Genetics of a difference in cuticular hydrocarbons between Drosophila pseudoobscura and D. persimilis

Integrating lipid metabolism, pheromone production and perception by Fruitless and Hepatocyte Nuclear Factor 4

Sexual attraction and perception are crucial for mating and reproductive success. In Drosophila melanogaster, the male-specific isoform of Fruitless (Fru), FruM, is a known master neuro-regulator of innate courtship behavior to control the perception of sex pheromones in sensory neurons. Here, we show that the non-sex-specific Fru isoform (FruCOM) is necessary for pheromone biosynthesis in hepatocyte-like oenocytes for sexual attraction. Loss of FruCOM in oenocytes resulted in adults with reduced levels of cuticular hydrocarbons (CHCs), including sex pheromones, and show altered sexual attraction and reduced cuticular hydrophobicity. We further identify Hepatocyte nuclear factor 4 (Hnf4) as a key target of FruCOM in directing fatty acid conversion to hydrocarbons. Fru or Hnf4 depletion in oenocytes disrupts lipid homeostasis, resulting in a sex-dimorphic CHC profile that differs from doublesex- and transformer-dependent CHC dimorphism. Thus, Fru couples pheromone perception and production in separate organs to regulate chemosensory communications and ensure efficient mating behavior.

Phenotypic disruption of cuticular hydrocarbon production in hybrids between sympatric species of Hawaiian picture-wing Drosophila

Interspecies hybrids can express phenotypic traits far outside the range of parental species. The atypical traits of hybrids provide insight into differences in the factors that regulate the expression of these traits in the parental species. In some cases, the unusual phenotypic traits of hybrids can lead to phenotypic dysfunction with hybrids experiencing reduced survival or reproduction. Cuticular hydrocarbons (CHCs) in insects are important phenotypic traits that serve several functions, including desiccation resistance and pheromones for mating. We used gas chromatography mass spectrometry to investigate the differences in CHC production between two closely related sympatric Hawaiian picture-wing Drosophila species, Drosophila heteroneura and D. silvestris, and their F1 and backcross hybrid offspring. CHC profiles differed between males of the two species, with substantial sexual dimorphism in D. silvestris but limited sexual dimorphism in D. heteroneura. Surprisingly, F1 hybrids did not produce three CHCs, and the abundances of several other CHCs occurred outside the ranges present in the two parental species. Backcross hybrids produced all CHCs with greater variation than observed in F1 or parental species. Overall, these results suggest that the production of CHCs was disrupted in F1 and backcross hybrids, which may have important consequences for their survival or reproduction.

Drosophila Sexual Attractiveness in Older Males Is Mediated by Their Microbiota

Age is well known to be a basis for female preference of males. However, the mechanisms underlying age-based choices are not well understood, with several competing theories and little consensus. The idea that the microbiota can affect host mate choice is gaining traction, and in this study we examine whether the male microbiota influences female preference for older individuals in the fruit fly Drosophila pseudoobscura. We find that an intact microbiota is a key component of attractiveness in older males. However, we found no evidence that this decrease in older male attractiveness was simply due to impaired microbiota generally reducing male quality. Instead, we suggest that the microbiota underlies an honest signal used by females to assess male age, and that impaired microbiota disrupt this signal. This suggests that age-based preferences may break down in environments where the microbiota is impaired, for example when individuals are exposed to naturally occurring antibiotics, extreme temperatures, or in animals reared in laboratories on antibiotic supplemented diet.

Divergence in female damselfly sensory structures is consistent with a species recognition function but shows no evidence of reproductive character displacement

Males and females transmit and receive signals prior to mating that convey information such as sex, species identity, or individual condition. In some animals, tactile signals relayed during physical contact between males and females before and during mating appear to be important for mate choice or reproductive isolation. This is common among odonates, when a male grasps a female's thorax with his terminal appendages prior to copulation, and the female subsequently controls whether copulation occurs by bending her abdomen to complete intromission. It has been hypothesized that mechanosensory sensilla on the female thoracic plates mediate mating decisions, but is has been difficult to test this idea. Here, we use North American damselflies in the genus Enallagma (Odonata: Coenagrionidae) to test the hypothesis that variation in female sensilla traits is important for species recognition. Enallagma anna and E. carunculatum hybridize in nature, but experience strong reproductive isolation as a consequence of divergence in male terminal appendage morphology. We quantified several mechanosensory sensilla phenotypes on the female thorax among multiple populations of both species and compared divergence in these traits in sympatry versus allopatry. Although these species differed in features of sensilla distribution within the thoracic plates, we found no strong evidence of reproductive character displacement among the sensilla traits we measured in regions of sympatry. Our results suggest that species-specific placement of female mechanoreceptors may be sufficient for species recognition, although other female sensory phenotypes might have diverged in sympatry to reduce interspecific hybridization.

Phylogeny, environment and sexual communication across the Drosophila genus

Social behaviour emerges from the local environment but is constrained by the animal's life history and its evolutionary lineage. In this perspective, we consider the genus Drosophila and provide an overview of how these constraints can shape how individuals interact. Our focus is restricted to visual and chemical signals and how their use varies across species during courtship - currently the only social behaviour well-studied across many Drosophila species. We broadly categorize species into four climatic groups - cosmopolitan, tropical, temperate and arid - which serve as discussion points as we review comparative behavioural and physiological studies and relate them to the abiotic conditions of a species environment. We discuss how the physiological and behavioural differences among many fly species may reflect life history differences as much as, or even more than, differences in phylogeny. This perspective serves not only to summarize what has been studied across drosophilids, but also to identify questions and outline gaps in the literature worth pursuing for progressing the understanding of behavioural evolution in Drosophila.

GENETICS OF SEXUAL ISOLATION AND COURTSHIP DYSFUNCTION IN MALE HYBRIDS OF DROSOPHILA PSEUDOOBSCURA AND DROSOPHILA PERSIMILIS

Despite the importance of sexual isolation to speciation, few studies have analyzed the genetic basis of interspecific mating discrimination, particularly using hybrid males. In this study, I investigated the genetic basis of sexual isolation using male hybrids of Drosophila pseudoobscura and D. persimilis. Hybrid male mating success was caused by interactions between the X-chromosome and autosomes (or Y-chromosome), and different arms of the X-chromosome contributed to mating success with females of each species. Further, although there was an X-chromosome component to mating success, its magnitude was not disproportionately large when compared with the proportion of the genome contained on this chromosome. Some hybrid males courted with an anomalously low intensity, so I simultaneously mapped the genetic basis of this "courtship dysfunction." The courtship dysfunction was caused by an interaction between the left arm of the X-chromosome in D. persimilis with the autosomes or Y-chromosome from D. pseudoobscura. Anomalous courtship behavior in interspecific hybrids can obscure the conclusions of studies of the genetics of sexual isolation, so courtship intensity should be evaluated in all such investigations.

Genetic architecture of natural variation in cuticular hydrocarbon composition in Drosophila melanogaster

Insect cuticular hydrocarbons (CHCs) prevent desiccation and serve as chemical signals that mediate social interactions. Drosophila melanogaster CHCs have been studied extensively, but the genetic basis for individual variation in CHC composition is largely unknown. We quantified variation in CHC profiles in the D. melanogaster Genetic Reference Panel (DGRP) and identified novel CHCs. We used principal component (PC) analysis to extract PCs that explain the majority of CHC variation and identified polymorphisms in or near 305 and 173 genes in females and males, respectively, associated with variation in these PCs. In addition, 17 DGRP lines contain the functional Desat2 allele characteristic of African and Caribbean D. melanogaster females (more 5,9-C27:2 and less 7,11-C27:2, female sex pheromone isomers). Disruption of expression of 24 candidate genes affected CHC composition in at least one sex. These genes are associated with fatty acid metabolism and represent mechanistic targets for individual variation in CHC composition.

DOI:http://dx.doi.org/10.7554/eLife.09861.001

The outermost layer of an insect’s body is called the epicuticle and is made of a blend of fat molecules. “Cuticular hydrocarbons” (or CHCs) are the most common fat molecules in the epicuticle, and play an important role in protecting the insect’s body from harsh, dry habitats. CHCs also have other roles in insect behavior. For example, these molecules act as chemical cues when insects search for mates (i.e. pheromones), and they can even contribute to camouflage.

Insects are amongst the most diverse groups of animals on Earth, and different species have different blends of CHC molecules in their epicuticles. Fruit flies are a useful model to understand the genetics of CHC production, including CHCs that act as sex pheromones. Previous research has analyzed the CHCs made by both sexes in several fruit fly strains. However this work was unable to uncover which genes influence how much of a given CHC an individual fly will make.

Dembeck et al. have now looked into CHC production in a collection of 205 different fly strains, all of which have already had their total genetic material sequenced and studied. Comparing these known sequences and looking for associations between genetic differences and particular CHCs uncovered 24 genes that may be involved in CHC manufacture. Only six of the genes had been identified previously. Dembeck et al. found that interfering with the activity of any of the 24 genes had a knock-on effect on many other CHCs present in the flies’ epicuticle.

These 24 genes could to be pieced together in a network that is needed to make and recycle CHCs. The complexity and flexibility of this network can explain in part how insects have been able to build epicuticles for almost every environment. These data set the stage for future work directed towards understanding the evolutionary significance of variation in CHC composition in many fruit fly populations.

DOI:http://dx.doi.org/10.7554/eLife.09861.002

Reproductive character displacement of epicuticular compounds and their contribution to mate choice in Drosophila subquinaria and Drosophila recens

Interactions between species can alter selection on sexual displays used in mate choice within species. Here we study the epicuticular pheromones of two Drosophila species that overlap partially in geographic range and are incompletely reproductively isolated. Drosophila subquinaria shows a pattern of reproductive character displacement against Drosophila recens, and partial behavioral isolation between conspecific sympatric versus allopatric populations, whereas D. recens shows no such variation in mate choice. First, using manipulative perfuming experiments, we show that females use pheromones as signals for mate discrimination both between species and among populations of D. subquinaria. Second, we show that patterns of variation in epicuticular compounds, both across populations and between species, are consistent with those previously shown for mating probabilities: pheromone compositions differ between populations of D. subquinaria that are allopatric versus sympatric with D. recens, but are similar across populations of D. recens regardless of overlap with D. subquinaria. We also identify differences in pheromone composition among allopatric regions of D. subquinaria. In sum, our results suggest that epicuticular compounds are key signals used by females during mate recognition, and that these traits have diverged among D. subquinaria populations in response to reinforcing selection generated by the presence of D. recens.

Multifaceted, cross-generational costs of hybridization in sibling Drosophila species

Maladaptive hybridization, as determined by the pattern and intensity of selection against hybrid individuals, is an important factor contributing to the evolution of prezygotic reproductive isolation. To identify the consequences of hybridization between Drosophila pseudoobscura and D. persimilis, we estimated multiple fitness components for F1 hybrids and backcross progeny and used these to compare the relative fitness of parental species and their hybrids across two generations. We document many sources of intrinsic (developmental) and extrinsic (ecological) selection that dramatically increase the fitness costs of hybridization beyond the well-documented F1 male sterility in this model system. Our results indicate that the cost of hybridization accrues over multiple generations and reinforcement in this system is driven by selection against hybridization above and beyond the cost of hybrid male sterility; we estimate a fitness loss of >95% relative to the parental species across two generations of hybridization. Our findings demonstrate the importance of estimating hybridization costs using multiple fitness measures from multiple generations in an ecologically relevant context; so doing can reveal intense postzygotic selection against hybridization and thus, an enhanced role for reinforcement in the evolution of populations and diversification of species.

Sexual selection and experimental evolution of chemical signals in Drosophila pseudoobscura

Our expectations for the evolution of chemical signals in response to sexual selection are uncertain. How are chemical signals elaborated? Does sexual selection result in complexity of the composition or in altered quantities of expression? We addressed this in Drosophila pseudoobscura by examining male and female cuticular hydrocarbons (CHs) after 82 generations of elevated (E) sexual selection or relaxed sexual selection through monogamy (M). The CH profile consisted of 18 different components. We extracted three eigenvectors using principal component analysis that explained 72% of the variation. principal component (PC)1 described the amount of CHs produced, PC2 the trade-off between short- and long-chain CHs and PC3 the trade-off between apparently arbitrary CHs. In both sexes, the amount of CHs produced was greater in flies from the E treatment. PC3 was also higher, indicating that sexual selection also influenced the evolution of CH composition. The sexes differed in all three PCs, indicating substantial sexual dimorphism in this species, although the magnitude of this dimorphism was not increased as a result of our experimental evolution. Collectively, our work provides direct evidence that sexual selection plays an important role in the evolution of CHs in D. pseudoobscura and that both increased quantity and overall composition are targeted.

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.

Time in a bottle: the evolutionary fate of species discrimination in sibling Drosophila species

Disadvantageous hybridization favors the evolution of prezygotic isolating behaviors, generating a geographic pattern of interspecific mate discrimination where members of different species drawn from sympatric populations exhibit stronger preference for members of their own species than do individuals drawn from allopatric populations. Geographic shifts in species' boundaries can relax local selection against hybridization; under such scenarios the fate of enhanced species preference is unknown. Lineages established from populations in the region of sympatry that have been maintained as single-species laboratory cultures represent cases where allopatry has been produced experimentally. Using such cultures dating from the 1950s, we assess how Drosophila pseudoobscura and D. persimilis mate preferences respond to relaxed natural selection against hybridization. We found that the propensity to hybridize generally declines with increasing time in experimental allopatry, suggesting that maintaining enhanced preference for conspecifics may be costly. However, our data also suggest a strong role for drift in determining mating preferences once secondary allopatry has been established. Finally, we discuss the interplay between populations in establishing the presence or absence of patterns consistent with reinforcement.

Sexual isolation between North American and Bogota strains of Drosophila pseudoobscura

Sexual isolation, the reduced ability of organisms of different species to successfully mate, is one of the reproductive barriers that prevent gene flow between different taxa. Various species-specific signals during courtship are attributed to the sexual isolation between them. Drosophila pseudoobscura has been widely used to study the behavioral and genetic mechanisms underlying selection for sexual isolation, as a model system for speciation. D. pseudoobscura and its sibling species, D. persmilis, live together in many locations but are reproductively isolated from one another. North American geographic strains of D. pseudoobscura from the American West mate at random. Several decades ago, D. pseudoobscura was collected in the vicinity of Bogota, Colombia, and later named the subspecies D. pseudoobscura bogotana. Nearly 5,000 matings were observed in this study. We analyzed mating behavior and cuticular hydrocarbon profiles as well as courtship within and between North American and Bogota strains of D. pseudoobscura. Here we report for the first time that Bogota strains of D. pseudoobscura do show statistically significant sexual isolation from North American strains. In addition, there are quantitative variations in cuticular hydrocarbons as well as in courtship behavior between Bogota and North American strains, and females of both North American and Bogota strains show strong preference for North American strain males having high mating propensities, suggesting that the Bogota strains are at an early stage that could lead to a separate species.

Signatures of selection and sex-specific expression variation of a novel duplicate during the evolution of the Drosophila desaturase gene family

The tempo and mode of evolution of loci with a large effect on adaptation and reproductive isolation will influence the rate of evolutionary divergence and speciation. Desaturase loci are involved in key biochemical changes in long-chain fatty acids. In insects, these have been shown to influence adaptation to starvation or desiccation resistance and in some cases act as important pheromones. The desaturase gene family of Drosophila is known to have evolved by gene duplication and diversification, and at least one locus shows rapid evolution of sex-specific expression variation. Here, we examine the evolution of the gene family in species representing the Drosophila phylogeny. We find that the family includes more loci than have been previously described. Most are represented as single-copy loci, but we also find additional examples of duplications in loci which influence pheromone blends. Most loci show patterns of variation associated with purifying selection, but there are strong signatures of diversifying selection in new duplicates. In the case of a new duplicate of desat1 in the obscura group species, we show that strong selection on the coding sequence is associated with the evolution of sex-specific expression variation. It seems likely that both sexual selection and ecological adaptation have influenced the evolution of this gene family in Drosophila.

Pervasive genetic associations between traits causing reproductive isolation in Heliconius butterflies

Ecological speciation proceeds through the accumulation of divergent traits that contribute to reproductive isolation, but in the face of gene flow traits that characterize incipient species may become disassociated through recombination. Heliconius butterflies are well known for bright mimetic warning patterns that are also used in mate recognition and cause both pre- and post-mating isolation between divergent taxa. Sympatric sister taxa representing the final stages of speciation, such as Heliconius cydno and Heliconius melpomene, also differ in ecology and hybrid fertility. We examine mate preference and sterility among offspring of crosses between these species and demonstrate the clustering of Mendelian colour pattern loci and behavioural loci that contribute to reproductive isolation. In particular, male preference for red patterns is associated with the locus responsible for the red forewing band. Two further colour pattern loci are associated, respectively, with female mating outcome and hybrid sterility. This genetic architecture in which ‘speciation genes’ are clustered in the genome can facilitate two controversial models of speciation, namely divergence in the face of gene flow and hybrid speciation.

Rapid evolution of sex pheromone-producing enzyme expression in Drosophila

A wide range of organisms use sex pheromones to communicate with each other and to identify appropriate mating partners. While the evolution of chemical communication has been suggested to cause sexual isolation and speciation, the mechanisms that govern evolutionary transitions in sex pheromone production are poorly understood. Here, we decipher the molecular mechanisms underlying the rapid evolution in the expression of a gene involved in sex pheromone production in Drosophilid flies. Long-chain cuticular hydrocarbons (e.g., dienes) are produced female-specifically, notably via the activity of the desaturase DESAT-F, and are potent pheromones for male courtship behavior in Drosophila melanogaster. We show that across the genus Drosophila, the expression of this enzyme is correlated with long-chain diene production and has undergone an extraordinary number of evolutionary transitions, including six independent gene inactivations, three losses of expression without gene loss, and two transitions in sex-specificity. Furthermore, we show that evolutionary transitions from monomorphism to dimorphism (and its reversion) in desatF expression involved the gain (and the inactivation) of a binding-site for the sex-determination transcription factor, DOUBLESEX. In addition, we documented a surprising example of the gain of particular cis-regulatory motifs of the desatF locus via a set of small deletions. Together, our results suggest that frequent changes in the expression of pheromone-producing enzymes underlie evolutionary transitions in chemical communication, and reflect changing regimes of sexual selection, which may have contributed to speciation among Drosophila.

Molecular evolution and functional diversification of fatty acid desaturases after recurrent gene duplication in Drosophila

Frequent gene duplications in the genome incessantly supply new genetic materials for functional innovation presumably driven by positive Darwinian selection. This mechanism in the desaturase gene family has been proposed to be important in triggering the pheromonal diversification in insects. With the recent completion of a dozen Drosophila genomes, a genome-wide perspective is possible. In this study, we first identified homologs of desaturase genes in 12 Drosophila species and noted that while gene duplication events are relatively frequent, gene losses are not scarce, especially in the desat1-desat2-desatF clade. By reconciling the gene tree with species phylogeny and the chromosomal synteny of the sequenced Drosophila genomes, at least one gene loss in desat2 and a minimum of six gene gains (resulting in seven desatF homologs, alpha-eta), three gene losses and one relocation in desatF were inferred. Upon branching off the ancestral desat1 lineage, both desat2 and desatF gained novel functions through accelerating protein evolution. The amino acid residues under positive selection located near the catalytic sites and the C-terminal region might be responsible for altered substrate selectivity between closely related species. The association between the expression pattern of desatF-alpha and the chemical composition of cuticular hydrocarbons implies that the ancestral function of desatF-alpha is the second desaturation at the four carbons after the first double bond in diene synthesis, and the shift from bisexual to female-specific expression in desatF-alpha occurred in the ancestral lineage of Drosophila melanogaster subgroup. A relationship between the number of expressed desatF homologs and the diene diversification has also been observed. These results suggest that the molecular diversification of fatty acid desaturases after recurrent gene duplication plays an important role in pheromonal diversity in Drosophila.

On the scent of speciation: the chemosensory system and its role in premating isolation

Chemosensory speciation is characterized by the evolution of barriers to genetic exchange that involve chemosensory systems and chemical signals. Here, we review some representative studies documenting chemosensory speciation in an attempt to evaluate the importance and the different aspects of the process in nature and to gain insights into the genetic basis and the evolutionary mechanisms of chemosensory trait divergence. Although most studies of chemosensory speciation concern sexual isolation mediated by pheromone divergence, especially in Drosophila and moth species, other chemically based behaviours (habitat choice, pollinator attraction) can also play an important role in speciation and are likely to do so in a wide range of invertebrate and vertebrate species. Adaptive divergence of chemosensory traits in response to factors such as pollinators, hosts and conspecifics commonly drives the evolution of chemical prezygotic barriers. Although the genetic basis of chemosensory speciation remains largely unknown, genomic approaches to chemosensory gene families and to enzymes involved in biosynthetic pathways of signal compounds now provide new opportunities to dissect the genetic basis of these complex traits and of their divergence among taxa.

Divergence between the Drosophila pseudoobscura and D. persimilis genome sequences in relation to chromosomal inversions

As whole-genome sequence assemblies accumulate, a challenge is to determine how these can be used to address fundamental evolutionary questions, such as inferring the process of speciation. Here, we use the sequence assemblies of Drosophila pseudoobscura and D. persimilis to test hypotheses regarding divergence with gene flow. We observe low differentiation between the two genome sequences in pericentromeric and peritelomeric regions. We interpret this result as primarily a remnant of the correlation between levels of variation and local recombination rate observed within populations. However, we also observe lower differentiation far from the fixed chromosomal inversions distinguishing these species and greater differentiation within and near these inversions. This finding is consistent with models suggesting that chromosomal inversions facilitate species divergence despite interspecies gene flow. We also document heterogeneity among the inverted regions in their degree of differentiation, suggesting temporal differences in the origin of each inverted region consistent with the inversions arising during a process of divergence with gene flow. While this study provides insights into the speciation process using two single-genome sequences, it was informed by lower throughput but more rigorous examinations of polymorphism and divergence. This reliance highlights the need for complementary genomic and population genetic approaches for tackling fundamental evolutionary questions such as speciation.

No evidence for learned mating discrimination in male Drosophila pseudoobscura

BACKGROUND

Since females often pay a higher cost for heterospecific matings, mate discrimination and species recognition are driven primarily by female choice. In contrast, frequent indiscriminate matings are hypothesized to maximize male fitness. However, recent studies show that previously indiscriminate males (e.g., Drosophila melanogaster and Poecilia reticulata) can learn to avoid heterospecific courtship. This ability of males to discriminate against heterospecific courtship may be advantageous in populations where two species co-occur if courtship or mating is costly.

RESULTS

Here, we tested whether Drosophila pseudoobscura males learn to discriminate against heterospecific females after being exposed to and rejected by D. persimilis females. In most of our assays, we failed to observe differences in D. pseudoobscura courtship intensity of heterospecific females by males that had previously courted heterospecific females vs. males that had been maintained in isolation.

CONCLUSION

We conclude that learning to avoid heterospecific courtship may not be universal, even within the genus Drosophila, and may possibly be dependent on the natural history of the species.

Cuticular hydrocarbons: their evolution and roles in Drosophila pheromonal communication

I review the recent literature on cuticular hydrocarbons (CHs) in Drosophila. First, the major structural features of CHs are examined in a variety of species with regard to phylogeny. The genetic bases of the CH variation between and within species have been investigated with some of the genes involved characterized and manipulated. The effect of non-genetic factors as temperature, food and development is also examined with regard to CH production. Using a model involving the stimulating or the inhibiting role of CHs, it is possible to speculate on the mechanisms of CH perception and on the role(s) that these substances could play on sexual isolation and on the evolution of pheromonal communication.

A test of the chromosomal rearrangement model of speciation in Drosophila pseudoobscura

Recent studies suggest that chromosomal rearrangements play a significant role in speciation by preventing recombination and maintaining species persistence despite interspecies gene flow. Factors conferring adaptation or reproductive isolation are maintained in rearranged regions in the face of hybridization, while such factors are eliminated from collinear regions. As a direct test of this rearrangement model, we evaluated the genetic basis of hybrid male sterility in a sympatric species pair, Drosophila pseudoobscura pseudoobscura and D. persimilis, and an allopatric species pair, D. pseudoobscura bogotana and D. persimilis. Our results are consistent with the proposed model: virtually all of the sterility factors in the former pair are associated with three inverted regions, whereas sterility factors are present in the collinear regions in the latter pair. These findings indicate recombination and selection may have eliminated sterility factors outside the inverted regions between D. p. pseudoobscura and D. persimilis, suggesting chromosomal rearrangements may facilitate species persistence despite hybridization.

Evolution of water conservation mechanisms in Drosophila

Flies of the genus Drosophila inhabit a wide range of habitats, from the tropics to deserts to boreal forests. The primary physiological mechanism allowing Drosophila and other insects to survive in arid habitats is a reduction in rates of water loss. To understand mechanisms of water retention in greater detail, we investigated the three main routes by which Drosophila lose water: excretion, cuticular transpiration and respiratory loss through the spiracles. Excretory losses comprised <6% of total water flux and did not differ between xeric (cactophilic) and mesic species. No consistent relationship was observed between water-loss rates and the composition, physical properties or amounts of cuticular hydrocarbons, suggesting that cuticular transpiration did not differ among species from different habitats. Metabolic rates and water-loss rates were highly correlated. Cactophilic Drosophila were less active, and female cactophiles had lower metabolic rates than female mesic species of the same size. They were also more likely to exhibit a pattern of cyclic CO(2) release that may help to conserve water. We conclude that lower overall rates of water loss are achieved primarily by reduction of respiratory losses.

Perspective: female remating, operational sex ratio, and the arena of sexual selection in Drosophila species

As commonly observed among closely related species within a variety of taxa, Drosophila species differ considerably in whether they exhibit sexual dimorphism in coloration or morphology. Those Drosophila species in which male external sexual characters are minimal or absent tend, instead, to have exaggerated ejaculate traits such as sperm gigantism or seminal nutrient donations. Underlying explanations for the interspecific differences in the presence of external morphological sexual dimorphism versus exaggerated ejaculate traits are addressed here by examining the opportunity for sexual selection on males to occur before versus after mating in 21 species of Drosophila. Female remating frequency, an important component of the operational sex ratio, differs widely among Drosophila species and appears to dictate whether the arena of sexual selection is prior to, as opposed to after, copulation. Infrequent female mating results in fewer mating opportunities for males and thus stronger competition for receptive females that favors the evolution of male characters that maximize mating success. On the other hand, rapid female remating results in overlapping ejaculates in the female reproductive tract, such that ejaculate traits which enhance fertilization success are favored. The strong association between female remating frequency in a given species and the presence of sexually selected external versus internal male characters indicates that the relationship be examined in other taxa as well.

Chromosomal inversions and the reproductive isolation of species

Recent genetic studies have suggested that many genes contribute to differences between closely related species that prevent gene exchange, particularly hybrid male sterility and female species preferences. We have examined the genetic basis of hybrid sterility and female species preferences in Drosophila pseudoobscura and Drosophila persimilis, two occasionally hybridizing North American species. Contrary to findings in other species groups, very few regions of the genome were associated with these characters, and these regions are associated also with fixed arrangement differences (inversions) between these species. From our results, we propose a preliminary genic model whereby inversions may contribute to the speciation process, thereby explaining the abundance of arrangement differences between closely related species that co-occur geographically. We suggest that inversions create linkage groups that cause sterility to persist between hybridizing taxa. The maintenance of this sterility allows the species to persist in the face of gene flow longer than without such inversions, and natural selection will have a greater opportunity to decrease the frequency of interspecies matings.

Courtship in Drosophila

Courtship is a complex behavior in Drosophila that recruits a wide range of genes for its realization, including those concerning sex determination, ion channels, and circadian rhythms. Results from different experimental approaches-behavioral and genetic comparisons between species, analysis of mutants and mosaics, and identification of specific sensory stimuli-sketch the outlines of a set of pleiotropic genes acting on a distributed system in the brain to produce the species-specific sequence of responses and actions.

The genetics of reproductive isolation and the potential for gene exchange between Drosophila pseudoobscura and D. persimilis via backcross hybrid males

Hybrid male sterility, hybrid inviability, sexual isolation, and a hybrid male courtship dysfunction reproductively isolate Drosophila pseudoobscura and D. persimilis. Previous studies of the genetic bases of these isolating mechanisms have yielded only limited information about how much and what areas of the genome are susceptible to interspecies introgression. We have examined the genetic basis of these barriers to gene exchange in several thousand backcross hybrid male progeny of these species using 14 codominant molecular genetic markers spanning the five chromosomes of these species, focusing particularly on the autosomes. Hybrid male sterility, hybrid inviability, and the hybrid male courtship dysfunction were all associated with X-autosome interactions involving primarily the inverted regions on the left arm of the X-chromosome and the center of the second chromosome. Sexual isolation from D. pseudoobscura females was primarily associated with the left arm of the X-chromosome, although both the right arm and the center of the second chromosome also contributed to it. Sexual isolation from D. persimilis females was primarily associated with the second chromosome. The absence of isolating mechanisms being associated with many autosomal regions, including some large inverted regions that separate the strains, suggests that these phenotypes may not be caused by genes spread throughout the genome. We suggest that gene flow between these species via hybrid males may be possible at loci spread across much of the autosomes.

Courtship songs of Drosophila pseudoobscura and D. persimilis. II. Genetics of species differences

Although male courtship songs have been repeatedly implicated in sexual isolation between numerous Drosophila species, no genetic studies have evaluated the genetic basis of differences between species beyond using quantitative genetic analyses of hybrids or surveying associations of song characters to five or fewer genetic markers. Here, we dissect the genetic basis of the difference between D. pseudoobscura and D. persimilis in two courtship song elements (interpulse interval and intrapulse frequency) using 15 molecular markers. We also evaluate the association between song elements and sexual isolation in these backcross hybrid males of these species. We find that song differences between these species are associated with at least two or three genomic regions, and the species difference in interpulse interval may be oligogenic. Courtship song differences are especially strongly associated with two inversions that differentiate these species. Further, we found that interpulse interval is strongly associated with mating success to D. pseudoobscura females, while intrapulse frequency is associated with mating success to D. persimilis females. Implications of these findings are discussed.

Courtship songs of Drosophila pseudoobscura and D. persimilis: analysis of variation

Differences in Drosophila courtship song elements are thought to confer species sexual isolation because of the low levels of variation within species, the large differences often seen between closely related species, and the results of experiments using synthetic songs in two species groups. In this study, we reanalyse the courtship song elements of D. pseudoobscura and D. persimilis. We confirmed the differences between these species in high-rate repetition song interpulse interval (IPI) and intrapulse frequency and in the amount of low-rate repetition song produced. We found that both X-chromosome and autosomal loci contribute to the differences between D. pseudoobscura and D. persimilis in their IPI and intrapulse frequency. No association was detected between IPI and hybrid male mating success to females of either species, although frequency was associated with mating success to D. persimilis females. Additionally, we found evidence of IPI periodicity in the song of D. persimilis, demonstrated differences in mean IPI between D. pseudoobscura populations, and showed a significant influence of temperature on IPI and intrapulse frequency. Such environmental effects may have obscured tests for population differentiation in courtship song parameters in other species, and we suggest the possibility that differences between Drosophila species in mean courtship song IPI might only rarely affect their sexual isolation. Copyright 1998 The Association for the Study of Animal Behaviour.