Figure 2. Diverse responses to CO₂ across nematode species.

A. CO₂ chemotaxis behavior varies across nematode species [84]. Phoretic C. elegans dauers, which seek insect vectors, entomopathogenic Ste. carpocapsae IJs, and passively ingested H. contortus IJs are attracted to CO₂, while skin-penetrating Str. stercoralis IJs are repelled by CO₂ [51,84]. Dauers and IJs were tested in a chemotaxis assay with 10% CO₂, in which the animals were given 1 hr to migrate in a CO₂ gradient. A positive chemotaxis index (CI) indicates attraction and a negative CI indicates repulsion. B. The BAG neurons are required for multiple CO₂-evoked behaviors across species. Left, BAG neurons are required for CO₂ chemotaxis in C. elegans adults and dauers regardless of whether CO₂ is attractive or repulsive [37,51]. BAG-ablated C. elegans adults were tested in a 20 min assay [37], whereas dauers were tested in a 10 min assay [51]. Right, BAG neurons are required for both CO₂ chemotaxis and CO₂-evoked jumping in Ste. carpocapsae IJs [51]. The BAG neurons in IJs were laser-ablated; wild-type animals were mock-ablated. IJs were tested in either a 1 hr chemotaxis assay or a jumping assay in which IJs were given 8 s to jump in response to a 10% CO₂ puff [51]. C. The response of Ste. scapterisci IJs to CO₂ shifts from repulsion to attraction as the IJs age [90]. IJs were tested in a 1 hr chemotaxis assay with 1% CO₂.