Altered Grooming Cycles in Transgenic Drosophila.

Head grooming in Drosophila consists of repeated sweeps of the legs across the head, comprising regular cycles. We used the GAL4-UAS system to study the effects of overexpressing shibirets1 and of Adar knockdown via RNA interference, on the period of head-grooming cycles in Drosophila. Overexpressing shibirets1 interferes with synaptic vesicle recycling and thus with cell communication, while Adar knockdown reduces RNA editing of neuronal transcripts for a large number of genes. All transgenic flies and their controls were tested at 22° to avoid temperature effects; in wild type, cycle frequency varied with temperature with a Q10 of 1.3. Two experiments were performed with transgenic shibirets1: (1) each fly was heat-shocked for 10 min at 30° immediately before testing at 22° and (2) flies were not heat shocked. In both experiments, cycle period was increased when shibirets1 was overexpressed in all neurons, but was not increased when shibirets1 was overexpressed in motoneurons alone. We hypothesize that grooming cycles in flies overexpressing shibirets1 are lengthened because of synaptic impairment in neural circuits that control head-grooming cycles. In flies with constitutive, pan-neuronal Adar knockdown, cycle period was more variable within individuals, but mean cycle period was not significantly altered. We conclude that RNA editing is essential for the maintenance of within-individual stereotypy of head-grooming cycles.

Commensal bacteria play a role in mating preference of Drosophila melanogaster.

Development of mating preference is considered to be an early event in speciation. In this study, mating preference was achieved by dividing a population of Drosophila melanogaster and rearing one part on a molasses medium and the other on a starch medium. When the isolated populations were mixed, "molasses flies" preferred to mate with other molasses flies and "starch flies" preferred to mate with other starch flies. The mating preference appeared after only one generation and was maintained for at least 37 generations. Antibiotic treatment abolished mating preference, suggesting that the fly microbiota was responsible for the phenomenon. This was confirmed by infection experiments with microbiota obtained from the fly media (before antibiotic treatment) as well as with a mixed culture of Lactobacillus species and a pure culture of Lactobacillus plantarum isolated from starch flies. Analytical data suggest that symbiotic bacteria can influence mating preference by changing the levels of cuticular hydrocarbon sex pheromones. The results are discussed within the framework of the hologenome theory of evolution.

SYMMETRY VERSUS ASYMMETRY IN SEXUAL ISOLATION EXPERIMENTS.

Two hypotheses predicting the ancestral or derived status of populations and based on asymmetrical mate discrimination (Kaneshiro, 1976; Watanabe and Kawanishi, 1979) were tested using nine laboratory populations of D. simulans, a highly outcrossed ancestral population, and eight populations derived from it via founder-flush-crash cycles. The data from individual mating tests using pairwise combinations of these populations fit the Kaneshiro hypothesis reasonably well, rejecting the Watanabe-Kawanishi hypothesis. However, more powerful tests rejected the Kaneshiro hypothesis for the data we analyzed. The values for derived females predicted by the Kaneshiro hypothesis were biased: they were consistently high for derived males and consistently low for ancestral males. We propose a hypothesis, based on variation in mating propensities and symmetrical mate discrimination. We assessed the power of Kaneshiro's and our hypotheses to predict the number of matings between derived females and derived males by plotting predicted vs. observed values and fitting these points to the expected line of unit slope passing through the origin. Predictions of our hypothesis explained more of the variance (r2 = 0.87) than predictions of the Kaneshiro model (r2 = 0.63). While asymmetrical sexual isolation undoubtedly occurs between some species, its existence cannot be determined simply by measuring mating frequencies in a single experiment.

OVIPOSITION SITE SELECTION BY DROSOPHILA MELANOGASTER AND DROSOPHILA SIMULANS.

The effects of texture and larval residues in the medium on oviposition site selection (OSS) by Drosophila melanogaster and Drosophila simulans were studied. Drosophila melanogaster laid over 95% of its eggs in sieved medium (vs. unsieved medium); D. simulans laid all of its eggs in sieved medium. Surgical removal of antennal segments, and of fore-, mid-, or hindtarsi did not affect this result, indicating that sense organs involved in discriminating between sieved and unsieved medium are not confined to only one of the tested structures. In a "multiple choice" experiment, females were allowed to lay eggs in sieved medium of three types: unconditioned (fresh) medium, medium conditioned by D. melanogaster larvae (i.e., medium containing larval residues of D. melanogaster), and medium conditioned by D. simulans larvae. This choice experiment was performed with D. melanogaster and with D. simulans, using three densities of females (10, 20, and 40 per experimental unit). Both species laid more eggs in unconditioned medium than in either of the conditioned media, and density had no effect. D. melanogaster laid more eggs near the edges of food patches than in the center, whereas D. simulans showed no preference for edge or center. Under crowded conditions, both species survived at a higher rate in conditioned media (egg-to-adult survival) than in unconditioned medium, leading to the anomalous conclusion that females of these species seem not to maximize the survival of their offspring. This anomaly was partially resolved by the finding that medium already containing larvae gave lower survival rates than unoccupied medium.