Negative gravitactic behavior of Caenorhabditis japonica dauer larvae.

Gravity on Earth is a constant stimulus and many organisms are able to perceive and respond to it. However, there is no clear evidence that nematodes respond to gravity. In this study, we demonstrated negative gravitaxis in a nematode using dauer larvae (DL) of Caenorhabditis japonica, which form an association with their carrier insect Parastrachia japonensis. Caenorhabditis japonica DL demonstrating nictation, a typical host-finding behavior, had a negative gravitactic behavior, whereas non-nictating C. japonica and C. elegans DL did not. The negative gravitactic index of nictating DL collected from younger nematode cultures was higher than that from older cultures. After a 24 h incubation in M9 buffer, nictating DL did not alter their negative gravitactic behavior, but a longer incubation resulted in less pronounced negative gravitaxis. These results are indicative of negative gravitaxis in nictating C. japonica DL, which is maintained once initiated, seems to be affected by the age of DL and does not appear to be a simple passive mechanism.