Drosophila C virus cycle during the development of two Drosophila melanogaster strains (Charolles and Champetières) after larval contamination by food.

Drosophila C virus (DCV) cycle during Drosophila melanogaster development was studied after feeding contamination at the first, most sensitive, instar (L1). Two Drosophila strains were examined and compared. Presence of DCVC in apparently healthy animals (L3 larvae bred on a contaminated rearing medium and adults coming from larvae which were grown on medium containing DCVC) was demonstrated by biological tests. Using the immunofluorescence technique, DCV was exhibited in the diseased Charolles larvae, in the lumen of the digestive tract and in the basal part of gut cells which is in contact with the haemolymph. On the contrary, in Charolles larvae which seemed 'healthy', DCV was exhibited only in the lumen of the digestive tract at the apical boundary of the gut cells. But DCV typical protein capsid was not shown in the tissues of Drosophila L3 and adults. However, C virus remained in Drosophila tissues even after host metamorphosis and would seem to interact with Drosophila cells. Hypotheses are proposed concerning the intracellular state of Drosophila C virus in this case.     

Intraspecific genetic variation in host vigour,viral load and disease tolerance during Drosophila C virus infection

Genetic variation for resistance and disease tolerance has been described in a range of species. In Drosophila melanogaster, genetic variation in mortality following systemic Drosophila C virus (DCV) infection is driven by large-effect polymorphisms in the restriction factor pastrel (pst). However, it is unclear if pst contributes to disease tolerance. We investigated systemic DCV challenges spanning nine orders of magnitude, in males and females of 10 Drosophila Genetic Reference Panel lines carrying either a susceptible (S) or resistant (R) pst allele. We find among-line variation in fly survival, viral load and disease tolerance measured both as the ability to maintain survival (mortality tolerance) and reproduction (fecundity tolerance). We further uncover novel effects of pst on host vigour, as flies carrying the R allele exhibited higher survival and fecundity even in the absence of infection. Finally, we found significant genetic variation in the expression of the JAK-STAT ligand upd3 and the epigenetic regulator of JAK-STAT G9a. However, while G9a has been previously shown to mediate tolerance of DCV infection, we found no correlation between the expression of either upd3 or G9a on fly tolerance or resistance. Our work highlights the importance of both resistance and tolerance in viral defence.     

Ecology and population genetics of Sonoran Desert Drosophila

Three species of cactophilic Drosophila endemic to the Sonoran Desert of North America, D. nigrospiracula, D. pachea and D. mettleri, experience marked differences in spatial resource availability, and the first two of these display significant differences in dispersal behaviour. We employed starch gel and cellulose acetate electrophoresis for eight allozyme loci to test for a relationship between these variables and genetic differentiation among geographical populations of each species. No evidence was found for population structure in any of the three species, populations of which were separated by geographical distances of up to 475 km. Allele frequencies for two loci, Mdh-1 and Est-2, in D. nigrospiracula and D. pachea were very similar to those obtained approximately 30 years ago by other workers, indicating that the polymorphisms are remarkably stable under the stressful and variable conditions of the desert environment. High longevity, dispersal and multiple female remating are likely to contribute to the apparent high level of gene flow in all three species.     

Limitations of the RAPD technique in phylogeny reconstruction in Drosophila

In this study the limitations of the RAPD technique for phylogenetic analysis of very closely related and less related species of Drosophila are examined. In addition, assumptions of positional homology of amplified fragments in different species are examined by cross-hybridization of RAPD fragments. It is demonstrated that in Drosophila the use of RAPD markers is very efficient in identification of species. For assessment of phylogenetic relationships, however, the method is limited to sibling species, and reliable measures for genetic distances cannot be obtained. Hybridization experiments demonstrate that fragments of similar length amplified from different species are not always derived from corresponding loci, and that not all RAPD fragments within the same amplification pattern are independent.     

Evolution of the Drosophila melanogaster-sigma virus system in a natural population from Tübingen

Summary In natural populations of D. melanogaster, usually, a minority of individuals are infected by a Rhabdovirus called sigma. This virus is not contagious but is vertically transmitted through the gametes. In D. melanogaster, a polymorphism for two alleles (O, permissive and P, restrictive) of a gene responsible for resistance to the virus is regularly observed in the wild. On the virus side two types are found, which differ in their sensitivity to the P allele: Type I is very sensitive, and Type II more resistant. Previous findings had led to the hypothesis that an invasion of Type II clones, starting from central France, might be spreading over European populations. This replacement of viral Type I by viral Type II in natural populations could be observed in Languedoc (southern France), where it led to a dramatic increase in the frequency of infected flies. The invasion hypothesis is confirmed by the data from samples collected at Tübingen, where the frequency of Type II clones increased from 0.27 to 0.93 over a 6-year period (1985–1991). However, over the same period, no increase in the frequency of infected flies was observed. The evolution of other viral characteristics is discussed.     

Functional genomics of cactus host shifts in Drosophila mojavensis

Understanding the genetic basis of adaptation to novel environments remains one of the major challenges confronting evolutionary biologists. While newly developed genomic approaches hold considerable promise for addressing this overall question, the relevant tools have not often been available in the most ecologically interesting organisms. Our study organism, Drosophila mojavensis, is a cactophilic Sonoran Desert endemic utilizing four different cactus hosts across its geographical range. Its well-known ecology makes it an attractive system in which to study the evolution of gene expression during adaptation. As a cactophile, D. mojavensis oviposits in the necrotic tissues of cacti, therefore exposing larvae and even adults to the varied and toxic compounds of rotting cacti. We have developed a cDNA microarray of D. mojavensis to examine gene expression associated with cactus host use. Using a population from the Baja California population we examined gene expression differences of third instar larvae when reared in two chemically distinct cactus hosts, agria (Stenocereus gummosus, native host) vs. organpipe (Stenocereus thurberi, alternative host). We have observed differential gene expression associated with cactus host use in genes involved in metabolism and detoxification.     

Interspecific transfer of P elements by crosses between Drosophila simulans and Drosophila mauritiana

Interspecific crosses were carried out between P element-transformed strains of D. simulans and a strain of D. mauritiana, a species devoid of this transposable element family. Four lines were established from hybrid females backcrossed with D. mauritiana males for four generations, and then maintained by intra-line mass mating. In situ hybridization of polytene chromosomes and southern blots showed that full-length and deleted P elements were present in all of the lines after 15 generations. We conclude that at least some of the P elements observed in two lines result from their transposition into D. mauritiana genome. Gonadal sterility, induced at 29°C in D. melanogaster by P elements also occurred with these two latter lines.     

The Wolbachia protein TomO interacts with a host RNA to induce polarization defects in Drosophila oocytes

Wolbachia is an endosymbiont prevalent in arthropods. To maximize its transmission thorough the female germline, Wolbachia induces in infected hosts male‐to‐female transformation, male killing, parthenogenesis, and cytoplasmic incompatibility, depending on the host species and Wolbachia strain involved. However, the molecular mechanisms underlying these host manipulations by Wolbachia remain largely unknown. The Wolbachia strain wMel, an inhabitant of Drosophila melanogaster, impairs host oogenesis only when transplanted into a heterologous host, for example, Drosophila simulans. We found that egg polarity defects induced by wMel infection in D. simulans can be recapitulated in the natural host D. melanogaster by transgenic overexpression of a variant of the Wolbachia protein Toxic manipulator of oogenesis (TomO), TomO_wMel^?HS, in the female germline. RNA immunoprecipitation assays demonstrated that TomO physically associates with orb mRNA, which, as a result, fails to interact with the translation repressor Cup. This leads to precocious translation of Orb, a posterior determinant, and thereby to the misspecification of oocytes and accompanying polarity defects. We propose that the ability of TomO to bind to orb mRNA might provide a means for Wolbachia to enter the oocyte located at the posterior end of the egg chamber, thereby accomplishing secure maternal transmission thorough the female germline.     

Migration of glial cells into retinal axon target field in Drosophila melanogaster

During the development of the Drosophila visual system, photoreceptor (retinal) axons (R axons) project retinotopically to their targets in the optic lobes. The establishment of this precise pattern of connections does not depend on interactions between adjacent axon bundles, suggesting that R axons rely on environmental signals for proper pathfinding. Glial cells that are located along the R-axon trajectory are likely candidates to provide guidance cues for R-axon navigation. This study defines the origin of lamina glia (L glia), and demonstrates that L glia migrate into the lamina over a considerable distance. Glia are located in positions at which the R axons make critical growth choices. In the absence of cues from the eye, several classes of glia migrate to their final positions within the optic lobe anlage and begin to differentiate. Our results are consistent with a role for the glia in providing guidance cues to the R axons. © 1996 John Wiley & Sons, Inc.     

Olfactory responses of Drosophila suzukii females to host plant volatiles

Drosophila suzukii Matsumura, an endemic pest in southeast Asia, has invaded Europe and the U.S.A. Unlike most of its closely related sibling species, the serrated ovipositor of D. suzukii permits ovipositing in undamaged fresh fruits. In the present study, volatiles are identified from host plants that are potentially involved in D. suzukii host recognition and oviposition behaviour. It is shown that mated females are attracted to volatiles emitted from intact fruits. The antennally‐active suite of compounds released from the fresh fruits is identified by gas chromatography coupled with electroantennographic detection, as well as gas chromatography‐mass spectrometry. In olfactometer bioassays, mated females are significantly attracted to an electroantennographically active volatile, isoamyl acetate, when tested at 10 µg of synthetic compound in a rubber septa, which has a release rate comparable to that of fresh fruits. In addition, a genomic survey shows that D. suzukii not only possesses the full repertoire of genes encoding odorant receptors activated by isoamyl acetate in D. melanogaster, but also that one of the genes, OR67a, is represented by five duplicated copies. These results indicate that D. suzukii uses olfactory cues to select oviposition sites. The identification of volatiles emitted by host fruits that attract D. suzukii may aid in the development of a selective and efficient synthetic lure for monitoring this pest. As a close relative of Drosophila melanogaster, D. suzukii provides a unique opportunity for understanding the physiological mechanisms involved in the shift of this species from use of rotten to ripe fruits for oviposition.     

Evolutionary relationships of Drosophila mojavensis geographic host races and their sister species Drosophila arizonae

The cactophilic Drosophila mojavensis species group living in the deserts and dry tropical forests of the southwestern United States and Mexico provides a valuable system for studies in diversification and speciation. Rigorous studies of the relationships between host races of D. mojavensis and the relationships among the members of the species group (D. mojavensis, Drosophila arizona, and Drosophila navojoa) are lacking. We used mitochondrial CO1 sequence data to address the phylogenetics and population genetics of this species group. In this study we have found that the sister species D. mojavensis and D. arizonae share no mitochondrial haplotypes and thus show no evidence for recent introgression. We estimate the divergence time between D. mojavensis and D. arizonae to be between 1.91 and 2.97 million years ago. D. arizonae shows little structure in our population genetic analyses but there is phylogenetic differentiation between southeastern and northern populations of D. arizonae. Drosophila mojavensis shows significant population and phylogenetic structure across the four geographic regions of its distribution. The mitochondrial data support an origin of D. mojavensis on the mainland with early differentiation into the populations now found in the Mojave Desert and the Mainland Sonoran Desert and later colonization of the Baja Peninsula, in contrast to previous models. Also, the sister clade to D. mojavensis/D. arizonae includes D. navojoa and Drosophila huaylasi. By defining the genetic relationships among these populations, we provide a foundation for more sophisticated hypothesis testing regarding the timing of early speciation events and host switches in this species group.     

Virus particles in an internal parasite, Portunion conformis (Crustacea: Isopoda: Entoniscidae), and its marine crab host, Hemigrapsus oregonensis

Based on morphological evidence and preliminary physicochemical data, we report the first picornavirus from crustacean hosts. The viral particles are widespread in the tissues of an isopodan parasitic castrator, Portunion conformis, and its shore crab host, Hemigrapsus oregonensis, collected in San Francisco Bay, California. Less frequently, infected cells of the parasitic isopod also contain larger viral particles.     

Open ground individuals and population structure in Drosophila subobscura Collin

The distribution of Drosophila subobscura adults within, and their movement through, open ground habitats has been investigated, along with the status of such individuals within populations as a whole. Movement occurs between open ground and woodland, to some extent as a result of passive transport by air currents, but flies can remain in open ground for at least several days at a time. There are no detectable differences in either the reproductive age-structure or the body-size distributions of flies collected from woodland and open ground habitats, although a significantly higher proportion of females are found in open ground than in woodland. The large number of gravid females in open ground provide the potential for breeding there, but circumstantial evidence suggests that this is unlikely to occur. Rather, their presence appears to reflect the foraging strategy adopted by females in response to their high nutritional requirements. The flux of flies through open ground between woodlands suggests that previous estimates of the genetically-important parameter, effective population size, have been seriously underestimated for natural D. subobscura populations.     

The impact of anaesthetic technique on survival and fertility in Drosophila

Abstract The consequences of ice and carbon dioxide anaesthetics on the survival and reproductive success of Drosophila simulans and Drosophila melanogaster were investigated after observations of high levels of mortality in D. simulans, possibly associated with brief chill coma. The association between brief chill coma and death is confirmed in female D. simulans but there is no mortality in male D. simulans or D. melanogaster of either sex. Mortality is unlikely to be associated with a strain specific cold intolerance because two geographically distinct populations of D. simulans were examined. In addition to the effect of ice anaesthesia, anaesthetizing newly‐eclosed male D. simulans with CO₂ causes a reduction in fertility, which is evident 9–13 days after anaesthesia. This finding is important given that CO₂ anaesthesia is a standard technique used in Drosophila and other insect cultures, and may have important consequences for studies of male fertility and sperm competition.     

Habituation of an odorant-induced startle response in Drosophila

Habituation is a fundamental form of behavioral plasticity that permits organisms to ignore inconsequential stimuli. Here we describe the habituation of a locomotor response to ethanol and other odorants in Drosophila, measured by an automated high-throughput locomotor tracking system. Flies exhibit an immediate and transient startle response upon exposure to a novel odor. Surgical removal of the antennae, the fly's major olfactory organs, abolishes this startle response. With repeated discrete exposures to ethanol vapor, the startle response habituates. Habituation is reversible by a mechanical stimulus and is not due to the accumulation of ethanol in the organism, nor to non-specific mechanisms. Ablation or inactivation of the mushroom bodies, central brain structures involved in olfactory and courtship conditioning, results in decreased olfactory habituation. In addition, olfactory habituation to ethanol generalizes to odorants that activate separate olfactory receptors. Finally, habituation is impaired in rutabaga, an adenylyl cyclase mutant isolated based on a defect in olfactory associative learning. These data demonstrate that olfactory habituation operates, at least in part, through central mechanisms. This novel model of olfactory habituation in freely moving Drosophila provides a scalable method for studying the molecular and neural bases of this simple and ubiquitous form of learning.     

Individual inversions or their combinations: which is the main selective target in a natural population of Drosophila subobscura?

It is generally accepted that chromosomal inversions have been key elements in adaptation and speciation processes. In this context, Drosophila subobscura has been, and still is, an excellent model species due to its rich chromosomal polymorphism. In this species, many analyses from natural populations have demonstrated the adaptive potential of individual inversions (and their overlapped combinations, the so‐called arrangements). However, little information is available on the evolutionary role of combinations generated by inversions located in homologous and nonhomologous chromosomes. The aim of this research was to ascertain whether these combinations are also a target for natural selection. For this objective, we have studied the inversion composition of homologous and nonhomologous chromosomes from a D. subobscura sample collected in a well‐studied population, Mount Avala (Serbia). No significant deviation from H‐W expectations was detected, and when comparing particular karyotypic combinations, likelihood ratios close to 1 were obtained. Thus, it seems that for each pair of homologous chromosomes inversions no deviation from randomness was detected. Finally, no linkage disequilibrium was observed between inversions located in different chromosomes of the karyotype. For all these reasons, it can be assumed that, at the cytological level, the individual inversions rather than their combinations in different chromosomes are the main target of selection.     

Analysis of inbreeding in a colonizing population of Drosophila subobscura

An analysis of the effects of inbreeding on the genetic structure of a colonizing population of Drosophila subobscura has been carried out. Species of Drosophila, particularly D. subobscura, may have lethal alleles associated with chromosomal inversions and our aim was to assess the extent to which the genome is balanced in this way. The frequencies of chromosomal inversions were compared between a large population and a set of 72 lines that were maintained by brother-sister mating for 10 generations. Fisher's matrix method was used to calculate the expected homozygosity in these inbred lines for 5 allozyme loci (Aph, Hk-1, Lap, Odh, and Pept-1) used as markers of large chromosomal segments. Furthermore, the expected rates of fixation corresponding to these allozyme loci were also calculated. The results show that the amount of homozygosis observed did not differ significantly from expectations (with the corresponding loss of lines as a consequence of the reduction in viability). However, two deviations from strict neutrality were observed: there was a heterozygote excess at the Lap locus, and the frequency of the O ₅ inversion (always associated with a lethal gene in colonizing populations) was higher than expected.