Velvety tree ant extract is a chemotaxis repellent for C. elegans

Abstract

Ants use a range of compounds for interspecies interactions, but the neurogenetic mechanisms mediating these interactions are unclear. Here, we used chemotaxis assays with the nematode Caenorhabditis elegans to test if ant compounds can be detected by the worm nervous system and which chemosensory neurons are required for detection. We found that C. elegans avoid the extracts of velvety tree ants ( Liometopum occidentale ), and this response requires osm-9 and tax-4 positive neurons. These experiments were conducted by undergraduate students in an upper-division laboratory course, demonstrating how simple behavior assays conducted in a classroom setting can provide practical research experiences and new insights into interspecies interactions.

Figure 1. Velvety tree ant ( Liometopum occidentale ) extract induces an osm-9 and tax-4 dependent repulsive chemotaxis response in Caenorhabditis elegans.

(A) A diagram of the aspirator students constructed to collect ants. (B) Ants collected and sorted by students on the Stanford University campus. (C) Chemotaxis assays were performed on circular plates divided into experimental (E) and solvent (X) quadrants. Worms were placed in the center. (D) The chemotaxis response of osm-9(ky10) knockout worms (green), tax-4(p678) knockout worms (beige), tax-4(p678);osm-9(ky10) double mutants (brown), and wild type worms (PD1074, dark blue) were tested in response to Argentine and velvety tree ant extracts. Asterisks indicate significant differences with a p-value below 0.05; n.s. (not significant) indicates comparisons with a p-value above 0.05. The number of assays per group are indicated as dots overlaid on the boxplots. (E) Representative total ion chromatogram of overlaid Argentine ant (black) and velvety tree ant (blue) chemical extracts. For velvety tree ants, the corresponding mass spectrum is included (insert) for the putative alarm pheromone detected at 10.17 min. For Argentine ants, the corresponding mass spectrum (insert) for iridomyrmecin, detected at 23.65 min, is included.