Nematology Monographs & Perspectives, 2015, Vol. 11, 331-352

Chapter 12

Pristionchus pacificus olfaction

Ray L. HONG Biology Department, California State University, Northridge, CA 91330, USA [email protected]

Introduction

Olfaction allows to detect chemical molecules from the environment from a distance and is probably the most important sensory modality in . Unlike gustation, which detects water-soluble chemicals by direct contact, olfactory cues allow nematodes to sense minute amounts of chemicals associated with nutrients, danger and potential hosts before engaging in relevant chemotaxis. Because of their similar culturing requirements in the laboratory, it was presumed that much of the superficial anatomical resemblances between the model systems and Pristionchus pacificus entailed similar odour preference profiles. It took almost a decade after the adoption of P.pacificus as a model system and the discovery of a species- specific association between Pristionchus species and before a systematic survey of the olfactory preferences revealed not only strong differences in the types of molecules and direction of responses among Pristionchus species, but also diametrically opposed odour profiles between P. pacificus and C. elegans (Hong & Sommer, 2006). Ensuing genetic studies identified a highly conserved protein kinase involved in the natural variation for an sex pheromone, but many more genes and genetic mutants need to be identified and characterised before it is possible to estimate the level of conservation between the two species. Below, I review recent advances and highlight gaps in our understanding of the molecular mechanisms of olfaction in these two nematode models.

© Koninklijke Brill NV, Leiden, 2015 331 R. L. Hong

Olfaction in C. elegans and P. pacificus

Investigations into the genetics of olfaction in C. elegans began in earnest when Bargmann and coworkers established the basic odour palate using the population chemotaxis assay (Bargmann et al., 1993). Young adult worms were washed three times with water or M9 buffer to remove food and placed onto large 10 cm diam. assay plates made of NGM (Nematode Growth Medium) agar without tryptone (Fig. 12.1A). The wild-type C. elegans N2 strain was tested for attraction to low molecular weight, commercially available compounds such as 2,3-butanedione (diacetyl), 2,3-pentanedione, benzaldehyde, pyrazine, isoamyl alcohol, 2-butanone and 3,4,5-trimethylthiazole (Fig. 12.1B). Because most chemicals were diluted in ethanol, 100% ethanol was used as the counter-attractant. After 1 h, the number of worms on the attractant and counter-attractant areas were scored. The result for each assay with 100-200 worms was then expressed as a chemotaxis

Fig. 12.1. Pristionchus chemosensation. A: Chemotaxis assays are conducted on 10 cm NGM plates without tryptone. Washed worms are placed on a spot equidistant from the attractant and counter-attractant. Sodium azide- immobilised worms are scored after most of the worms have dispersed from the loading site; B: The Oriental , Exomala orientalis, is a host for P. pacificus in Japan; P. pacificus is particularly attracted to insect pheromones (ZTDO, ETDA, methyl myristate) and plant volatiles (β-caryophyllene and nicotinic acid) whilst C. elegans is primarily attracted to bacterial catabolites.

332 Nematology Monographs & Perspectives