The genetic basis of photoperiodism and its evolutionary divergence among populations of the pitcher-plant mosquito, Wyeomyia smithii

Clock-talk: have we forgotten about geographic variation?

Wyeomyia smithii, the pitcher-plant mosquito, has evolved from south to north and from low to high elevations in eastern North America. Along this seasonal gradient, critical photoperiod has increased while apparent involvement of the circadian clock has declined in concert with the evolutionary divergence of populations. Response to classical experiments used to test for a circadian basis of photoperiodism varies as much within and among populations of W. smithii as have been found in the majority of all other insects and mites. The micro-evolutionary processes revealed within and among populations of W. smithii, programmed by a complex underlying genetic architecture, illustrate a gateway to the macro-evolutionary divergence of biological timing among species and higher taxa in general.

Climate, landscape, and life history jointly predict multidecadal community mosquito phenology

Phenology of adult host-seeking female mosquitoes is a critical component for understanding potential for vector-borne pathogen maintenance and amplification in the natural environment. Despite this importance, long-term multi-species investigations of mosquito phenologies across environments and differing species' life history traits are rare. Here we leverage long-term mosquito control district monitoring data to characterize annual phenologies of 7 host-seeking female mosquito species over a 20-year time period in suburban Illinois, USA. We also assembled data on landscape context, categorized into low and medium development, climate variables including precipitation, temperature and humidity, and key life history traits, i.e. overwintering stage and Spring-Summer versus Summer-mid-Fallseason fliers. We then fit linear mixed models separately for adult onset, peak abundances, and flight termination with landscape, climate and trait variables as predictors with species as a random effect. Model results supported some expectations, including warmer spring temperatures leading to earlier onset, warmer temperatures and lower humidity leading to earlier peak abundances, and warmer and wetter fall conditions leading to later termination. However, we also found sometimes complex interactions and responses contrary to our predictions. For example, temperature had generally weak support on its own, impacting onset and peak abundance timing; rather temperature has interacting effects with humidity or precipitation. We also found higher spring precipitation, especially in low development contexts, generally delayed adult onset, counter to expectations. These results emphasize the need to consider how traits, landscape and climatic factors all interact to determine mosquito phenology, when planning management strategies for vector control and public health protection.

Artificial selection on reproductive timing in hatchery salmon drives a phenological shift and potential maladaptation to climate change

The timing of breeding migration and reproduction links generations and substantially influences individual fitness. In salmonid fishes, such phenological events (seasonal return to freshwater and spawning) vary among populations but are consistent among years, indicating local adaptation in these traits to prevailing environmental conditions. Changing reproductive phenology has been observed in many populations of Atlantic and Pacific salmon and is sometimes attributed to adaptive responses to climate change. The sockeye salmon spawning in the Cedar River near Seattle, Washington, USA, have displayed dramatic changes in spawning timing over the past 50 years, trending later through the early 1990s, and becoming earlier since then. We explored the patterns and drivers of these changes using generalized linear models and mathematical simulations to identify possible environmental correlates of the changes, and test the alternative hypothesis that hatchery propagation caused inadvertent selection on timing. The trend toward later spawning prior to 1993 was partially explained by environmental changes, but the rapid advance in spawning since was not. Instead, since its initiation in 1991, the hatchery has, on average, selected for earlier spawning, and, depending on trait heritability, could have advanced spawning by 1-3 weeks over this period. We estimated heritability of spawning date to be high (h 2 ~0.8; 95% CI: 0.5-1.1), so the upper end of this range is not improbable, though at lower heritabilities a smaller effect would be expected. The lower reproductive success of early spawners and relatively low survival of early emerging juveniles observed in recent years suggest that artificial and natural selection are acting in opposite directions. The fitness costs of early spawning may be exacerbated by future warming; thus, the artificially advanced phenology could reduce the population's productivity. Such artificial selection is known in many salmon hatcheries, so there are broad consequences for the productivity of wild populations comingled with hatchery-produced fish.

Natural Variation and Genetics of Photoperiodism in Wyeomyia smithii

Seasonal change in the temperate and polar regions of Earth determines how the world looks around us and, in fact, how we live our day-to-day lives. For biological organisms, seasonal change typically involves complex physiological and metabolic reorganization, the majority of which is regulated by photoperiodism. Photoperiodism is the ability of animals and plants to use day length or night length, resulting in life-historical transformations, including seasonal development, migration, reproduction, and dormancy. Seasonal timing determines not only survival and reproductive success but also the structure and organization of complex communities and, ultimately, the biomes of Earth. Herein, a small mosquito, Wyeomyia smithii, that lives only in the water-filled leaves of a carnivorous plant over a wide geographic range, is used to explore the genetic and evolutionary basis of photoperiodism. Photoperiodism in W. smithii is considered in the context of its historical biogeography in nature to examine the startling finding that recent rapid climate change can drive genetic change in plants and animals at break-neck speed, and to challenge the ponderous 80+ year search for connections between daily and seasonal time-keeping mechanisms. Finally, a model is proposed that reconciles the seemingly disparate 24-h daily clock driven by the invariant rotation of Earth about its axis with the evolutionarily flexible seasonal timer orchestrated by variable seasonality driven by the rotation of Earth about the Sun.

TESTS OF INBREEDING EFFECTS ON HOST-SHIFT POTENTIAL IN THE PHYTOPHAGOUS BEETLE OPHRAELLA COMMUNA

Although inbreeding, on average, decreases additive genetic variance, some inbred populations may show an increase in phenotypic variance for some characters. In those populations with increased phenotypic variance, character changes by peak shifts may occur because of the effects of the higher variance on the adaptive landscape. A population's increased phenotypic variance may place it in the domain of attraction of a new adaptive peak or increase the likelihood of a selection-driven peak shift as the landscape of mean fitness flattens. The focus of this study was to test for increased variance, in inbred populations, in a behavioral character involved in adaptive diversification and probably speciation. We examined the effect of inbreeding on feeding responses of the leaf beetle Ophraella communa in a series of inbred lineages across a range of levels of inbreeding (f = 0.25, 0.375, 0.5). We measured the feeding response of inbred lineages of O. communa on its normal host, Ambrosia artemisiifolia, and on two novel plants, Chrysopsis villosa and Iva frutescens, that are the hosts of other Ophraella species. The results show that feeding responses on the different plants are not correlated, indicating that the feeding responses to the different plants are to some degree genetically independent. Despite apparent genetic variation in lineage feeding responses, we could not statistically demonstrate increases in phenotypic variance within the lineages. Thus, the experimental results do not support the idea that host shifts in this beetle evolved by peak shifts in bottlenecked populations.

REPRODUCTIVE CHARACTERISTICS OF THE FLOWER BREEDING DROSOPHILA HIBISCI BOCK (DROSOPHILIDAE) IN EASTERN AUSTRALIA: GENETIC AND ENVIRONMENTAL DETERMINANTS OF OVARIOLE NUMBER

Quantitative genetic analysis of the ovariole number of the Australian Hibiscus flower-breeding Drosophila hibisci Bock was conducted on populations from two localities along a latitudinal cline in ovariole number previously observed in the species (Starmer et al., in press). Parental strains, F₁ , F_1r (reciprocal), F₂ , backcross, and backcross reciprocal generations were used in a line-cross (generation means) analysis. This analysis revealed both additive and epistatic effects as important determinants of variation in ovariole number when larvae were reared at 25°C. Maternal effects and maternal-by-progeny genetic interactions were not significant. These results are comparable to previous studies that document epistatic components as genetic determinants of ovariole number in D. melanogaster. Parallel studies on ovariole number in D. hibisci parental and hybrid generations (F₁ and F_1r ) reared as larvae at three temperatures (18°, 21.5°, and 25°C) showed environmental effects and genotype-by-environment interactions as significant influences on the phenotype. Maternal effects were present when temperature of larval development was considered and significant, nonlinear environmental effects were detected. Field collections of D. hibisci females showed that field conditions result in significant departure of ovariole number from comparable laboratory reared females. The significant epistatic genetic effects, genotype-by-environment interactions, and maternal effects indicate that the genetic architecture of traits, such as ovariole number, may be more complex than often acknowledged and thus may be compatible with Wright's view of a netlike relationship between the genome and complex characters (Wright 1968).

EFFECTS OF POSTGLACIAL RANGE EXPANSION ON ALLOZYME AND QUANTITATIVE GENETIC VARIATION OF THE PITCHER-PLANT MOSQUITO, WYEOMYIA SMITHII

We determined allozyme variability of 34 populations of the pitcher-plant mosquito, Wyeomyia smithii, from Florida (30°N) to northern Manitoba (54°N) and compared allozyme variability with the additive genetic variance for preadult development time and photoperiodic response determined previously for six populations over a similar range (30-50°N). Phylogenetic analysis of allozymes shows a well-defined split between Gulf Coast and lowland North Carolina populations, similar to previously observed phylogeographic patterns in a wide variety of taxa. A deeper split in the phylogeny of W. smithii coincides with the location of the maximum extent of the Laurentide Ice Sheet. Furthermore, both average heterozygosity and patterns of isolation-by-distance decline in populations north of the former glacial border. It is likely that northern populations are the result of a range expansion that occurred subsequent to the late-Wisconsin retreat of the Laurentide Ice Sheet and that these populations have not yet reached a drift-migration equilibrium. The northern decline in allozyme heterozygosity contrasts sharply with the northern increase in additive genetic variance of development time and photoperiodic response found in previous studies. These previous studies also showed that the genetic divergence of populations has involved stochastic variation in the contribution of dominance and epistasis to the genetic architecture underlying demographic traits, including preadult development time, and photoperiodic response. When taken together, the present and prior studies identify the genetic processes underlying the lack of concordance between geographic patterns of allozyme and quantitative genetic variation in natural populations of W. smithii. In the presence of nonadditive genetic variation, isolation and drift can result in opposite patterns of genetic variation for structural genes and quantitative traits.

Effects of temperature variability on community structure in a natural microbial food web

Climate change research has demonstrated that changing temperatures will have an effect on community-level dynamics by altering species survival rates, shifting species distributions, and ultimately, creating mismatches in community interactions. However, most of this work has focused on increasing temperature, and still little is known about how the variation in temperature extremes will affect community dynamics. We used the model aquatic community held within the leaves of the carnivorous plant, Sarracenia purpurea, to test how food web dynamics will be affected by high temperature variation. We tested the community response of the first (bacterial density), second (protist diversity and composition), and third trophic level (predator mortality), and measured community respiration. We collected early and late successional stage inquiline communities from S. purpurea from two North American and two European sites with similar average July temperature. We then created a common garden experiment in which replicates of these communities underwent either high or normal daily temperature variation, with the average temperature equal among treatments. We found an impact of temperature variation on the first two, but not on the third trophic level. For bacteria in the high-variation treatment, density experienced an initial boost in growth but then decreased quickly through time. For protists in the high-variation treatment, alpha-diversity decreased faster than in the normal-variation treatment, beta-diversity increased only in the European sites, and protist community composition tended to diverge more in the late successional stage. The mortality of the predatory mosquito larvae was unaffected by temperature variation. Community respiration was lower in the high-variation treatment, indicating a lower ecosystem functioning. Our results highlight clear impacts of temperature variation. A more mechanistic understanding of the effects that temperature, and especially temperature variation, will have on community dynamics is still greatly needed.

Evolutionary Divergence of Circadian and Photoperiodic Phenotypes in the Pitcher-Plant Mosquito, Wyeomyia smithii

For decades, chronobiologists have investigated the relationship between the circadian clock that mediates daily activities and the photoperiodic timer that mediates seasonal activities. The main experiment used to infer a circadian basis for photoperiodic time measurement is the Nanda-Hamner protocol (NH). Herein, the authors compare additive and nonadditive (dominance and epistasis) genetic effects that lead to the divergence of populations of the pitcher-plant mosquito, Wyeomyia smithii, for critical photoperiod (CPP) and amplitude of the rhythmic response to NH for 3 temporal-geographic scales: 1) Over geological time between populations in northern and southern clades, 2) over millennial time between populations within the northern clade, and 3) over generational time between lines selected for long and short CPP from within a single population. The authors show that the pattern of additive, dominance, and epistatic effects depends on the time scale over which populations or lines have diverged. Patterns for genetic differences between populations for CPP and response to NH reveal similarities over geological and millennial time scales but differences over shorter periods of evolution. These results, and the observation that neither the period nor amplitude of the NH rhythm are significantly correlated with CPP among populations, lead the authors to conclude that the rhythmic response to NH has evolved independently of photoperiodic response in populations of W. smithii. The implication is that in this species, genetic modification of the circadian clock has not been the basis for the adaptive modification of photoperiodic time measurement over the climatic gradient of North America.

Epistasis and maternal effects in experimental adaptation to chronic nutritional stress in Drosophila

Based on ecological and metabolic arguments, some authors predict that adaptation to novel, harsh environments should involve alleles showing negative (diminishing return) epistasis and/or that it should be mediated in part by evolution of maternal effects. Although the first prediction has been supported in microbes, there has been little experimental support for either prediction in multicellular eukaryotes. Here we use a line-cross design to study the genetic architecture of adaptation to chronic larval malnutrition in a population of Drosophila melanogaster that evolved on an extremely nutrient-poor larval food for 84 generations. We assayed three fitness-related traits (developmental rate, adult female weight and egg-to-adult viability) under the malnutrition conditions in 14 crosses between this selected population and a nonadapted control population originally derived from the same base population. All traits showed a pattern of negative epistasis between alleles improving performance under malnutrition. Furthermore, evolutionary changes in maternal traits accounted for half of the 68% increase in viability and for the whole of 8% reduction in adult female body weight in the selected population (relative to unselected controls). These results thus support both of the above predictions and point to the importance of nonadditive effects in adaptive microevolution.

Footprints in time: comparative quantitative trait loci mapping of the pitcher-plant mosquito, Wyeomyia smithii

Identifying regions of the genome contributing to phenotypic evolution often involves genetic mapping of quantitative traits. The focus then turns to identifying regions of 'major' effect, overlooking the observation that traits of ecological or evolutionary relevance usually involve many genes whose individual effects are small but whose cumulative effect is large. Herein, we use the power of fully interfertile natural populations of a single species of mosquito to develop three quantitative trait loci (QTL) maps: one between two post-glacially diverged populations and two between a more ancient and a post-glacial population. All demonstrate that photoperiodic response is genetically a highly complex trait. Furthermore, we show that marker regressions identify apparently 'non-significant' regions of the genome not identified by composite interval mapping, that the perception of the genetic basis of adaptive evolution is crucially dependent upon genetic background and that the genetic basis for adaptive evolution of photoperiodic response is highly variable within contemporary populations as well as between anciently diverged populations.

Hybrid vigor in the biological control agent, Longitarsus jacobaeae

Hybridization is an important evolutionary mechanism that can increase the fitness and adaptive potential of populations. A growing body of evidence supports its importance as a key factor contributing to rapid evolution in invasive species, but the effects of hybridization have rarely been assessed in intentionally introduced biological control agents. We investigated hybrids between a Swiss and an Italian population of the beetle, Longitarsus jacobaeae, a biological control agent of Jacobaea vulgaris, by reciprocally crossing individuals in the laboratory. Phenological traits of F1 and F2 hybrid lineages showed intermediate values relative to their parental populations, with some maternal influence. Fitness of the F2 generation, measured as lifetime fecundity, was higher than that of the Italian parent in one of the lineages and higher than that of either parent in the other hybrid lineage. The increased fecundity of hybrids may benefit tansy ragwort biological control by increasing the establishment success and facilitating a more rapid population buildup in the early generations. Even though the long-term consequences of hybridization in this and other systems are hard to predict, intentional hybridization may be a useful tool in biological control strategies as it would promote similar microevolutionary processes operating in numerous targeted invasive species.

Rapid adaptive evolution of photoperiodic response during invasion and range expansion across a climatic gradient

Abstract Understanding the mechanisms of adaptation to spatiotemporal environmental variation is a fundamental goal of evolutionary biology. This issue also has important implications for anticipating biological responses to contemporary climate warming and determining the processes by which invasive species are able to spread rapidly across broad geographic ranges. Here, we compare data from a historical study of latitudinal variation in photoperiodic response among Japanese and U.S. populations of the invasive Asian tiger mosquito Aedes albopictus with contemporary data obtained using comparable methods. Our results demonstrated rapid adaptive evolution of the photoperiodic response during invasion and range expansion across ∼15° of latitude in the United States. In contrast to the photoperiodic response, size-based morphological traits implicated in climatic adaptation in a wide range of other insects did not show evidence of adaptive variation in Ae. albopictus across either the U.S. (invasive) or Japanese (native) range. These results show that photoperiodism has been an important adaptation to climatic variation across the U.S. range of Ae. albopictus and, in conjunction with previous studies, strongly implicate the photoperiodic control of seasonal development as a critical evolutionary response to ongoing contemporary climate change. These results also emphasize that photoperiodism warrants increased attention in studies of the evolution of invasive species.

Genetic correlations and the evolution of photoperiodic time measurement within a local population of the pitcher-plant mosquito, Wyeomyia smithii

The genetic relationship between the daily circadian clock and the seasonal photoperiodic timer remains a subject of intense controversy. In Wyeomyia smithii, the critical photoperiod (an overt expression of the photoperiodic timer) evolves independently of the rhythmic response to the Nanda-Hamner protocol (an overt expression of the daily circadian clock) over a wide geographical range in North America. Herein, we focus on these two processes within a single local population in which there is a negative genetic correlation between them. We show that antagonistic selection against this genetic correlation rapidly breaks it down and, in fact, reverses its sign, showing that the genetic correlation is due primarily to linkage and not to pleiotropy. This rapid reversal of the genetic correlation within a small, single population means that it is difficult to argue that circadian rhythmicity forms the necessary, causal basis for the adaptive divergence of photoperiodic time measurement within populations or for the evolution of photoperiodic time measurement among populations over a broad geographical gradient of seasonal selection.

Microarrays reveal early transcriptional events during the termination of larval diapause in natural populations of the mosquito, Wyeomyia smithii

BACKGROUND

The mosquito Wyeomyia smithii overwinters in a larval diapause that is initiated, maintained and terminated by day length (photoperiod). We use a forward genetic approach to investigate transcriptional events involved in the termination of diapause following exposure to long-days.

METHODS/PRINCIPAL FINDINGS

We incorporate a novel approach that compares two populations that differentially respond to a single day length. We identify 30 transcripts associated with differential response to day length. Most genes with a previously annotated function are consistent with their playing a role in the termination of diapause, in downstream developmental events, or in the transition from potentially oxygen-poor to oxygen-rich environments. One gene emerges from three separate forward genetic screens as a leading candidate for a gene contributing to the photoperiodic timing mechanism itself (photoperiodic switch). We name this gene photoperiodic response gene 1 (ppdrg1). WsPpdrg1 is up-regulated under long-day response conditions, is located under a QTL for critical photoperiod and is associated with critical photoperiod after 25 generations of recombination from a cross between extreme phenotypes.

CONCLUSIONS

Three independent forward genetic approaches identify WsPpdrg1 as a gene either involved in the photoperiodic switch mechanism or very tightly linked to a gene that is. We conclude that continued forward genetic approaches will be central to understanding not only the molecular basis of photoperiodism and diapause, but also the evolutionary potential of temperate and polar animal populations when confronted with rapid climate change.

Nonadditive genetic effects in animal behavior

Heritabilities, commonly used to predict evolutionary potential, are notoriously low for behaviors. Apart from strong contributions of environmental variance in reducing heritabilities, the additive genetic components can be very low, especially when they are camouflaged by nonadditive genetic effects. We first report the heritabilities of courtship traits in founder-flush and control populations of the housefly (Musca domestica L.). We estimated the heritability of each male and female display through the regression of the courtships involving daughters and sons (with randomly selected mates) onto the "midparental" courtship values of their parents. Overall, the average heritability was significantly (P = .012) higher for the parent-daughter assays than for the parent-son assays. We attributed the low (even negative) heritabilities to genotype-by-environment interactions whereby the male's behavior is influenced by the "environment" of his mating partner's preferences for the display, generating epistasis through indirect genetic effects. Moreover, bottlenecked lines had up to 800% of the heritability of the controls, suggesting "conversion" of additive genetic variance from nonadditive components. Second, we used line-cross assays on separate populations that had been selected for divergence in mating behavior to identify dominance and epistasis through heterosis and outbreeding depression in courtship. Finally, our literature review confirms the prevalence of such low heritabilities (i.e., a conservative mean of 0.38) and nonadditive genetics in other behavioral repertoires (64% of the studies). We conclude that animal behavior is especially prone to the gamut of quantitative genetic complexities that can result in negative heritabilities, negative selection responses, inbreeding depression, conversion, heterosis, and outbreeding depression.

Phenotypic evolution from genetic polymorphisms in a radial network architecture

Background

The genetic architecture of a quantitative trait influences the phenotypic response to natural or artificial selection. One of the main objectives of genetic mapping studies is to identify the genetic factors underlying complex traits and understand how they contribute to phenotypic expression. Presently, we are good at identifying and locating individual loci with large effects, but there is a void in describing more complex genetic architectures. Although large networks of connected genes have been reported, there is an almost complete lack of information on how polymorphisms in these networks contribute to phenotypic variation and change. To date, most of our understanding comes from theoretical, model-based studies, and it remains difficult to assess how realistic their conclusions are as they lack empirical support.

Results

A previous study provided evidence that nearly half of the difference in eight-week body weight between two divergently selected lines of chickens was a result of four loci organized in a 'radial' network (one central locus interacting with three 'radial' loci that, in turn, only interacted with the central locus). Here, we study the relationship between phenotypic change and genetic polymorphism in this empirically detected network. We use a model-free approach to study, through individual-based simulations, the dynamic properties of this polymorphic and epistatic genetic architecture. The study provides new insights to how epistasis can modify the selection response, buffer and reveal effects of major loci leading to a progressive release of genetic variation. We also illustrate the difficulty of predicting genetic architecture from observed selection response, and discuss mechanisms that might lead to misleading conclusions on underlying genetic architectures from quantitative trait locus (QTL) experiments in selected populations.

Conclusion

Considering both molecular (QTL) and phenotypic (selection response) data, as suggested in this work, provides additional insights into the genetic mechanisms involved in the response to selection. Such dissection of genetic architectures and in-depth studies of their ability to contribute to short- or long-term selection response represents an important step towards a better understanding of the genetic bases of complex traits and, consequently, of the evolutionary properties of populations.

Quantitative trait loci associated with photoperiodic response and stage of diapause in the pitcher-plant mosquito, Wyeomyia smithii

A wide variety of temperate animals rely on length of day (photoperiodism) to anticipate and prepare for changing seasons by regulating the timing of development, reproduction, dormancy, and migration. Although the molecular basis of circadian rhythms regulating daily activities is well defined, the molecular basis for the photoperiodic regulation of seasonal activities is largely unknown. We use geographic variation in the photoperiodic control of diapause in the pitcher-plant mosquito Wyeomyia smithii to create the first QTL map of photoperiodism in any animal. For critical photoperiod (CPP), we detect QTL that are unique, a QTL that is sex linked, QTL that overlap with QTL for stage of diapause (SOD), and a QTL that interacts epistatically with the circadian rhythm gene, timeless. Results presented here confirm earlier studies concluding that CPP is under directional selection over the climatic gradient of North America and that the evolution of CPP is genetically correlated with SOD. Despite epistasis between timeless and a QTL for CPP, timeless is not located within any detectable QTL, indicating that it plays an ancillary role in the evolution of photoperiodism in W. smithii. Finally, we highlight one region of the genome that includes loci contributing to CPP, SOD, and hormonal regulation of development.

POPULATION DIFFERENTIATION IN THE BEETLE TRIBOLIUM CASTANEUM. I. GENETIC ARCHITECTURE

We used joint-scaling analyses in conjunction with rearing temperature variation to investigate the contributions of additive, non-additive, and environmental effects to genetic divergence and incipient speciation among 12 populations of the red flour beetle, Tribolium castaneum, with small levels of pairwise nuclear genetic divergence (0.033 < Nei's D < 0.125). For 15 population pairs we created a full spectrum of line crosses (two parental, two reciprocal F1's, four F2's, and eight backcrosses), reared them at multiple temperatures, and analyzed the numbers and developmental defects of offspring. We assayed a total of 219,388 offspring from 5147 families. Failed crosses occurred predominately in F2's, giving evidence of F2 breakdown within this species. In all cases where a significant model could be fit to the data on offspring number, we observed at least one type of digenic epistasis. We also found maternal and cytoplasmic effects to be common components of divergence among T. castaneum populations. In some cases, the most complex model tested (additive, dominance, epistatic, maternal, and cytoplasmic effects) did not provide a significant fit to the data, suggesting that linkage or higher order epistasis is involved in differentiation between some populations. For the limb deformity data, we observed significant genotype-by-environment interaction in most crosses and pure parent crosses tended to have fewer deformities than hybrid crosses. Complexity of genetic architecture was not correlated with either geographic distance or genetic distance. Our results support the view that genetic incompatibilities responsible for postzygotic isolation, an important component of speciation, may be a natural but serendipitous consequence of nonadditive genetic effects and structured populations.

Quantitative trait loci with age-specific effects on fecundity in Drosophila melanogaster

Life-history theory and evolutionary theories of aging assume the existence of alleles with age-specific effects on fitness. While various studies have documented age-related changes in the genetic contribution to variation in fitness components, we know very little about the underlying genetic architecture of such changes. We used a set of recombinant inbred lines to map and characterize the effects of quantitative trait loci (QTL) affecting fecundity of Drosophila melanogaster females at 1 and 4 weeks of age. We identified one QTL on the second chromosome and one or two QTL affecting fecundity on the third chromosome, but these QTL affected fecundity only at 1 week of age. There was more genetic variation for fecundity at 4 weeks of age than at 1 week of age and there was no genetic correlation between early and late-age fecundity. These results suggest that different loci contribute to the variation in fecundity as the organism ages. Our data provide support for the mutation accumulation theory of aging as applied to reproductive senescence. Comparing the results from this study with our previous work on life-span QTL, we also find evidence that antagonistic pleiotropy may contribute to the genetic basis of senescence in these lines as well.

The role of epistatic gene interactions in the response to selection and the evolution of evolvability

It has been argued that the architecture of the genotype-phenotype map determines evolvability, but few studies have attempted to quantify these effects. In this article we use the multilinear epistatic model to study the effects of different forms of epistasis on the response to directional selection. We derive an analytical prediction for the change in the additive genetic variance, and use individual-based simulations to understand the dynamics of evolvability and the evolution of genetic architecture. This shows that the major determinant for the evolution of the additive variance, and thus the evolvability, is directional epistasis. Positive directional epistasis leads to an acceleration of evolvability, while negative directional epistasis leads to canalization. In contrast, pure non-directional epistasis has little effect on the response to selection. One consequence of this is that the classical epistatic variance components, which do not distinguish directional and non-directional effects, are useless as predictors of evolutionary dynamics. The build-up of linkage disequilibrium also has negligible effects. We argue that directional epistasis is likely to have major effects on evolutionary dynamics and should be the focus of empirical studies of epistasis.

Geographic and developmental variation in expression of the circadian rhythm gene, timeless, in the pitcher-plant mosquito, Wyeomyia smithii

Expression of the circadian rhythm gene timeless was investigated in the pitcher-plant mosquito, Wyeomyia smithii (Coq.), and was found to vary with time of day, instar of diapause, and latitude of origin. The temporal pattern of timeless expression differed between the two diapausing instars and was significantly higher in southern (38-40 degrees N) than in northern (50 degrees N) populations, when diapausing instar was held constant. Expression of timeless is therefore both developmentally and evolutionarily variable. This result provides the first example of a latitudinal difference in the expression of timeless, suggesting that, along with evidence from other insects, timeless has the potential to affect photoperiodic response and its adaptive evolution in temperate seasonal environments.

Epistasis underlying a fitness trait within a natural population of the pitcher-plant mosquito, Wyeomyia smithii

We selected on divergent photoperiodic response in three separate lines from a natural population of the pitcher-plant mosquito, Wyeomyia smithii. Line crosses reveal that there exists within a population, diverse epistatic variation for a fitness trait that could contribute to adaptive potential following founder events or rapid climate change.

The Genetic Architecture of House Fly Mating Behavior

This chapter summarizes several experimental approaches used to identify the effects of dominance, epistasis, and genotype-by-environment interactions in the genetic architecture of the mating behavior of the common house fly (Musca domestica L.). Quantitative genetic investigations of mating behavior hold special intrigue for unraveling the complexities of fitness traits, with applications to theory on sexual selection and speciation. Besides being well suited to large-scale quantitative genetic protocols, the house fly has a remarkably complex courtship repertoire, affording special opportunities for studies on communication, social interactions, and learning. Increased additive genetic variances for the courtship repertoire of experimentally bottlenecked populations provided evidence for the presence of dominance and/or epistasis. Negative genetic variances in these populations suggested genotype-by-environment interactions, where the environment is the mating partner. Line cross assays of populations that had been subjected to selection for divergent courtship repertoire confirmed that both dominance and epistasis have significant effects. These crosses also showed more directly that the expression of the male's genotype is dependent upon the preferences of his mating partner. Repeatability studies also detailed how males alter their courtship performances with successive encounters within and across females, such that the males learn to improve their techniques in securing copulations. A review of 41 animal behavior studies found that a wide range of traits and taxa have dominance, epistasis, and genotype-y-environment interactions, although house fly courtship may remain a unique model where learning is an intersexually selected trait. Future development of more sophisticated molecular techniques for the M. domestica genome will help unravel the underlying biochemical and developmental pathways of these quantitative genetic interactions for a more complete understanding of the processes of inbreeding depression, outbreeding depression, and pleiotropy.

FROM MICRO- TO MACROEVOLUTION THROUGH QUANTITATIVE GENETIC VARIATION: POSITIVE EVIDENCE FROM FIELD CRICKETS

Quantitative genetics has been introduced to evolutionary biologists with the suggestion that microevolution could be directly linked to macroevolutionary patterns using, among other parameters, the additive genetic variance/ covariance matrix (G) which is a statistical representation of genetic constraints to evolution. However, little is known concerning the rate and pattern of evolution of G in nature, and it is uncertain whether the constraining effect of G is important over evolutionary time scales. To address these issues, seven species of field crickets from the genera Gryllus and Teleogryllus were reared in the laboratory, and quantitative genetic parameters for morphological traits were estimated from each of them using a nested full-sibling family design. We used three statistical approaches (T method, Flury hierarchy, and Mantel test) to compare G matrices or genetic correlation matrices in a phylogenetic framework. Results showed that G matrices were generally similar across species, with occasional differences between some species. We suggest that G has evolved at a low rate, a conclusion strengthened by the consideration that part of the observed across-species variation in G can be explained by the effect of a genotype by environment interaction. The observed pattern of G matrix variation between species could not be predicted by either morphological trait values or phylogeny. The constraint hypothesis was tested by comparing the multivariate orientation of the reconstructed ancestral G matrix to the orientation of the across-species divergence matrix (D matrix, based on mean trait values). The D matrix mainly revealed divergence in size and, to a much smaller extent, in a shape component related to the ovipositor length. This pattern of species divergence was found to be predictable from the ancestral G matrix in agreement with the expectation of the constraint hypothesis. Overall, these results suggest that the G matrix seems to have an influence on species divergence, and that macroevolution can be predicted, at least qualitatively, from quantitative genetic theory. Alternative explanations are discussed.

The contribution of an hourglass timer to the evolution of photoperiodic response in the pitcher-plant mosquito, Wyeomyia smithii

Photoperiodism, the ability to assess the length of day or night, enables a diverse array of plants, birds, mammals, and arthropods to organize their development and reproduction in concert with the changing seasons in temperate climatic zones. For more than 60 years, the mechanism controlling photoperiodic response has been debated. Photoperiodism may be a simple interval timer, that is, an hourglasslike mechanism that literally measures the length of day or night or, alternatively, may be an overt expression of an underlying circadian oscillator. Herein, we test experimentally whether the rhythmic response in Wyeomyia smithii indicates a causal, necessary relationship between circadian rhythmicity and the evolutionary modification of photoperiodic response over the climatic gradient of North America, or may be explained by a simple interval timer. We show that a day-interval timer is sufficient to predict the photoperiodic response of W. smithii over this broad geographic range and conclude that rhythmic responses observed in classical circadian-based experiments alone cannot be used to infer a causal role for circadian rhythmicity in the evolution of photoperiodic time measurement. More importantly, we argue that the pursuit of circadian rhythmicity as the central mechanism that measures the duration of night or day has distracted researchers from consideration of the interval-timing processes that may actually be the target of natural selection linking internal photoperiodic time measurement to the external seasonal environment.

EVOLUTION OF ADDITIVE AND NONADDITIVE GENETIC VARIANCE IN DEVELOPMENT TIME ALONG A CLINE IN DROSOPHILA SERRATA

Latitudinal clines provide natural systems that may allow the effect of natural selection on the genetic variance to be determined. Ten clinal populations of Drosophila serrata collected from the eastern coast of Australia were used to examine clinal patterns in the trait mean and genetic variance of the life-history trait egg-to-adult development time. Development time significantly lengthened from tropical areas to temperate areas. The additive genetic variance for development time in each population was not associated with latitude but was associated with the population mean development time. Additive genetic variance tended to be larger in populations with more extreme development times and appeared to be consistent with allele frequency change. In contrast, the nonadditive genetic variance was not associated with the population mean but was associated with latitude. Levels of nonadditive genetic variance were greatest in the region of the cline where the gradient in the change in mean was greatest, consistent with Barton's (1999) conjecture that the generation of linkage disequilibrium may become an important component of the genetic variance in systems with a spatially varying optimum.

The influence of variable rates of inbreeding on fitness, environmental responsiveness, and evolutionary potential

We manipulated experimental populations of the housefly (Musca domestica L.) under three inbreeding schemes (fast, slow, and punctuated) to partition out the influences of different means and variances in the rate of inbreeding, per generation, while controlling for the final level of inbreeding as a constant. One treatment used constant fast inbreeding (11% per generation; Ne = 4 for 4 generations), for a comparison to one that was consistently slow (3% per generation; Ne = 16 for 14 generations). The third followed a model for serial founder-flush events. Each founder-flush episode involved a one-generation pulse of fast inbreeding (Ne = 4) followed by two generations of very low (or no) inbreeding, yielding high intergenerational variation (i.e., for an average inbreeding rate of 4% per generation). Allozyme assays showed that we achieved the intended final inbreeding coefficient of about 37%. All inbreeding schemes decreased fitness levels in terms of egg-to-adult viability, development time, and male mating success relative to the outbred control. The consistently fast inbreeding protocol had more pronounced reductions in fitness, relative to the other two inbreeding schemes. In comparison to the fast and punctuated regimes, the consistently slow protocol preserved evolutionary potential (as assayed by the genetic divergence of subpopulations exposed to different environments) in egg-to-adult viability, and (albeit anecdotally) reduced the extinction probabilities, especially in a novel environment. The punctuated treatment did not optimize the potential for purge as predicted, but instead reduced fitness, evolutionary potential, and environmental responsiveness (as measured by genotype-by-environment interactions). This founder-flush treatment also had the highest extinction probabilities. Longer periods of population flush might be necessary to purge effectively in a punctuated scheme. We conclude that the rate of inbreeding, independent from the final level, can have important effects on population fitness, environmental responsiveness, and evolutionary potential.

Circadian rhythmicity and photoperiodism in the pitcher-plant mosquito: adaptive response to the photic environment or correlated response to the seasonal environment?

Many plants and animals use the length of day or photoperiod to cue their seasonal patterns of development, reproduction, dormancy, and migration. Among temperate arthropods, the median or critical photoperiod increases with latitude or altitude. Concomitantly, in beetles, moths, mites, flies, and mosquitoes, there is a declining expression of a rhythmic, presumably circadian-based, component of photoperiodic response. It has been proposed that the long summer days in the north select for a reduced response to light by the circadian clock, which results in this declining rhythmic expression and, consequently, longer northern critical photoperiods. However, these patterns might also be due to direct, seasonal selection on the critical photoperiod itself, which results in a correlated reduction in the rhythmic component as a result of internal physiological constraints within the organism. Using standard light duration and selection experiments, we show that evolution of photoperiodic time measurement in the mosquito, Wyeomyia smithii, results from the direct response of critical photoperiod to seasonal selection and a correlated response of the rhythmic component of photoperiodic time measurement. We conclude that expression of the circadian clock is necessary neither for the central mechanism of photoperiodic time measurement nor for the adaptive modification of critical photoperiod.

Genetic shift in photoperiodic response correlated with global warming

To date, all altered patterns of seasonal interactions observed in insects, birds, amphibians, and plants associated with global warming during the latter half of the 20th century are explicable as variable expressions of plastic phenotypes. Over the last 30 years, the genetically controlled photoperiodic response of the pitcher-plant mosquito, Wyeomyia smithii, has shifted toward shorter, more southern daylengths as growing seasons have become longer. This shift is detectable over a time interval as short as 5 years. Faster evolutionary response has occurred in northern populations where selection is stronger and genetic variation is greater than in southern populations. W. smithii represents an example of actual genetic differentiation of a seasonality trait that is consistent with an adaptive evolutionary response to recent global warming.

Seasonally varying diet quality and the quantitative genetics of development time and body size in birch feeding insects

Genetic variance-covariance structures (G), describing genetic constraints on microevolutionary changes of populations, have a central role in the current theories of life-history evolution. However, the evolution of Gs in natural environments has been poorly documented. Resource quality and quantity for many animals and plants vary seasonally, which may shape genetic architectures of their life histories. In the mountain birch-insect herbivore community, leaf quality of birch for insect herbivores declines profoundly during both leaf growth and senescence, but remains stable during midsummer. Using six sawfly species specialized on the mountain birch foliage, we tested the ways in which the seasonal variation in foliage quality of birch is related to the genetic architectures of larval development time and body size. In the species consuming mature birch leaves of stable quality, that is, without diet-imposed time constraints for development time, long development led to high body mass. This was revealed by the strongly positive phenotypic and genetic correlations between the traits. In the species consuming growing or senescing leaves, on the other hand, the rapidly deteriorating leaf quality prevented the larvae from gaining high body mass after long development. In these species, the phenotypic and genetic correlations between development time and final mass were negative or zero. In the early-summer species with strong selection for rapid development, genetic variation in development time was low. These results show that the intuitively obvious positive genetic relationship between development time and final body mass is a probable outcome only when the constraints for long development are relaxed. Our study provides the first example of a modification in guild-wide patterns in the genetic architectures brought about by seasonal variation in resource quality.

Nuclear and cytoplasmic contributions to intraspecific divergence in an annual legume

The genetic architecture of trait differentiation was evaluated between two ecologically distinct populations of Chamaecrista fasciculata. Individuals from Maryland and Illinois populations were crossed to create 10 types of seed: Maryland and Illinois parents, reciprocal F1 and F2 hybrids, and backcrosses to Maryland and to Illinois on reciprocal F1 hybrids. Reciprocal crosses created hybrid generation seeds with both Maryland and Illinois cytoplasmic backgrounds. Experimental individuals were grown in a common garden near the site of the Maryland population. In the garden, plants from the Illinois population flowered, set fruit, and died earlier than those from Maryland, likely reflecting adaptations to differences in growing season length between the two populations. Although reproductive components at the flower and whole plant level differed between the two populations, reproductive output as measured by fruit and seed production was similar. Cytoplasmic genes had a subtle but pervasive effect on population differentiation; hybrids with Maryland cytoplasm were significantly differentiated from those with Illinois cytoplasm when all characters were evaluated jointly. The nuclear genetic architecture of population differentiation was evaluated with joint scaling tests. Depending on the trait, both additive and nonadditive genetic effects contributed to population differentiation. Intraspecific genetic differentiation in this wild plant species appears to reflect a complex genetic architecture that includes the contribution of additive, dominance, and epistatic components in addition to subtle cytoplasmic effects.

Geographic variation and the evolution of reproductive allocation in the pitcher-plant mosquito, Wyeomyia smithii

We measured the egg size of six geographic populations of the pitcher-plant mosquito, Wyeomyia smithii, from Florida (30 degrees N) to Ontario (49 degrees N). Populations from northern latitudes produced larger eggs than populations from southern latitudes. Egg size increased with increasing latitude more rapidly when larvae were reared under low rather than high density. One southern (30 degrees N) and one northern (49 degrees N) population of W. smithii that persisted through 10 generations of selection for increased persistence under conditions of chronic thermal- and nutrient-limiting stress (conditions similar to southern rather than northern habitats) produced smaller eggs more rapidly than unselected control lines. However, there were no differences in lifetime fecundity or fertility between control and selected lines. Thus, laboratory evolution in an environment representative of extreme southern latitudes caused evolutionary changes consistent with geographic patterns of egg size. These results implicate temperature as a selective factor influencing the geographic variation of egg size in W. smithii, and demonstrate a novel trade-off in reproductive allocation between egg size and egg maturation time.

Population differentiation in an annual legume: genetic architecture

The presence or absence of epistasis, or gene interaction, is explicitly assumed in many evolutionary models. Although many empirical studies have documented a role of epistasis in population divergence under laboratory conditions, there have been very few attempts at quantifying epistasis in the native environment where natural selection is expected to act. In addition, we have little understanding of the frequency with which epistasis contributes to the evolution of natural populations. In this study we used a quantitative genetic design to quantify the contribution of epistasis to population divergence for fitness components of a native annual legume, Chamaecrista fasciculata. The design incorporated the contrast of performance of F2 and F3 segregating progeny of 18 interpopulation crosses with the F1 and their parents. Crosses were conducted between populations from 100 m to 2000 km apart. All generations were grown for two seasons in the natural environment of one of the parents. The F1 often outperformed the parents. This F1 heterosis reveals population structure and suggests that drift is a major contributor to population differentiation. The F2 generation demonstrated that combining genes from different populations can sometimes have unexpected positive effects. However, the F3 performance indicated that combining genes from different populations decreased vigor beyond that due to the expected loss of heterozygosity. Combined with previous data, our results suggest that both selection and drift contribute to population differentiation that is based on epistatic genetic divergence. Because only the F3 consistently expressed hybrid breakdown, we conclude that the epistasis documented in our study reflects interactions among linked loci.

Evolutionary responses to environmental stress by the pitcher-plant mosquito, wyeomyia smithii

We performed truncation selection for increased fitness (rc) under conditions of chronic stress from the combined effects of low nutrients and high temperature, representative of extremes likely to be encountered in nature by the pitcher-plant mosquito, Wyeomyia smithii. We performed selection on geographical parental populations and their hybrids to determine whether hybridization would facilitate or constrain adaptation under our selection protocol. The stressful environment decreased fitness (rc) by 54% averaged across all populations relative to near-optimal conditions. After approximately 10 generations of selection under chronically stressful conditions, exactly one-half of the parental and one-half of the hybrid populations had gone extinct. Thus hybridization had no effect on the likelihood of population persistence. Fitness (rc) of the surviving populations did not show any response to selection. Despite initial hybrid vigour under stressful conditions, the fitness (rc) of surviving hybrid populations was either equal to, or worse than, the fitness (rc) of surviving parental populations after approximately 10 generations of selection. These results suggest that outcrossing populations to augment genetic variation and facilitate adaptation to a rapidly changing environment may not be useful over longer time scales, even in cases where hybridization does initially increase fitness. Although we detected no direct response to selection for increased fitness (rc) under lifetime chronic stress, selected populations showed a strong correlated response for survivorship through transient, acute heat and desiccation shock. In evaluating how organisms might respond to future climate change, biologists must maintain a clear distinction between lifelong performance in chronically stressful environments and short-term survivorship through transient, acute stress.