Homeostasis of internal carbon dioxide (CO2) and oxygen (O2) levels is fundamental to all animals. Here we examine the CO 2 response of the nematode Caenorhabditis elegans. This species inhabits rotting material, which typically has a broad CO 2 concentration range. We show that well fed C. elegans avoid CO 2 levels above 0.5%. Animals can respond to both absolute CO 2 concentrations and changes in CO 2 levels within seconds. Responses to CO 2 do not reflect avoidance of acid pH but appear to define a new sensory response. Sensation of CO 2 is promoted by the cGMPgated ion channel subunits TAX-2 and TAX-4, but other pathways are also important. Robust CO 2 avoidance in well fed animals requires inhibition of the DAF-16 forkhead transcription factor by the insulin-like receptor DAF-2. Starvation, which activates DAF-16, strongly suppresses CO 2 avoidance. Exposure to hypoxia (<1% O2) also suppresses CO 2 avoidance via activation of the hypoxiainducible transcription factor HIF-1. The npr-1 215V allele of the naturally polymorphic neuropeptide receptor npr-1, besides inhibiting avoidance of high ambient O 2 in feeding C. elegans, also promotes avoidance of high CO 2. C. elegans integrates competing O 2 and CO2 sensory inputs so that one response dominates. Food and allelic variation at NPR-1 regulate which response prevails. Our results suggest that multiple sensory inputs are coordinated by C. elegans to generate different coherent foraging strategies.carbon dioxide sensing ͉ natural variation ͉ oxygen sensing C O 2 is an important sensory cue for many organisms. Insects can use elevated CO 2 as part of an alarm signal or to find food (1-3). In fungi, high CO 2 can induce filamentation (4) and regulate sporulation (5). Nematode parasites of plants and animals can follow CO 2 gradients to locate their hosts (6, 7). Internal CO 2 levels also provide important signals. For example, insects and mammals monitor internal CO 2 to modulate respiratory exchange (8-10). This homeostatic function prevents respiratory poisoning and pH changes in body fluids, which can occur if CO 2 levels rise above 5% (11).Several mechanisms have been implicated in sensing CO 2 . In Drosophila, avoidance of high CO 2 is mediated by a pair of odorant receptors (2, 12, 13). Artificially activating neurons expressing these receptors elicits the escape response (14). Less is known about how insects monitor internal CO 2 to control opening of spiracles (15). In mammals internal CO 2 levels regulate breathing, diuresis, blood pH, and blood flow (8). In most cases the molecular sensors involved are unclear although pH changes associated with hydration of CO 2 are thought to be important. Carbonic anhydrases, which catalyze the hydration of CO 2 to produce H ϩ and HCO 3 Ϫ , are widely expressed in mammals. HCO 3 Ϫ has been shown to regulate the activity of a family of adenylate cyclases that is conserved from bacteria to man (16). However, the role of these enzymes in CO 2 signaling in animals is unclear. In fungi an HCO 3 Ϫ -regulated adenylate cy...

SummaryHomeostatic control of body fluid CO2 is essential in animals but is poorly understood. C. elegans relies on diffusion for gas exchange and avoids environments with elevated CO2. We show that C. elegans temperature, O2, and salt-sensing neurons are also CO2 sensors mediating CO2 avoidance. AFD thermosensors respond to increasing CO2 by a fall and then rise in Ca2+ and show a Ca2+ spike when CO2 decreases. BAG O2 sensors and ASE salt sensors are both activated by CO2 and remain tonically active while high CO2 persists. CO2-evoked Ca2+ responses in AFD and BAG neurons require cGMP-gated ion channels. Atypical soluble guanylate cyclases mediating O2 responses also contribute to BAG CO2 responses. AFD and BAG neurons together stimulate turning when CO2 rises and inhibit turning when CO2 falls. Our results show that C. elegans senses CO2 using functionally diverse sensory neurons acting homeostatically to minimize exposure to elevated CO2.

a b s t r a c tInnate immune mechanisms are well conserved throughout evolution, and many theoretical concepts, molecular pathways and gene networks are applicable to invertebrate model organisms as much as vertebrate ones. Drosophila immunity research benefits from an easily manipulated genome, a fantastic international resource of transgenic tools and over a quarter century of accumulated techniques and approaches to study innate immunity. Here we present a short collection of ways to challenge the fruit fly immune system with various pathogens and parasites, as well as read-outs to assess its functions, including cellular and humoral immune responses. Our review covers techniques for assessing the kinetics and efficiency of immune responses quantitatively and qualitatively, such as survival analysis, bacterial persistence, antimicrobial peptide gene expression, phagocytosis and melanisation assays. Finally, we offer a toolkit of Drosophila strains available to the research community for current and future research.

Eater is an EGF-like repeat transmembrane receptor of the Nimrod family and is expressed in Drosophila hemocytes. Eater was initially identified for its role in phagocytosis of both Gram-positive and Gram-negative bacteria. We have deleted eater and show that it appears to be required for efficient phagocytosis of Gram-positive but not Gram-negative bacteria. However, the most striking phenotype of eater deficient larvae is the near absence of sessile hemocytes, both plasmatocyte and crystal cell types. The eater deletion is the first loss of function mutation identified that causes absence of the sessile hemocyte state. Our study shows that Eater is required cell-autonomously in plasmatocytes for sessility. However, the presence of crystal cells in the sessile compartment requires Eater in plasmatocytes. We also show that eater deficient hemocytes exhibit a cell adhesion defect. Collectively, our data uncovers a new requirement of Eater in enabling hemocyte attachment at the sessile compartment and points to a possible role of Nimrod family members in hemocyte adhesion.