Life has evolved under the gravitational field of Earth since it began; so it is fascinating and fundamental to find out how living things respond to an environment with different gravity. Experiments on Drosophila melanogaster
, the common fruitfly, in microgravity conditions on-board the Columbia Space Shuttle (STS-65) [1
] and during the Cervantes mission on the International Space Station (ISS) [2
] showed a striking increase in the frequency of locomotor activity and walking speed, compared with controls performed on the ground.
is an ideal model organism on which to study the effects of gravity: the flies are small enough that many individuals can be contained in compact cells suitable for space-flight, yet complex enough to possess a sophisticated gravity sense mechanism [3
]. Their use is ubiquitous in studies of biological developmental processes and in endeavours to understand cellular mechanisms in higher organisms, and they have been used in a number of studies on the origin of the biological gravity sense mechanism [4
]. The motility of these flies has been linked to molecular ageing responses that could be of significance for future human space exploration [6
In these experiments, we set out to investigate the walks of D. melanogaster (henceforth referred to as ‘fruitflies’) in a ground-based ‘simulation’ of the microgravity environment in space. We have used a relatively new technique called diamagnetic levitation to provide a pseudo-weightless environment, which requires a powerful magnetic field with a large field gradient to levitate the flies. Ground-based experiments are essential for selecting feasible and interesting experiments for space-flight studies. A primary aim of our experiments is to demonstrate the usefulness of diamagnetic levitation as a viable alternative to more established ground-based techniques for simulating the effects of microgravity on a complex organism, such as the random positioning machine, or parabolic flights.
An additional aim of the experiments is to validate findings of the original space-flight experiments on fruitflies. In the experiments aboard the Columbia Space Shuttle [1
], six groups of 50 male flies remained in orbit for nearly 15 days. The flies were hatched and incubated on the ground until they reached the adult stage. Approximately 8 h after launch, two of the groups were installed in a 1g
centrifuge aboard the Shuttle. Every 2 days the containers were transferred to a glove box, where they were recorded with a video camera for 15 min to observe their behaviour. All groups of flies (including those from the 1g
centrifuge) showed pronounced increases in the frequency of locomotor activity and walking speed. It was necessary to remove the flies from the 1g
centrifuge aboard the Shuttle during periods of video recording. Hence, the centrifuge cannot be regarded as a control in these experiments, at least as far as the behaviour is concerned. Any behavioural abnormalities were identified by comparison with experiments performed on the ground. Although great efforts were made to ensure a close match between the environmental conditions of the flies on the ground and those of their counterparts on the Shuttle, the flies on the ground did not experience the conditions of the Shuttle launch, which were difficult to reproduce exactly in the ground-based experimental control. Indeed, subsequent experiments performed on the ISS showed that the behaviour of the flies on the ISS was sensitive to launch procedures [2
]. There is thus some doubt about the root cause of the observed motility increase in microgravity.
By using diamagnetic levitation, we were able to ensure that all groups of flies were treated in the same way, except for their differing positions in the magnetic field, and that all experiments were performed simultaneously.
In diamagnetic levitation, diamagnetic materials such as water and many organic-based materials including oils, plastics and biological material are levitated using a strong, spatially varying magnetic field [7
]. Diamagnetic material is weakly repelled from magnetic fields, compared with the more commonly known ‘magnetic’ (i.e. ferromagnetic) materials such as iron, which are strongly attracted to a magnetic field. The diamagnetic force, balancing the weight of the levitating object, acts at the molecular level throughout the body of the object, just as the centrifugal force balances the gravitational force on an object in Earth orbit. The forces on a diamagnetically levitated object differ importantly from those on a floating, neutrally buoyant object (such as the forces on an SCUBA diver), in that the weight of the levitated object is balanced throughout the body of the object, not just at its surface.
Levitation of water and organic materials was reported first by Beaugnon & Tournier [8
]. The potential of diamagnetic levitation for studying living organisms in weightlessness was first demonstrated by Valles et al.
] who levitated frog embryos, and by Geim and colleagues, who levitated a variety of small living organisms, including a live frog ([12
], see also [15
]). Levitation has since also been used in studies of micro-organisms [16
], single-cell cultures [20
], biomolecule aggregation in vitro
] and protein crystal growth [25
We used a superconducting solenoid magnet with a vertical bore to levitate fruitflies. Water, being diamagnetic, is repelled by the strong magnetic field generated by the solenoid, with a force given by the product of the magnetic field strength and the field gradient [13
]. Because the field is strongest in the central region of the solenoid bore, the diamagnetic force acts in the direction opposite to gravity in the upper region of the bore, and in the same direction as gravity in the lower region. In our magnet, when the magnetic field at the geometric centre of the solenoid is 16.5 T, water levitates approximately 80 mm above the centre of the solenoid, where the diamagnetic force is equal in magnitude to the gravitational force on it. The technique of stable diamagnetic levitation has been described in detail elsewhere [8
]. The flies levitate at approximately the same position as water, owing to their high water content (in the region of 70 per cent by mass), as in any animal, and because the dry mass of the insect has a magnetic susceptibility similar to that of water. Further details about the levitation forces are given in §4 and the electronic supplementary material.
Temporal and spatial variations in the walking patterns of fruitflies have been studied as indicators of brain activity in response to environmental cues [27
]. Here, we concentrate on the measurement of the velocity of the flies and the mean square distance travelled as a function of time, in different effective gravities in the magnetic field.
The fruitflies were confined within three cylindrical ‘arenas’, 25 mm in diameter and 10 mm tall, stacked inside the magnet bore, one at the centre of the solenoid, one near the top of the solenoid, and one near the bottom, as shown in . Flies in the central arena experienced normal gravity. In the arena located near the top of the solenoid, where the diamagnetic force balances the gravitational force, flies experienced pseudo-weightless conditions. Below the normal gravity arena was a pseudo-hypergravity arena where the gravitational and magnetic forces sum together so that the effective weight of the flies is twice that outside the magnet [18
]. For convenience, we label the three arenas inside the bore as 0g
* and 2g
*, as shown in ; the asterisk indicates the presence of a strong magnetic field (16.5 T in 1g
*, 11.5 T in 0g
* and 2g
*). We label a zero magnetic field control, outside the magnet, as ‘1g
’. The flies could not escape from the arenas. We used the calculated effective gravity of water in the magnetic field, computed from the solenoid geometry [26
] and measurements of the magnetic field, to determine the vertical position of each of the three arenas in the magnet bore. The 0g
* arena was placed to enclose the stable levitation point of water [26
]. The 1g
* and 2g
* arenas were placed to enclose the point where the effective gravity on water is 9.8 ms−2
and 19.6 ms−2
, respectively. Additional details about the apparatus are given in §5; details on the calculation of the effective gravity are given in the electronic supplementary material.
Figure 1. (a) One of the arenas contained within a transparent plastic tube (diameter: 25 mm; height: 50 mm), viewed from the side. The arena floor is a semi-solid culture medium (off-white material at the bottom of the tube), which provided a food source and maintained (more ...)