The quantitative genetic basis of male mating behavior in Drosophila melanogaster

Courtship song differs between African and European populations of Drosophila melanogaster and involves a strong effect locus

The courtship song of Drosophila melanogaster has long served as excellent model system for studies of animal communication and differences in courtship song have been demonstrated among populations and between species. Here, we report that flies of African and European origin, which diverged approximately 13,000 years ago, show significant genetic differentiation in the use of slow versus fast pulse song. Using a combination of quantitative trait mapping and population genetic analysis we detected a single strong QTL underlying this trait and we identified candidate genes that may contribute to the evolution of this trait. Song trait variation between parental strains of our recombinant inbred panel enabled detection of genomic intervals associated with six additional song traits, some of which include known courtship-related genes. These findings improve the prospects for further genetic insights into the evolution of reproductive behavior and the biology underlying courtship song.

Predicting thresholds for population replacement gene drives

BACKGROUND

Threshold-dependent gene drives (TDGDs) could be used to spread desirable traits through a population, and are likely to be less invasive and easier to control than threshold-independent gene drives. Engineered Genetic Incompatibility (EGI) is an extreme underdominance system previously demonstrated in Drosophila melanogaster that can function as a TDGD when EGI agents of both sexes are released into a wild-type population.

RESULTS

Here we use a single generation fitness assay to compare the fecundity, mating preferences, and temperature-dependent relative fitness to wild-type of two distinct genotypes of EGI agents. We find significant differences in the behavior/performance of these EGI agents that would not be predicted a priori based on their genetic design. We report a surprising temperature-dependent change in the predicted threshold for population replacement in an EGI agent that drives ectopic expression of the developmental morphogen pyramus.

CONCLUSIONS

The single-generation fitness assay presented here could reduce the amount of time required to estimate the threshold for TDGD strategies for which hybrid genotypes are inviable. Additionally, this work underscores the importance of empirical characterization of multiple engineered lines, as behavioral differences can arise in unique genotypes for unknown reasons.

The Evolutionary Importance of Intraspecific Variation in Sexual Communication Across Sensory Modalities

The evolution of sexual communication is critically important in the diversity of arthropods, which are declining at a fast pace worldwide. Their environments are rapidly changing, with increasing chemical, acoustic, and light pollution. To predict how arthropod species will respond to changing climates, habitats, and communities, we need to understand how sexual communication systems can evolve. In the past decades, intraspecific variation in sexual signals and responses across different modalities has been identified, but never in a comparative way. In this review, we identify and compare the level and extent of intraspecific variation in sexual signals and responses across three different modalities, chemical, acoustic, and visual, focusing mostly on insects. By comparing causes and possible consequences of intraspecific variation in sexual communication among these modalities, we identify shared and unique patterns, as well as knowledge needed to predict the evolution of sexual communication systems in arthropods in a changing world.

The complex genetic architecture of male mate choice evolution between Drosophila species

Mate choice behaviors are among the most important reproductive isolating barriers in many animals. Little is known about the genetic basis of reproductively isolating behaviors, but examples to date provide evidence that they can have a simple genetic basis. However, it is unclear if these results indicate that individual genes with large effects are common, or are instead due to ascertainment biases. Here, we present the results of a QTL mapping study for the most important behavioral isolating barrier between Drosophila simulans and D. sechellia: male mate choice. Our QTL results initially suggested that differences in male mate choice may be due to a couple loci with large effects. However, as we divided the largest-effect QTL using stable introgression strains, we found evidence of multiple interacting loci. We further find that separate regions of the genome control different aspects of male choice. Taken together, our results suggest that the genetic architecture of mate choice behavior, in this case, is more complex than QTL mapping suggested, highlighting potential challenges to future mapping studies. We discuss the implications of these results as they relate to signal-receiver coevolution, mate choice, and reproductive isolation.

The Loci of Behavioral Evolution: Evidence That Fas2 and tilB Underlie Differences in Pupation Site Choice Behavior between Drosophila melanogaster and D. simulans

The behaviors of closely related species can be remarkably different, and these differences have important ecological and evolutionary consequences. Although the recent boom in genotype-phenotype studies has led to a greater understanding of the genetic architecture and evolution of a variety of traits, studies identifying the genetic basis of behaviors are, comparatively, still lacking. This is likely because they are complex and environmentally sensitive phenotypes, making them difficult to measure reliably for association studies. The Drosophila species complex holds promise for addressing these challenges, as the behaviors of closely related species can be readily assayed in a common environment. Here, we investigate the genetic basis of an evolved behavioral difference, pupation site choice, between Drosophila melanogaster and D. simulans. In this study, we demonstrate a significant contribution of the X chromosome to the difference in pupation site choice behavior between these species. Using a panel of X-chromosome deficiencies, we screened the majority of the X chromosome for causal loci and identified two regions associated with this X-effect. We then collect gene disruption and RNAi data supporting a single gene that affects pupation behavior within each region: Fas2 and tilB. Finally, we show that differences in tilB expression correlate with the differences in pupation site choice behavior between species. This evidence associating two genes with differences in a complex, environmentally sensitive behavior represents the first step toward a functional and evolutionary understanding of this behavioral divergence.

The Genetics of Male Pheromone Preference Difference Between Drosophila melanogaster and Drosophila simulans

Species of flies in the genus Drosophila differ dramatically in their preferences for mates, but little is known about the genetic or neurological underpinnings of this evolution. Recent advances have been made to our understanding of one case: pheromone preference evolution between the species D. melanogaster and D. simulans. Males of both species are very sensitive to the pheromone 7,11-HD that is present only on the cuticle of female D. melanogaster. In one species this cue activates courtship, and in the other it represses it. This change in valence was recently shown to result from the modification of central processing neurons, rather than changes in peripherally expressed receptors, but nothing is known about the genetic changes that are responsible. In the current study, we show that a 1.35 Mb locus on the X chromosome has a major effect on male 7,11-HD preference. Unfortunately, when this locus is divided, the effect is largely lost. We instead attempt to filter the 159 genes within this region using our newfound understanding of the neuronal underpinnings of this phenotype to identify and test candidate genes. We present the results of these tests, and discuss the difficulty of identifying the genetic architecture of behavioral traits and the potential of connecting these genetic changes to the neuronal modifications that elicit different behaviors.

Social behavior and aging: A fly model

The field of behavioral genetics has recently begun to explore the effect of age on social behaviors. Such studies are particularly important, as certain neuropsychiatric disorders with abnormal social interactions, like autism and schizophrenia, have been linked to older parents. Appropriate social interaction can also have a positive impact on longevity, and is associated with successful aging in humans. Currently, there are few genetic models for understanding the effect of aging on social behavior and its potential transgenerational inheritance. The fly is emerging as a powerful model for identifying the basic molecular mechanisms underlying neurological and neuropsychiatric disorders. In this review, we discuss these recent advancements, with a focus on how studies in Drosophila melanogaster have provided insight into the effect of aging on aspects of social behavior, including across generations.

Genetic basis of allochronic differentiation in the fall armyworm

BACKGROUND

Very little is known on how changes in circadian rhythms evolve. The noctuid moth Spodoptera frugiperda (Lepidoptera: Noctuidae) consists of two strains that exhibit allochronic differentiation in their mating time, which acts as a premating isolation barrier between the strains. We investigated the genetic basis of the strain-specific timing differences to identify the molecular mechanisms of differentiation in circadian rhythms.

RESULTS

Through QTL analyses we identified one major Quantitative trait chromosome (QTC) underlying differentiation in circadian timing of mating activity. Using RADtags, we identified this QTC to be homologous to Bombyx mori C27, on which the clock gene vrille is located, which thus became the major candidate gene. In S. frugiperda, vrille showed strain-specific polymorphisms. Also, vrille expression differed significantly between the strains, with the rice-strain showing higher expression levels than the corn-strain. In addition, RT-qPCR experiments with the other main clock genes showed that pdp1, antagonist of vrille in the modulatory feedback loop of the circadian clock, showed higher expression levels in the rice-strain than in the corn-strain.

CONCLUSIONS

Together, our results indicate that the allochronic differentiation in the two strains of S. frugiperda is associated with differential transcription of vrille or a cis-acting gene close to vrille, which contributes to the evolution of prezygotic isolation in S. frugiperda.

Genetic variation but weak genetic covariation between pre- and post-copulatory episodes of sexual selection in Drosophila melanogaster

When females mate polyandrously, male reproductive success depends both on the male's ability to attain matings and on his ability to outcompete rival males in the fertilization of ova post-copulation. Increased investment in  ejaculate components may trade off with investment in precopulatory traits due to resource allocation. Alternatively, pre- and post-copulatory traits could be positively related if individuals can afford to invest heavily in traits advantageous at both episodes of selection. There is empirical evidence for both positive and negative associations between pre- and post-copulatory episodes, but little is known about the genetic basis of these correlations. In this study, we measured morphological, chemical and behavioural precopulatory male traits and investigated their relationship with measures of male fitness (male mating success, remating inhibition and offensive sperm competitiveness) across 40 isofemale lines of Drosophila melanogaster. We found significant variation among isofemale lines, indicating a genetic basis for most of the traits investigated. However, we found weak evidence for genetic correlations between precopulatory traits and our indices of male fitness. Moreover, pre- and post-copulatory episodes of selection were uncorrelated, suggesting selection may act independently at the different episodes to maximize male reproductive success.

Multiple-Line Inference of Selection on Quantitative Traits

Trait differences between species may be attributable to natural selection. However, quantifying the strength of evidence for selection acting on a particular trait is a difficult task. Here we develop a population genetics test for selection acting on a quantitative trait that is based on multiple-line crosses. We show that using multiple lines increases both the power and the scope of selection inferences. First, a test based on three or more lines detects selection with strongly increased statistical significance, and we show explicitly how the sensitivity of the test depends on the number of lines. Second, a multiple-line test can distinguish between different lineage-specific selection scenarios. Our analytical results are complemented by extensive numerical simulations. We then apply the multiple-line test to QTL data on floral character traits in plant species of the Mimulus genus and on photoperiodic traits in different maize strains, where we find a signature of lineage-specific selection not seen in two-line tests.

Gr33a modulates Drosophila male courtship preference

In any gamogenetic species, attraction between individuals of the opposite sex promotes reproductive success that guarantees their thriving. Consequently, mate determination between two sexes is effortless for an animal. However, choosing a spouse from numerous attractive partners of the opposite sex needs deliberation. In Drosophila melanogaster, both younger virgin females and older ones are equally liked options to males; nevertheless, when given options, males prefer younger females to older ones. Non-volatile cuticular hydrocarbons, considered as major pheromones in Drosophila, constitute females' sexual attraction that act through males' gustatory receptors (Grs) to elicit male courtship. To date, only a few putative Grs are known to play roles in male courtship. Here we report that loss of Gr33a function or abrogating the activity of Gr33a neurons does not disrupt male-female courtship, but eliminates males' preference for younger mates. Furthermore, ectopic expression of human amyloid precursor protein (APP) in Gr33a neurons abolishes males' preference behavior. Such function of APP is mediated by the transcription factor forkhead box O (dFoxO). These results not only provide mechanistic insights into Drosophila male courtship preference, but also establish a novel Drosophila model for Alzheimer's disease (AD).

Aging-related neurodegeneration eliminates male courtship choice in Drosophila

Choices between 2 options, one liked and one disliked, are effortless for an animal, whereas those among 2 equally liked options are more difficult to determine and might depend on an unknown mechanism. Here, we report that in the fruit fly Drosophila melanogaster, both younger virgin females and older ones are "liked options" to males. However, when given the choice, males tend to be fastidious and prefer younger virgin females to older ones. Besides, aging eliminates males' preference for younger mates, which can be mimicked by ectopically expressing the human amyloid precursor protein in their central nervous system. Furthermore, we examined the effect of neurodegeneration in Drosophila courtship circuit and confirmed that male courtship preference for younger mates was abrogated by neurodegeneration. Our work, thus characterizes a novel choice behavior that can be decisions after comparison and also reveals the critical role of neurodegeneration in this behavior, which provides new insights on decision-making mechanisms.

Sexual traits are sensitive to genetic stress and predict extinction risk in the stalk-eyed fly, Diasemopsis meigenii

The handicap principle predicts that sexual traits are more susceptible to inbreeding depression than nonsexual traits. However, this hypothesis has received little testing and results are inconsistent. We used 11 generations of full-sibling mating to test the effect of inbreeding on sexual and nonsexual traits in the stalk-eyed fly Diasemopsis meigenii. Consistent with the theoretical predictions, the male sexual trait (eyespan) decreased more than nonsexual traits (female eyespan and male wing length), even after controlling for body size variation. In addition, male eyespan was a reliable predictor of line extinction, unlike other nonsexual traits. After 11 generations, inbred lines were crossed to generate inbred and outbred families. All morphological traits were larger in outbred individuals than inbred individuals. This heterosis was greater in male eyespan than in male wing length, but not female eyespan. The elevated response in male eyespan to genetic stress mirrored the result found using environmental stress during larval development and suggests that common mechanisms underlie the patterns observed. Overall, these results support the hypothesis that male sexual traits suffer more from inbreeding depression than nonsexual traits and are in line with predictions based on the handicap principle.

Evidence for no sexual isolation between Drosophila albomicans and D. nasuta

Sexual isolation, the reduced tendency to mate, is one of the reproductive barriers that prevent gene flow between different species. Various species-specific signals during courtship contribute to sexual isolation between species. Drosophila albomicans and D. nasuta are closely related species of the nasuta subgroup within the Drosophila immigrans group and are distributed in allopatry. We analyzed mating behavior and courtship as well as cuticular hydrocarbon profiles within and between species. Here, we report that these two species randomly mated with each other. We did not observe any sexual isolation between species or between strains within species by multiple-choice tests. Significant difference in the courtship index was detected between these two species, but males and females of both species showed no discrimination against heterospecific partners. Significant quantitative variations in cuticular hydrocarbons between these two species were also found, but the cuticular hydrocarbons appear to play a negligible role in both courtship and sexual isolation between these two species. In contrast to the evident postzygotic isolation, the lack of sexual isolation between these two species suggests that the evolution of premating isolation may lag behind that of the intergenomic incompatibility, which might be driven by intragenomic conflicts.

Analysis of natural variation reveals neurogenetic networks for Drosophila olfactory behavior

Understanding the relationship between genetic variation and phenotypic variation for quantitative traits is necessary for predicting responses to natural and artificial selection and disease risk in human populations, but is challenging because of large sample sizes required to detect and validate loci with small effects. Here, we used the inbred, sequenced, wild-derived lines of the Drosophila melanogaster Genetic Reference Panel (DGRP) to perform three complementary genome-wide association (GWA) studies for natural variation in olfactory behavior. The first GWA focused on single nucleotide polymorphisms (SNPs) associated with mean differences in olfactory behavior in the DGRP, the second was an extreme quantitative trait locus GWA on an outbred advanced intercross population derived from extreme DGRP lines, and the third was for SNPs affecting the variance among DGRP lines. No individual SNP in any analysis was associated with variation in olfactory behavior by using a strict threshold accounting for multiple tests, and no SNP overlapped among the analyses. However, combining the top SNPs from all three analyses revealed a statistically enriched network of genes involved in cellular signaling and neural development. We used mutational and gene expression analyses to validate both candidate genes and network connectivity at a high rate. The lack of replication between the GWA analyses, small marginal SNP effects, and convergence on common cellular networks were likely attributable to epistasis. These results suggest that fully understanding the genotype-phenotype relationship requires a paradigm shift from a focus on single SNPs to pathway associations.

One quantitative trait locus for intra- and interspecific variation in a sex pheromone

Even though premating isolation is hypothesized to be a major driving force in speciation, its genetic basis is poorly known. In the noctuid moth Heliothis subflexa, one group of sex pheromone components, the acetates, emitted by the female, plays a crucial isolating role in preventing interspecific matings to males of the closely related Heliothis virescens, in which females do not produce acetates and males are repelled by them. We previously found intraspecific variation in acetates in H. subflexa: females in eastern North America contain significantly more acetates than females in Western Mexico. Here we describe the persistence of this intraspecific variation in laboratory-reared strains and the identification of one major quantitative trait locus (QTL), explaining 40% of the variance in acetate amounts. We homologized this intraspecific QTL to our previously identified interspecific QTL using restriction-associated DNA (RAD) tags. We found that a major intraspecific QTL overlaps with one of the two major interspecific QTL. To identify candidate genes underlying the acetate variation, we investigated a number of gene families with known or suspected acetyl- or acyltransferase activity. The most likely candidate genes did not map to our QTL, so that we currently hypothesize that a transcription factor underlies this QTL. Finding a single, large QTL that impacts variation in pheromone blends between and within species is, to our knowledge, the first such example for traits that have been demonstrated to affect premating isolation.

Heat stress survival in the pre-adult stage of the life cycle in an intercontinental set of recombinant inbred lines of Drosophila melanogaster

In insects, pre-adult stages of the life cycle are exposed to variation in temperature that may differ from that in adults. However, the genetic basis for adaptation to environmental temperature could be similar between the pre-adult and the adult stages of the life cycle. Here, we tested quantitative trait loci (QTL) for heat-stress survival in larvae of D. melanogaster, with and without a mild-heat-stress pre-treatment. Two sets of recombinant inbred lines derived from lines artificially selected for high and low levels of knockdown resistance to high temperature in young flies were used as mapping population. There was no apparent increase in heat-shock survival between heat-pretreated and non-pretreated larvae. There was a positive correlation between the two experimental conditions of heat-shock survival (with and without a heat pre-treatment) except for males from one set of lines. Several QTL were identified involving all three major chromosomes. Many QTL for larval thermotolerance overlapped with thermotolerance-QTL identified in previous studies for adults. One new thermotolerance-QTL was found but these QTL explained only a small fraction of the phenotypic variance and were only significant in larvae that received no heat pre-treatment. Several candidate genes mapped within QTL ranges. We discuss an overall co-localization for thermotolerance-QTL between the adult fly in previous studies and the pre-adult stage of the life cycle in this study.

The genetic basis of female mate preference and species isolation in Drosophila

The processes that underlie mate choice have long fascinated biologists. With the advent of increasingly refined genetic tools, we are now beginning to understand the genetic basis of how males and females discriminate among potential mates. One aspect of mate discrimination of particular interest is that which isolates one species from another. As behavioral isolation is thought to be the first step in speciation, and females are choosy more often than males in this regard, identifying the genetic variants that influence interspecies female mate choice can enhance our understanding of the process of speciation. Here, we review the literature on female mate choice in the most widely used model system for studies of species isolation Drosophila. Although females appear to use the same traits for both within- and between-species female mate choice, there seems to be a different genetic basis underlying these choices. Interestingly, most genomic regions that cause females to reject heterospecific males fall within areas of low recombination. Likely, candidate genes are those that act within the auditory or olfactory system, or within areas of the brain that process these systems.

Fine-scale genetic analysis of species-specific female preference in Drosophila simulans

Behavioural differences are thought to be the first components to contribute to species isolation, yet the precise genetic basis of behavioural isolation remains poorly understood. Here, we used a combination of behaviour assays and genetic mapping to provide the first refined map locating candidate genes for interspecific female preference isolating Drosophila simulans from D. melanogaster. First, we tested whether two genes identified as affecting D. melanogaster female intraspecific mate choice also affect interspecific mate choice; neither of these genes was found to contribute to species-specific female preference. Next, we used deficiency mapping to locate genes on the right arm of the third chromosome for species-specific female preference and identified five small significant regions that contain candidate genes contributing to behavioural isolation. All five regions were located in areas that would have low interspecific recombination, which mirrors the results of other behavioural isolation studies that used quantitative trait locus (QTL) mapping, but without the potential concern of bias towards regions of low recombination that QTL mapping may have. As this model system may be refined to the individual gene level using the same methodology, this initial map we provide may potentially serve as a ready template for the identification and characterization of the first behavioural isolation genes.

Degree of adaptive male mate choice is positively correlated with female quality variance

When the cost of reproduction for males and variance in female quality are high, males are predicted to show adaptive mate choice. Using Drosophila melanogaster, we test this prediction and show that sperm limited males preferentially mated with young and/or well fed females. The preferred females had higher reproductive output – direct evidence of adaptive precopulatory male mate choice. Our most striking finding is the strong positive correlation between the degree of mating bias showed by the males and the variance in the fitness of the females. We discuss the possible mechanism for such adaptive male mate choice and propose that such choice has important consequences with respect to the existing understanding of the mating system and the evolution of aging.

Reproductive behavior and fitness components in male Drosophila melaogaster are non-linearly affected by the number of male co-inhabitants early in adult life

Although multiple lines of evidence suggest that early adult life is very important in shaping the reproductive behavior of males, few studies have looked at the fitness consequences of the variation in reproductive behavior induced by differences in early life experience of males. Using a long term laboratory adapted population of Drosophila melanogaster Meigen (Diptera: Drosophilidae), early life experience, in terms of co-inhabitant numbers, was found to affect male mating behavior and at least one fitness component. However, in contrast to previous studies, a non-linear relationship was found between early life experience and fitness components and a significant effect of co-inhabitant number on copulation duration and sperm defense. Both these traits showed a sharp increase as the co-inhabitant numbers changed from 1 to 16. However, there was a decline in the trait values as the co-inhabitant number increased further. The probable causes for the observed non-linear pattern of responses are discussed.

Identification of X-linked quantitative trait loci affecting cold tolerance in Drosophila melanogaster and fine mapping by selective sweep analysis

Drosophila melanogaster is a cosmopolitan species that colonizes a great variety of environments. One trait that shows abundant evidence for naturally segregating genetic variance in different populations of D. melanogaster is cold tolerance. Previous work has found quantitative trait loci (QTL) exclusively on the second and the third chromosomes. To gain insight into the genetic architecture of cold tolerance on the X chromosome and to compare the results with our analyses of selective sweeps, a mapping population was derived from a cross between substitution lines that solely differed in the origin of their X chromosome: one originates from a European inbred line and the other one from an African inbred line. We found a total of six QTL for cold tolerance factors on the X chromosome of D. melanogaster. Although the composite interval mapping revealed slightly different QTL profiles between sexes, a coherent model suggests that most QTL overlapped between sexes, and each explained around 5-14% of the genetic variance (which may be slightly overestimated). The allelic effects were largely additive, but we also detected two significant interactions. Taken together, this provides evidence for multiple QTL that are spread along the entire X chromosome and whose effects range from low to intermediate. One detected transgressive QTL influences cold tolerance in different ways for the two sexes. While females benefit from the European allele increasing their cold tolerance, males tend to do better with the African allele. Finally, using selective sweep mapping, the candidate gene CG16700 for cold tolerance colocalizing with a QTL was identified.

The Genetic Basis of Sexually Selected Variation

Sexually selected traits contribute greatly to phenotypic diversity, yet we have historically understood little about their genetic basis and how that basis may affect their evolution. Recent work in developmental and quantitative genetics has provided both mechanistic and statistical descriptions of genotype-phenotype maps for sexually selected traits. These studies expose generally complex genetic architectures; genotype-phenotype maps are polygenic with allelic effects that are pleiotropic and highly context-dependent. At the same time, developments in quantitative genetics have provided new insights into the microevolutionary potential of standing variation and indicate genetic constraints on the contemporary evolution of male sexually selected characters, mate preferences, and also male mating success itself. Understanding the extent to which these constraints are a function of genetic architecture will require a tighter integration of developmental, molecular, and quantitative genetic approaches in a variety of model systems. Emerging genomic technologies offer an unprecedented opportunity to deepen our understanding of sexual selection as an evolutionary process.

Quantitative trait locus mapping of gravitaxis behaviour in Drosophila melanogaster

Drosophila melanogaster, like other organisms, move and orient themselves in response to the earth's gravitational force. The ability to sense and respond to gravity is essential for an organism to navigate and thrive in its environment. The genes underlying this behaviour in Drosophila remain elusive. Using 88 recombinant inbred lines, we have identified four quantitative trait loci (QTLs) that contribute to adult gravitaxis (geotaxis) behaviour in Drosophila. Candidate genes of interest were selected from the QTLs of highest significance based on their function in chordotonal organ formation. Quantitative complementation tests with these candidate genes revealed a role for skittles in adult gravitaxis behaviour in D. melanogaster.

Multiple hybridization events between Drosophila simulans and Drosophila mauritiana are supported by mtDNA introgression

The study of speciation has advanced considerably in the last decades because of the increased application of molecular tools. In particular, the quantification of gene flow between recently diverged species could be addressed. Drosophila simulans and Drosophila mauritiana diverged, probably allopatrically, from a common ancestor approximately 250,000 years ago. However, these species share one mitochondrial DNA (mtDNA) haplotype indicative of a recent episode of introgression. To study the extent of gene flow between these species, we took advantage of a large sample of D. mauritiana and employed a range of different markers, i.e. nuclear and mitochondrial sequences, and microsatellites. This allowed us to detect two new mtDNA haplotypes (MAU3 and MAU4). These haplotypes diverged quite recently from haplotypes of the siII group present in cosmopolitan populations of D. simulans. The mean divergence time of the most diverged haplotype (MAU4) is approximately 127,000 years, which is more than 100,000 years before the assumed speciation time. Interestingly, we also found some evidence for gene flow at the nuclear level because an excess of putatively neutral loci shows significantly reduced differentiation between D. simulans and D. mauritiana. Our results suggest that these species are exchanging genes more frequently than previously thought.

Effect of laboratory acclimation on the variation of reproduction-related characters in Drosophila melanogaster

The natural variation of sex-specific characters between populations can favor their behavioral isolation, eventually leading to the formation of new species. Marked variations for male courtship, mating and the production of sex pheromones – three complex characters potentially inducing sexual isolation – were found between Drosophila melanogaster populations of various origins acclimated for many generations in research laboratories. However, the natural variation of these three characters between natural populations and their evolution after long-term acclimation in the laboratory remains unknown. We measured many traits involved in these characters in six stocks initiated with distinct populations sampled in a restricted geographic area. Several sex-specific traits varied between stocks freshly brought back to the laboratory. After 100 generations spent in the laboratory without any experimental selection, traits varied in a strain-dependent manner. This variation was not related to a reduction of their variance except for copulation duration. This indicates that reproduction-related characters can diverge between neighboring D. melanogaster populations, and differently adapt to stable laboratory conditions.

The genetics of quantitative traits: challenges and prospects

A major challenge in current biology is to understand the genetic basis of variation for quantitative traits. We review the principles of quantitative trait locus mapping and summarize insights about the genetic architecture of quantitative traits that have been obtained over the past decades. We are currently in the midst of a genomic revolution, which enables us to incorporate genetic variation in transcript abundance and other intermediate molecular phenotypes into a quantitative trait locus mapping framework. This systems genetics approach enables us to understand the biology inside the 'black box' that lies between genotype and phenotype in terms of causal networks of interacting genes.

Age, but not experience, affects courtship gene expression in male Drosophila melanogaster

Mutation screens in model organisms have helped identify the foundation of many fundamental organismal phenotypes. An emerging question in evolutionary and behavioral biology is the extent to which these “developmental” genes contribute to the subtle individual variation that characterizes natural populations. A related question is whether individual differences arise from static differences in gene expression that arose during previous life stages, or whether they are due to dynamic regulation of expression during the life stage under investigation. Here, we address these questions using genes that have been discovered to control the development of normal courtship behavior in male Drosophila melanogaster. We examined whether these genes have static or dynamic expression in the heads of adult male flies of different ages and with different levels of social experience. We found that 16 genes of the 25 genes examined were statically expressed, and 9 genes were dynamically expressed with changes related to adult age. No genes exhibited rapid dynamic expression changes due to social experience or age*experience interaction. We therefore conclude that a majority of fly “courtship” genes are statically expressed, while a minority are regulated in adults with respect to age, but not with respect to relevant social experience. These results are consistent with those from a recent microarray analysis that found none of the canonical courtship genes changed expression in male flies after brief exposure to females.

Quantitative trait loci for aggressive behavior in Drosophila melanogaster

Aggressive behavior is observed across animal taxa and is likely to be evolutionarily conserved. Although potentially advantageous, aggression can have social and health consequences in humans, and is a component of a number of psychiatric disorders. As a complex genetic trait, it is modulated by numerous quantitative trait loci (QTL) with allelic effects that can vary in direction and magnitude and that are sensitive to environmental perturbations. Assays to quantify aggressive behavior in Drosophila melanogaster have been developed, making this an ideal model system in which to dissect the genomic architecture underlying manifestation of and variation in aggressive behavior. Here, we map QTL affecting variation in aggression between two wild-type Drosophila strains. We identified a minimum of five QTL in a genomewide scan: two on chromosome 2 and three on chromosome 3. At least three and possibly all five of these QTL interact epistatically. We used deficiency complementation mapping to subdivide two linked, epistatically interacting QTL of large effect on chromosome 3 into at least six QTL, and complementation tests to mutations identified four candidate quantitative trait genes. Extensive epistasis poses a serious challenge for understanding the genetic basis of complex traits.

Genetic architecture of quantitative traits in mice, flies, and humans

We compare and contrast the genetic architecture of quantitative phenotypes in two genetically well-characterized model organisms, the laboratory mouse, Mus musculus, and the fruit fly, Drosophila melanogaster, with that found in our own species from recent successes in genome-wide association studies. We show that the current model of large numbers of loci, each of small effect, is true for all species examined, and that discrepancies can be largely explained by differences in the experimental designs used. We argue that the distribution of effect size of common variants is the same for all phenotypes regardless of species, and we discuss the importance of epistasis, pleiotropy, and gene by environment interactions. Despite substantial advances in mapping quantitative trait loci, the identification of the quantitative trait genes and ultimately the sequence variants has proved more difficult, so that our information on the molecular basis of quantitative variation remains limited. Nevertheless, available data indicate that many variants lie outside genes, presumably in regulatory regions of the genome, where they act by altering gene expression. As yet there are very few instances where homologous quantitative trait loci, or quantitative trait genes, have been identified in multiple species, but the availability of high-resolution mapping data will soon make it possible to test the degree of overlap between species.

Variations at a quantitative trait locus (QTL) affect development of behavior in lead-exposed Drosophila melanogaster

We developed Drosophila melanogaster as a model to study correlated behavioral, neuronal and genetic effects of the neurotoxin lead, known to affect cognitive and behavioral development in children. We showed that, as in vertebrates, lead affects both synaptic development and complex behaviors (courtship, fecundity, locomotor activity) in Drosophila. By assessing differential behavioral responses to developmental lead exposure among recombinant inbred Drosophila lines (RI), derived from parental lines Oregon R and Russian 2b, we have now identified a genotype by environment interaction (GEI) for a behavioral trait affected by lead. Drosophila Activity Monitors (TriKinetics, Waltham, MA), which measure activity by counting the number of times a single fly in a small glass tube walks through an infrared beam aimed at the middle of the tube, were used to measure activity of flies, reared from eggs to 4 days of adult age on either control or lead-contaminated medium, from each of 75 RI lines. We observed a significant statistical association between the effect of lead on Average Daytime Activity (ADA) across lines and one marker locus, 30AB, on chromosome 2; we define this as a Quantitative Trait Locus (QTL) associated with behavioral effects of developmental lead exposure. When 30AB was from Russian 2b, lead significantly increased locomotor activity, whereas, when 30AB was from Oregon R, lead decreased it. 30AB contains about 125 genes among which are likely "candidate genes" for the observed lead-dependent behavioral changes. Drosophila are thus a useful, underutilized model for studying behavioral, synaptic and genetic changes following chronic exposure to lead or other neurotoxins during development.

The genetic architecture of complex behaviors: lessons from Drosophila

Complex behaviors are affected by multiple interacting loci with individually small and environmentally sensitive effects. Understanding the genetic architecture of behavioral traits begins with identifying the genes regulating these traits, mapping the subset of genetically varying quantitative trait loci (QTLs) in natural populations, and pinpointing the molecular polymorphisms defining QTL alleles. Drosophila brings an impressive toolkit to the challenge of genetically dissecting complex traits: P transposable element mutagenesis to identify genes regulating these traits; artificial selection from natural populations to create extreme trait phenotypes; high resolution mapping to identify positional candidate genes corresponding to QTLs; linkage disequilibrium mapping to identify molecular polymorphism(s) that functionally define QTL alleles; and whole genome transcriptional profiling to postulate networks of interacting genes affecting complex traits. Studies in Drosophila have revealed large numbers of pleiotropic genes that interact epistatically to regulate behavioral traits, and that can have sex- and environment-specific effects. These observations offer valuable lessons for understanding the genetic basis of variation for complex behaviors in other organisms, including humans.

Genetic architecture of hybrid male sterility in Drosophila: analysis of intraspecies variation for interspecies isolation

BACKGROUND

The genetic basis of postzygotic isolation is a central puzzle in evolutionary biology. Evolutionary forces causing hybrid sterility or inviability act on the responsible genes while they still are polymorphic, thus we have to study these traits as they arise, before isolation is complete.

METHODOLOGY/PRINCIPAL FINDINGS

Isofemale strains of D. mojavensis vary significantly in their production of sterile F(1) sons when females are crossed to D. arizonae males. We took advantage of the intraspecific polymorphism, in a novel design, to perform quantitative trait locus (QTL) mapping analyses directly on F(1) hybrid male sterility itself. We found that the genetic architecture of the polymorphism for hybrid male sterility (HMS) in the F(1) is complex, involving multiple QTL, epistasis, and cytoplasmic effects.

CONCLUSIONS/SIGNIFICANCE

The role of extensive intraspecific polymorphism, multiple QTL, and epistatic interactions in HMS in this young species pair shows that HMS is arising as a complex trait in this system. Directional selection alone would be unlikely to maintain polymorphism at multiple loci, thus we hypothesize that directional selection is unlikely to be the only evolutionary force influencing postzygotic isolation.

Reproductive behaviour evolves rapidly when intralocus sexual conflict is removed

Background

Intralocus sexual conflict can inhibit the evolution of each sex towards its own fitness optimum. In a previous study, we confirmed this prediction through the experimental removal of female selection pressures in Drosophila melanogaster, achieved by limiting the expression of all major chromosomes to males. Compared to the control populations (C_1-4) where the genomes are exposed to selection in both sexes, the populations with male-limited genomes (ML_1-4) showed rapid increases in male fitness, whereas the fitness of females expressing ML-evolved chromosomes decreased [1].

Methodology/Principal Findings

Here we examine the behavioural phenotype underlying this sexual antagonism. We show that males expressing the ML genomes have a reduced courtship level but acquire the same number of matings. On the other hand, our data suggest that females expressing the ML genomes had reduced attractiveness, stimulating a lower rate of courtship from males. Moreover, females expressing ML genomes tend to display reduced yeast-feeding behaviour, which is probably linked to the reduction of their fecundity.

Conclusion/Significance

These results suggest that reproductive behaviour is shaped by opposing selection on males and females, and that loci influencing attractiveness and foraging were polymorphic for alleles with sexually antagonistic expression patterns prior to ML selection. Hence, intralocus sexual conflict appears to play a role in the evolution of a wide range of fitness-related traits and may be a powerful mechanism for the maintenance of genetic variation in fitness.

Fluorescent sperm marking to improve the fight against the pest insect Ceratitis capitata (Wiedemann; Diptera: Tephritidae)

The Sterile Insect Technique (SIT) involving area-wide release of mass-reared and sterilized pest insects has proven successful to reduce, control and eradicate economically important pest species, such as the Mediterranean fruit fly (medfly). For the efficient application, effective monitoring to assess the number and mating success of the released medflies is essential. Here, we report sperm-specific marking systems based on the spermatogenesis-specific Ceratitis capitata beta2-tubulin (Ccbeta2t) promoter. Fluorescent sperm can be isolated from testes or spermathecae. The marking does not cause general disadvantages in preliminary laboratory competitiveness assays. Therefore, transgenic sperm marking could serve as a major improvement for monitoring medfly SIT programs. The use of such harmless transgenic markers will serve as an ideal initial condition to transfer insect transgenesis technology from the laboratory to field applications. Moreover, effective and easily recognizable sperm marking will make novel studies possible on medfly reproductive biology which will help to further improve SIT programs.

Of flies and man: Drosophila as a model for human complex traits

Understanding the genetic and environmental factors affecting human complex genetic traits and diseases is a major challenge because of many interacting genes with individually small effects, whose expression is sensitive to the environment. Dissection of complex traits using the powerful genetic approaches available with Drosophila melanogaster has provided important lessons that should be considered when studying human complex traits. In Drosophila, large numbers of pleiotropic genes affect complex traits; quantitative trait locus alleles often have sex-, environment-, and genetic background-specific effects, and variants associated with different phenotypic are in noncoding as well as coding regions of candidate genes. Such insights, in conjunction with the strong evolutionary conservation of key genes and pathways between flies and humans, make Drosophila an excellent model system for elucidating the genetic mechanisms that affect clinically relevant human complex traits, such as alcohol dependence, sleep, and neurodegenerative diseases.

Natural genetic variation in complex mating behaviors of male Drosophila melanogaster

Mating behavior, including courtship and copulation, is a main component of male fitness, especially in species with no parental care. Variation in this behavior can thus be a target for mate choice and sexual selection, and can lead to evolution. The fruit fly, Drosophila melanogaster, has well-documented complex male courtship comprised of a sequence behaviors, and is an ideal model for behavior-genetic analysis. In order to evaluate genetic differences in the temporal pattern of mating behavior, we developed a high-throughput method that allows us to document the progression of male courtship and copulation using an ordinal scale (male mating progression scale, MMP). Using this method, we document natural genetic variation in the temporal pattern of behavior that was not detected using other metrics. This method was robust enough to detect genetic variation in this trait for males placed with both virgin and mated female targets.

The genetic architecture of a female sexual ornament

Understanding the evolution of sexual ornaments, and particularly that of female sexual ornaments, is an enduring challenge in evolutionary biology. Key to this challenge are establishing the relationship between ornament expression and female reproductive investment, and determining the genetic basis underpinning such relationship. Advances in genomics provide unprecedented opportunities to study the genetic architecture of sexual ornaments in model species. Here, we present a quantitative trait locus (QTL) analysis of a female sexual ornament, the comb of the fowl, Gallus gallus, using a large-scale intercross between red junglefowl and a domestic line, selected for egg production. First, we demonstrate that female somatic investment in comb reflects female reproductive investment. Despite a trade-off between reproductive and skeletal investment mediated by the mobilization of skeletal minerals for egg production, females with proportionally large combs also had relatively high skeletal investment. Second, we identify a major QTL for bisexual expression of comb mass and several QTL specific to female comb mass. Importantly, QTL for comb mass were nonrandomly clustered with QTL for female reproductive and skeletal investment on chromosomes one and three. Together, these results shed light onto the physiological and genetic architecture of a female ornament.

Antagonistic pleiotropic effects reduce the potential adaptive value of the FRIGIDA locus

Although the occurrence of epistasis and pleiotropy is widely accepted at the molecular level, its effect on the adaptive value of fitness-related genes is rarely investigated in plants. Knowledge of these features of a gene is critical to understand the molecular basis of adaptive evolution. Here we investigate the importance of pleiotropy and epistasis in determining the adaptive value of a candidate gene using the gene FRI (FRIGIDA), which is thought to be the major gene controlling flowering time variation in Arabidopsis thaliana. The effect of FRI on flowering time was analyzed in an outbred population created by randomly mating 19 natural accessions of A. thaliana. This unique population allows the estimation of FRI effects independent of any linkage association with other loci due to demographic processes or to coadapted genes. It also allows for the estimation of pleiotropic effects of FRI on fitness and inflorescence architecture. We found that FRI explains less variation in flowering time than previously observed among natural accessions, and interacts epistatically with the FLC locus. Although early flowering plants produce more fruits under spring conditions, and nonfunctional alleles of FRI were associated with early flowering, variation at FRI was not associated with fitness. We show that nonfunctional FRI alleles have negative pleiotropic effects on fitness by reducing the numbers of nodes and branches on the inflorescence. We propose that these antagonistic pleiotropic effects reduce the adaptive value of FRI, and helps explain the maintenance of alternative life history strategies across natural populations of A. thaliana.

Natural genetic variation in cuticular hydrocarbon expression in male and female Drosophila melanogaster

Cuticular hydrocarbons (CHCs) act as contact pheromones in Drosophila melanogaster and are an important component of several ecological traits. Segregating genetic variation in the expression of CHCs at the population level in D. melanogaster is likely to be important for mate choice and climatic adaptation; however, this variation has never been characterized. Using a panel of recombinant inbred lines (RILs) derived from a natural population, we found significant between-line variation for nearly all CHCs in both sexes. We identified 25 QTL in females and 15 QTL in males that pleiotropically influence CHC expression. There was no evidence of colocalization of QTL for homologous traits across the sexes, indicating that sexual dimorphism and low intersex genetic correlations between homologous CHCs are a consequence of largely independent genetic control. This is consistent with a pattern of divergent sexual and natural selection between the sexes.

Increased male mating rate in Drosophila is associated with Wolbachia infection

The maternally inherited bacterium Wolbachia pipientis infects 25-75% of arthropods and manipulates host reproduction to improve its transmission. One way Wolbachia achieves this is by inducing cytoplasmic incompatibility (CI), where crosses between infected males and uninfected females are inviable. Infected males suffer reduced fertility through CI and reduced sperm production. However, Wolbachia induce lower levels of CI in nonvirgin males. We examined the impact of Wolbachia on mating behaviour in male Drosophila melanogaster and D. simulans, which display varying levels of CI, and show that infected males mate at a higher rate than uninfected males in both species. This may serve to increase the spread of Wolbachia, or alternatively, may be a behavioural adaptation employed by males to reduce the level of CI. Mating at high rate restores reproductive compatibility with uninfected females resulting in higher male reproductive success thus promoting male promiscuity. Increased male mating rates also have implications for the transmission of Wolbachia.

Dissecting the complex genetic basis of mate choice

The genetic analysis of mate choice is fraught with difficulties. Males produce complex signals and displays that can consist of a combination of acoustic, visual, chemical and behavioural phenotypes. Furthermore, female preferences for these male traits are notoriously difficult to quantify. During mate choice, genes not only affect the phenotypes of the individual they are in, but can influence the expression of traits in other individuals. How can genetic analyses be conducted to encompass this complexity? Tighter integration of classical quantitative genetic approaches with modern genomic technologies promises to advance our understanding of the complex genetic basis of mate choice.

Genetics of divergence in male wing pigmentation and courtship behavior between Drosophila elegans and D. gunungcola

Many sex-specific traits involved in mating consist of functionally coordinated morphologies and behaviors. How the components of these complex traits evolve and become coordinated during evolution is unknown. In order to understand how such trait complexes evolve and diversify, we must decipher the genetic underpinnings of their components. In this study, we begin to elucidate the genetic architecture underlying differences in functionally related male pigmentation and behavior between two Asian Drosophila melanogaster group species, D. elegans and D. gunungcola. D. elegans possesses a male-specific wing melanin spot and a stereotypical wing display element in male courtship, whereas D. gunungcola lacks both of these traits. Using reciprocal F1 male hybrids, we demonstrate that the X-chromosome contains a major locus or loci required for wing spot formation and that autosomal loci largely determine the male courtship display. Using phenotypic and genetic analysis of backcross progeny, we further demonstrate that both the wing spot and courtship differences between the two species are polygenic and both depend at least in small part on genetic factors on both the X and the autosomes. Finally, we find that male wing spot size and courtship wing display are highly correlated in backcross progeny, suggesting that linkage or pleiotropy may have been involved in their coordinated evolution.

Quantitative trait loci for locomotor behavior in Drosophila melanogaster

Locomotion is an integral component of most animal behaviors and many human diseases and disorders are associated with locomotor deficits, but little is known about the genetic basis of natural variation in locomotor behavior. Locomotion is a complex trait, with variation attributable to the joint segregation of multiple interacting quantitative trait loci (QTL), with effects that are sensitive to the environment. We assessed variation in a component of locomotor behavior (locomotor reactivity) in a population of 98 recombinant inbred lines of Drosophila melanogaster and mapped four QTL affecting locomotor reactivity by linkage to polymorphic roo transposable element insertion sites. We used complementation tests of deficiencies to fine map these QTL to 12 chromosomal regions and complementation tests of mutations to identify 13 positional candidate genes affecting locomotor reactivity, including Dopa decarboxylase (Ddc), which catalyzes the final step in the synthesis of serotonin and dopamine. Linkage disequilibrium mapping in a population of 164 second chromosome substitution lines derived from a single natural population showed that polymorphisms at Ddc were associated with naturally occurring genetic variation in locomotor behavior. These data implicate variation in the synthesis of bioamines as a factor contributing to natural variation in locomotor reactivity.