Prevalence of a virus inducing behavioural manipulation near species range border.

The densities of conspecific individuals may vary through space, especially at the edge of species range. This variation in density is predicted to influence the diffusion of species-specific horizontally transmitted symbionts. However, to date there is very little data on how parasite prevalence varies around the border of a host species. Using a molecular epidemiology approach, we studied the prevalence of a vertically and horizontally transmitted virus at the edge of the geographic range of its insect host, the Drosophila parasitoid wasp Leptopilina boulardi. L. boulardi is a Mediterranean parasitoid species showing a recent range expansion to the north (in France). The LbFV virus manipulates the behaviour of females, increasing their tendency to lay additional eggs in already parasitized Drosophila larvae (superparasitism). This is beneficial for the virus because it allows the virus to be horizontally transferred during superparasitism. We show that LbFV prevalence is very high in central populations, intermediate in marginal populations and almost absent from newly established peripheral populations of L. boulardi. We failed to detect any influence of temperature and diapause on viral transmission efficiency but we observed a clear relationship between prevalence and parasitoid density, and between parasitoid density and the occurrence of superparasitism, as predicted by our epidemiological model. Viral strains were all efficient at inducing the behavioural manipulation and viral gene sequencing revealed very low sequence variation. We conclude that the prevalence reached by the virus critically depends on density-dependent factors, i.e. superparasitism, underlying the selective pressures acting on the virus to manipulate the behaviour of the parasitoid.

Induction by chlorpyrifos, of the confusion of males in discriminating female sexual pheromones used for mate finding by two sympatric trichogramma species (Hymenoptera: Trichogrammatidae).

Chlorpyrifos is one of the most widely used insecticides worldwide. It has been shown to have deleterious effects on survival of nontarget insects, but its impact on behavior has received less attention. In this study, we investigated the sublethal effects of this insecticide on sexual pheromone discrimination in two Trichogramma species. In these species, sexual pheromones arrest partners belonging to the same species. This specificity is important for reproduction efficacy because interspecific matings are sterile. We used two sympatric and closely related species of Trichogramma to study how two doses (LD 20 and LD 0.1) of chlorpyrifos can alter the discrimination by males and the emission by females of pheromones from both species. When exposed to the LD 20, the males of both species showed a decrease in the discrimination of conspecific sexual pheromones. For one of the two species (T. evanescens Westwood), this decrease even led to a total annihilation of discrimination. A dose as low as the LD 0.1, inducing no apparent mortality, induced the same decrease in pheromone discrimination for T. semblidis Aurivillius males. However, no effect was observed on the discrimination by males of sexual pheromones emitted by females either exposed to an LD 20 or an LD 0.1 of chlorpyrifos. By decreasing the discrimination by males of sexual pheromones, chlorpyrifos may induce interspecific interactions and attempts at copulating that would decrease the fitness of parasitoids. The implications of these results in relation to environmental pollution, the mode of action of the insecticide and the status of natural enemy of Trichogramma are discussed.

Ecological and genetic interactions in Drosophila-parasitoids communities: a case study with D. melanogaster, D. simulans and their common Leptopilina parasitoids in south-eastern France.

Drosophila species are attacked by a number of parasitoid wasps, which constitute an important factor of population regulation. Since Drosophila melanogaster and Drosophila simulans share common parasitoid species, their ecology and evolution can hardly be understood without considering parasitoids. After a short review of data available on Drosophila-parasitoid interactions involving D. melanogaster and D. simulans as hosts, we report field and laboratory experiments investigating the ecological role of Leptopilina parasitoids in Drosophila communities of southern France. Seasonal survey of species abundance shows that strong interspecific interactions occur at both tropic levels. D. simulans progressively replaces D. melanogaster in southern areas suggesting competitive displacement. Parasitoids are responsible for very high Drosophila mortality (up to 90% in some fruits). Field data emphasize the importance of selective pressure that parasitoids exert on Drosophila communities. The two Leptopilina parasites (L. heterotoma and L boulardi) have different local abundances, which vary in time, and they also compete for hosts. We show that parasitoids can mediate the coexistence of D. melanogaster and D. simulans in the laboratory, and thus may contribute to their puzzling coexistence in the field. Conversely, hosts exert selective pressures on parasitoids, and development on either D. melanogaster or D. simulans strongly affects fitness of adult wasps in a temperature-dependent fashion. Local variation in host species abundance and diversity could thus account for the genetic differentiation we observed in one parasitoid species. Despite laboratory studies cannot fully explain complex field situations, it is clear that the ecology and evolution of Drosophila populations and communities, especially D. melanogaster and D. simulans, are strongly constrained by parasitoids, which should receive more attention.

Adaptive significance of a circadian clock: temporal segregation of activities reduces intrinsic competitive inferiority in Drosophila parasitoids.

Most organisms show self-sustained circadian oscillations or biological clocks which control their daily fluctuations in behavioural and physiological activities. While extensive progress has been made in understanding the molecular mechanisms of biological clocks, there have been few clear demonstrations of the fitness value of endogenous rhythms. This study investigated the adaptive significance of circadian rhythms in a Drosophila parasitoid community. The activity rhythms of three sympatric Drosophila parasitoids are out of phase, the competitively inferior parasitoid species being active earlier than the superior competitor. This temporal segregation appears at least partially determined by endogenous periods of the clock which also vary between species and which correlate the time of activity. This earlier activity of the inferior competitor significantly reduces its intrinsic competitive disadvantage when multiparasitism occurs, thus suggesting that natural selection acting on the phase of the rhythm could substantially deviate the endogenous period from the optimal ca. 24 h period. This study demonstrates that temporal segregation of competing species could be endogenously controlled, which undoubtedly favours their coexistence in nature and also shows how natural selection can act on biological clocks to shape daily activity patterns.

Genetic variation in locomotor activity rhythm among populations of Leptopilina heterotoma (Hymenoptera: Eucoilidae), a larval parasitoid of Drosophila species.

The locomotor activity rhythm of Leptopilina heterotoma, a parasitoid insect of Drosophila larvae, was investigated under laboratory conditions. Under LD 12:12, the locomotor activity of females shows a clear rhythm which persists under continuous darkness (circadian rhythm). However, comparative study of five populations indicates that both the rate of activity and the profile of the rhythm vary according to the origin of females. The Mediterranean populations (Tunisia and Antibes) show two peaks of activity, at the beginning and at the end of the photophase, whereas more northern populations (Lyon and the Netherlands) are mostly active during the afternoon. Females originating from the area of Lyon have a very low level of activity. Reciprocal crosses (F1 hybrids and backcrosses) between the French and the Tunisian strains demonstrated the genetic basis of these variations and the biparental inheritance of the trait. This genetic variability is interpreted as a consequence of selective pressures and suggests a local adaptation of natural populations in host foraging behavior. The selective factors which could act on the daily organization of parasitoid behaviors are discussed.

Interest of TOF PET for clinical applications.

The interest of PET is now well established in medical research. Recently PET has proved to be also useful in the clinical domain e.g. for the diagnosis of epilepsy, myocardial disease etc... Its use has been so far limited by the cost of the radionuclide production and the cost of the PET itself. However, inexpensive, compact cyclotrons and centralized distribution of F18 are beginning to facilitate the availability of radionuclides. Thanks to its performances in terms of high random rate, sensitivity gain, random reduction, TOF is becoming practical for clinical use, especially with new types of architecture allowing the reduction of the cost of the detection head, so far the main obstacle to the use of TOF PET in clinical domain.

Correlated responses in lines of Drosophila melanogaster selected for different oviposition behaviors.

D. melanogaster was subjected to selection for two different traits of oviposition behaviour in relation to oocyte retention in ovaries, i.e. the aptitude to stop laying in response to an unsuitable substrate, and the occurrence of a dusk peak in the circadian oviposition rhythm. Selection for high and low lines was rapidly successful for both characters. Each selected line was also tested for the non-selected trait. Results showed a genetic correlation between the two behaviours, suggesting a common mechanism for the control of oviposition, independently of the origin and the duration of oocyte retention.

[Genetic variability of the circadian activity pattern of Drosophila in a natural population]. / Variabilité génétique du profil circadien d'activité de la Drosophile dans une population naturelle.

With an actographic recorder (IR emitter), the genetic variability in the circadian rhythm of locomoter activity has been analysed within a natural population of D. melanogaster. The activity pattern shows differences between sexes and environmental conditions (photoperiod LD or continuous darkness). In free running conditions, the period and the amplitude of the activity peaks differ between isofemale lines.

Reproductive strategy in Drosophila melanogaster: Significance of a genetic divergence between temperate and tropical populations.

Reproductive capacities of tropical and temperate populations of D. melanogaster were compared using three complementary techniques: (1) measure of egg production by females grown in the laboratory under uncrowded conditions and provided as adults with abundant food; (2) study of egg production of flies of unknown ages, collected in nature and then kept in similar conditions; and (3) analysis of ovarian activity of wild females dissected just after their capture.Tropical populations showed a lower fecundity in the laboratory and this was also observed in laboratory reared adults. On the average, flies also appeared to be older in the tropics than in temperate countries. These data, together with ecological observations showing that tropical populations live in a more predictable and stable environment, suggest that temperature populations are r-selected, while tropical ones are K-selected. The study of ovarian activity of wild females failed however to confirm this expectation. Tropical flies, which have a lower genetic fecundity, generally appeared to produce more propagules than did temperate flies. Such a contradiction shows how the ideas of r- and K-selection are difficult to apply to natural populations of Drosophila. Population density and interindividual competition are probably not the main selective forces in nature. Attention must also be paid to the necessity of exploring the environment to find resources, to the role of predation and parasitism, and to the occurrence in temperate countries of seasonal fluctuations with different selective pressures on successive generations.

Potential advantages of a cesium fluoride scintillator for a time-of-flight positron camera.

In order to improve the quality of positron tomographic imaging, a time-of-flight technique combined with a classical reconstruction method has been investigated. The decay time of NaI(Tl) and bismuth germanate (BGO) scintillators is too long for this application, and efficiency of the plastic scintillators is too low. Cesium fluoride appears to be a very promising detector material. This paper presents preliminary results obtained with a time-of-flight technique using CsF scintillators. The expected advantages were realized.

[Influence of light condition modification on the circadian rhythm of vitellogenesis and ovulation in Drosophila melanogaster].

PIP: The egg production and laying rhythms in the fruit fly were examined in constant darkness, a photoperiod of LD 12:12 and after switching from photoperiodic conditions to darkness. The flys were adult F1 heterozygotes (Brazzaville x Libreville); they were examined at 9, 14, and 19 hours and classified according to the terminology of King. In alternating light and dark there was a rhythm in egg production, with decreasing numbers of immature and increasing numbers of mature eggs during the photoperiod. Flies reared in total darkness kept their egg production rhythm for at least 5 days, but lost their rhythm of oviposition which ordinarily took place at the beginning of darkness. Removal of an egg-laying substrate also resulted in egg retention. Therefore, vitellogenesis has an endogenous circadian rhythm, while oviposition is dependent on light conditions.^ieng