Technical Abstract: Blends of ascarosides regulate dispersal in nematodes Presenter: Dr. Fatma Kaplan Dispersal is an important behavior for many organisms. It can easily be observed when infectious juveniles of entomopathogenic nematodes (Steinernema and Heterorhabditis) leave a consumed insect host. Dauer larvae of the phylogenetically related and well studied nematode C. elegans show a similar behavior when exposed to crowding and an unfavorable food situation. Here we show that C. elegans as well as S. feltiae dispersal is regulated by ascaroside semiochemicals, but contrary to the well studied C. elegans dauer formation, the actual natural blends of ascarosides might be important for this behavior. Four previously known ascarosides were identified in a dauer forming growth media and when C. elegans dauer were exposed to a synthetic blend of these components (ascr#2, ascr#3, ascr#8 and IcasC5) twice as many nematodes moved away from the food compared to a control with just food. Furthermore, the same blend was also recognized by S. feltiae IJs and by J2s of plant parasitic nematodes (Meloidogyne spp). A major component of the S. feltiae blend found in insect cadavers after dispersal was ascr#9 while none of the C. elegans components were detected. However, ascr#9 is a structural analog of asc#2 which is a comparable major component of the C. elegans dispersal pheromone. Similarly C. elegans dauer responded to a blend where asc#9 was substituted for asc#2. We propose that ascaroside blends represent common communication systems among nematodes where to a great extent common components are utilized but with species specific blends. Thus, nematodes sharing the same habitat can monitor and respond to other nematodes signals for example to avoid already depleted food sources. We anticipate the use of dispersal blends in a broad strategy to control parasitic nematode species by deterring them from a host.