Gravitaxis in Drosophila melanogaster: a forward genetic screen

Perception of the earth's gravitational force is essential for most forms of animal life. However, little is known of the molecular mechanisms and neuronal circuitry underlying gravitational responses. A forward genetic screen using Drosophila melanogaster that provides insight into these characteristics is described here. Vertical choice mazes combined with additional behavioral assays were used to identify mutants specifically affected in gravitaxic responses. Twenty-three mutants were selected for molecular analysis. As a result, 18 candidate genes are now implicated in the gravitaxic behavior of flies. Many of these genes have orthologs across the animal kingdom, while some are more specific to Drosophila and invertebrates. One gene (yuri) located close to a known locus for gravitaxis has been the subject of more extensive analysis including confirmation by transgenic rescue.     

Divergence in behaviour between the European corn borer,Ostrinia nubilalis,and its sibling species Ostrinia scapulalis: adaptation to human harvesting?

Divergent adaptation to host plant species may be the major mechanism driving speciation and adaptive radiations in phytophagous insects. Host plants can differ intrinsically in a number of attributes, but the role of natural enemies in host plant specialization is often underappreciated. Here, we report behavioural divergence between the European corn borer (ECB, Ostrinia nubilalis) and its sibling species Ostrinia scapulalis, in relation to a major enemy: humans. Harvesting maize imposes selective mortality on Ostrinia larvae: those located above the cut-off line of the stalk face almost certain death. We show that ECB larvae diapause closer to the ground than those of O. scapulalis, which is sympatric but feeds mainly on weeds. The difference in diapause height results from genetically determined differences in geotactic behaviour. ECB larvae descend towards the ground specifically at harvest time, increasing their chances of surviving harvesting by about 50 per cent over O. scapulalis larvae. Natural enemies appear as a major driver of host-plant specialization in this example, stressing the need to consider ‘tri-trophic’ ecological niches to understand insect diversification. Our results also strongly suggest that geotaxis evolved as a singular instance of behavioural resistance in a major agricultural pest.     

Pharmacological evidence for GABAergic regulation of specific behaviors in Drosophila melanogaster

We have identified several GABAergic‐modulated behaviors in Drosophila melanogaster by employing a pharmacological approach to disrupt GABA transporter function in vivo. Systemic treatment of adult female flies with the GABA transport inhibitors DL ‐2,4‐diaminobutyric acid (DABA) or R,S‐nipecotic acid (NipA), resulted in diminished locomotor activity, deficits in geotaxis, and the induction of convulsive behaviors with a secondary loss of the righting reflex. Pharmacological evidence suggested that the observed behavioral phenotypes were specific to disruption of GABA transporter function and GABAergic activity. The effects of GABA reuptake inhibitors on locomotor activity were dose dependent, pharmacologically distinct, and paralleled their known effects in mammalian systems. Recovery of normal locomotor activity and the righting reflex in DABA‐ and NipA‐treated flies was achieved by coadministration of bicuculline (BIC), a GABA receptor antagonist that supresses GABAergic activity in mammals. Recovery of these behaviors was also achieved by coadministration of gabapentin, an anticonvulsant agent that interacts with mammalian GABAergic systems. Finally, behavioral effects were selective because other specific behaviors such as feeding activity and female sexual receptivity were not affected. Related pharmacological analyses performed in vitro on isolated Drosophila synaptic plasma membrane vesicles demonstrated high affinity, saturable uptake mechanisms for [³H]‐GABA; further competitive inhibition studies with DABA and NipA demonstrated their ability to inhibit [³H]‐GABA transport. The existence of experimentally accessible GABA transporters in Drosophila that share conserved pharmacological properties with their mammalian counterparts has resulted in the identification of specific behaviors that are modulated by GABA. © 2002 Wiley Periodicals, Inc. J Neurobiol 50: 245–261, 2002; DOI 10.1002/neu.10030     

Vertical Migration of the Rice White-Tip Nematode,Aphelenchoides besseyi

In the laboratory, the vertical migration of Aphelenchoides besseyi was favored by rough surfaces and an inverse water gradient. In relation th gravity, the nematode migrated down or up equally, a circumstance suggesting that a geotaxis was not involved. Stems of rice seedlings were effective surfaces for vertical migration of nematodes only when the stems were continuously supplied with moisture.     

GEOTAXIS/PHOTOTAXIS AND BIOCHEMICAL PATTERNS IN HETEROCAPSA (=CACHONINA) ILLDEFINA (DINOPHYCEAE) DURING DIEL VERTICAL MIGRATIONS

Two separate experiments with Heterocapsa (= Cachonina ) illdefina Herman et Sweeney, one with and the other without water volume replacement, were performed in a 250-L laboratory mesocosm (45-cm diameter × 150-cm height) to examine how diel vertical migration (DVM) relates to taxis sign and strength and to cellular biochemical state. Although only the cell population grown with water volume replacement maintained a division per day over the course of the experiment, periodic measurements during both experiments demonstrated that cells aggregating at the surface during the light period generally were deficient in all measured biochemical constituents compared to cells obtained from a midcolumn depth. More specifically, H. illdefina cells that aggregated at the surface during the light period in both experiments exhibited weakened positive geotaxis but strengthened positive phototaxis and were very deficient in lipid and free amino acid compared to midcolumn cells. Cells sampled at midcolumn during the light period exhibited similar but weaker taxes changes compared to surface samples, and geotaxis strength was inversely correlated with cell diameter, cellular DNA and protein content, and RNA/DNA ratio. In comparison, published data on Gymnodinium breve Davis, a harmful algal bloom species, showed that cells aggregating at the surface during the light period generally exhibited weakened negative geotaxis and strengthened positive phototaxis and were very deficient in lipid and chl a compared to midcolumn cells. Although the persistent tendency toward negative geotaxis was weaker in midcolumn subpopulations throughout the day, its strength was inversely correlated with cell diameter and cellular lipid content. The combined results for both species support a revised conceptual model of optimized DVM in autotrophic marine dinoflagellates incorporating generalized expressions of taxis and biochemical state of individual cells.     

Poor geotaxis correlated with haematoporphyrin‐induced peroxidation of brain lipids as a predictor of medfly longevity reduction

Under illumination conditions, porphyrins generate cytotoxic radicals in cells. Our study evaluated the effects of haematoporphyrin IX (HP IX) in a laboratory population of male Ceratitis capitata (Wiedemann) (Diptera: Tephritidae) during exposure to a low fluence rate (39 μE m^?2 s^?1) of light. We found that exposing flies to HP IX for at least 5 days was sufficient to cause irreversible damage that led to anticipated death, as also provoked by chronic exposure to the same concentration. To identify early indicators of the accelerated senescence, we analysed both in vitro and in vivo parameters. The thiobarbituric acid reactive substances content in the heads of treated flies revealed a significant increase in lipid hydroperoxides at day 10, whereas this occurred several days later in controls. In addition, a significant decrease in glycogen content was observed at 15 days of age, 5 days before the reduction observed in the control group. This decrease has been associated with a decline in locomotor activity. Differences in the distribution of flies in the rearing flasks were observed, reflecting an impairment of the motility and climbing capacity of HP IX‐treated flies. This finding was also corroborated by a geotactic response assay (a rapid iterative negative geotaxis or RING assay). The results presented here demonstrate that low‐lethal oxidative stress can anticipate the senescence of flies, which can be predicted using a simple and fast behavioural test, such as the RING assay.     

Effects of gravity on positive phototaxis in fruit fly Drosophila melanogaster

Geotaxis and phototaxis are movements in response to gravity and light, respectively, and are commonly observed in nature. The interactions between these two types of movement have been shown to confer ecological advantages to many taxa. Although several studies have been conducted on phototaxis and geotaxis in various organisms, reports on the interactions between positive phototaxis and negative geotaxis are lacking. In the fruit fly, Drosophila melanogaster, any direct interactions that exist between positive phototaxis and negative geotaxis are yet to be determined and the ecological significance of such interactions remains unclear. In the present study, the effects of gravity on positive phototaxis in a Y‐maze were investigated using the Canton‐S wild type and gravity‐sensing‐deficient pyx³ mutant fruit flies. Gravity sensing was not necessary for horizontal positive phototaxis, but was required for vertical positive phototaxis. These results suggest that gravitoreception may selectively modulate positive phototaxis depending on the vertical and horizontal movements of the fruit flies.     

Motor Responses to Polarized Light and Gravity Sensing in Euglena gracilis*†

SYNOPSIS. Euglena gracilis strain Z has a motor response which results in orientation with respect to the polarization of a light stimulus. Cells swim preferentially in a direction perpendicular to the plane of polarization of the stimulus. If 2 polarized stimuli are given from opposite directions, the preferred direction is, under certain circumstances, at right angles to the directions of both stimuli. Euglena also preferentially assumes an orientation that is at right angles to the force of gravity. The relationships between these responses and phototactic movements oriented with respect to the direction of the stimulus are discussed.