Gap-climbing fruit flies reveal components of goal-driven behaviors

Like humans, other animals are faced with everyday obstacles in their physical environments and must engage appropriate decision-making and motor skills to deal with them. Navigating these obstacles can involve highly complex events in mammals and other vertebrates, but in new work, researchers have employed an ingenious obstacle-based system for studying the control and structure of goal-oriented motor programs in the fruit fly Drosophila.

The findings are reported in Current Biology by Roland Strauss, of the University of Würzburg, and Simon Pick, of the University of Ulm, both in Germany.

In their studies, the researchers presented freely walking flies with a chasm in their path. The flies had shortened wings and could not fly over the gap, and they were thus forced to decide whether to attempt to cross the gap “by foot.” The researchers found that the flies first visually measure the gap width and then only cross the gap if it is a traversable width. If the chasm is short, flies engage in an extraordinary crossing behavior that the researchers showed involve three motor programs. High-speed video analyses revealed that flies can flexibly combine, and iteratively improve, these three behavioral adaptations in order to traverse gaps much broader than their own body size. The decisive phase for the climbing success of a fly is the formation of a “bridge” with the hind and middle legs still holding on to the starting side and the front legs just about reaching the edge of the target side. Forward shifting of the body into the gap, primarily by little hind-leg steps, contributes the most to gap-crossing. Auxiliary actions of the middle legs keep the body close to a horizontal position and help in reaching the other side. In the last step, the front legs stretch as much as possible to grasp the other side.

In some ways, the motor programs employed by the flies show high similarity to vertebrate motor programs–for example, when a person is trying to reach a jar on a shelf high above her head, she will reach out with her arm and hand, will straighten her back, and tiptoe.

Studying the underlying control principles and neuronal circuits in invertebrates rather than in mice or cats involves fewer ethical problems and is less-expensive research.

Using noninvasive neurogenetic techniques for the analysis of the underlying motor-control system, Pick and Strauss showed that in order to reach a goal, flies compose complex motor patterns from different behavioral subunits. In their genetic analysis, the researchers found mutant flies with defects specifically in deciding whether to cross, another group of mutants performing the correct motor program at a wrong position, and still another group of mutants performing climbing actions at the correct position but without one of the three motor components. This goal-directed flexibility of the fly’s motor programs characterized in the new work profoundly changes our view of how insect behavior is composed and employed in response to the environment.