gynodioecious population of rose clover (Trifolium hirtum). Seeds collected from a roadside population were used to create an experimental population that consisted of seeds of open-pollinated individuals (S₀), and seeds obtained after one (S₁) and two (S₂) generations of selfing. Seeds from the three groups were equally likely to germinate. However, inbreeding effects were observed in the vigor and survivorship of the seedlings. The proportion of the transplanted seedlings that reached the reproductive stage was 0.42, 0.31, and 0.14 for the S₀, S₁, and S₂ groups, indicating a reduction of 26% and 66% by one and two generations of selfing, respectively. Inbreeding effects on reproductive components of fitness were nonsignificant. Groups were compared by multiplicative fitness functions based on germination, survival to reproduction, and fecundity. The relative fitness of the S₁ and S₂ groups were 0.64 and 0.23 relative to the S₀ group, representing a reduction in lifetime fitness of 36% and 77%, respectively. The differences in relative fitness among the groups were caused mainly by differences in survivorship. Thus, inbreeding depression is apparently an important factor in the maintenance of the gynodioecious breeding system in rose clover.     

The capacity for increase at a low temperature of several Australian populations of Sitophilus oryzae (L.)

The capacity for increase (I_c) of one laboratory and seven field populations of young adult S. oryzae from different sites in Australia was determined over a thirty-week period at 15°C, a supposedly marginal temperature, in wheat of 14% moisture content. The average value of I_c was 0.0838 ± 0.0017 and the populations comprised three significantly different groups. Variations in the net rate of increase per generation (R₀) which averaged 34.7±1.71, had a major effect on the value of r_c whereas variation in the cohort generation time (T_c), which averaged 41.6±.23 weeks had only a minor effect. The survivorship of adults of the populations did not differ significantly and 93% of females were alive after thirty weeks. Estimates of r_c based on a fifteen-week period and on an amended value of Tc differed little from those over thirty weeks. The capacity for increase at 15°C of a given population was found to be correlated with its fertility at 27°C and with its body-weight rather than with its cold-tolerance, as evidenced by chill-coma temperature, or with its previous temperature-history. The temperature experienced by the immature stages of weevils had a profound effect on r_c in that weevils reared at 15 and 27°C had respective values of 0.0350 and 0.0707 when subsequently cultured at 15°C. Because all observed values of r_c were higher than expected, possible errors in method were considered. An alternative estimate of R₀ agreed closely with that observed and indicated that only 17% of the immature stages survived at 15°C. Metabolic heating due to the higher than expected population density shortened the duration of the immature stages by 7% but did not affect their survival.     

Tolerance to deer herbivory and resistance to insect herbivores in the common evening primrose (Oenothera biennis)

The evolution of plant defence in response to herbivory will depend on the fitness effects of damage, availability of genetic variation and potential ecological and genetic constraints on defence. Here, we examine the potential for evolution of tolerance to deer herbivory in Oenothera biennis while simultaneously considering resistance to natural insect herbivores. We examined (i) the effects of deer damage on fitness, (ii) the presence of genetic variation in tolerance and resistance, (iii) selection on tolerance, (iv) genetic correlations with resistance that could constrain evolution of tolerance and (v) plant traits that might predict defence. In a field experiment, we simulated deer damage occurring early and late in the season, recorded arthropod abundances, flowering phenology and measured growth rate and lifetime reproduction. Our study showed that deer herbivory has a negative effect on fitness, with effects being more pronounced for late‐season damage. Selection acted to increase tolerance to deer damage, yet there was low and nonsignificant genetic variation in this trait. In contrast, there was substantial genetic variation in resistance to insect herbivores. Resistance was genetically uncorrelated with tolerance, whereas positive genetic correlations in resistance to insect herbivores suggest there exists diffuse selection on resistance traits. In addition, growth rate and flowering time did not predict variation in tolerance, but flowering phenology was genetically correlated with resistance. Our results suggest that deer damage has the potential to exert selection because browsing reduces plant fitness, but limited standing genetic variation in tolerance is expected to constrain adaptive evolution in O. biennis.