Catecholamine receptor polymorphisms affect decision-making in C. elegans

Abstract

Innate behaviours are flexible: they change rapidly in response to transient environmental conditions, and are modified slowly by changes in the genome. A classical flexible behaviour is the exploration-exploitation decision, which describes the time at which foraging animals choose to abandon a depleting food supply. We have used quantitative genetic analysis to examine the decision to leave a food patch in Caenorhabditis elegans. Here we show that patch-leaving is a multigenic trait regulated in part by naturally occurring non-coding polymorphisms in tyra-3 (tyramine receptor 3), which encodes a G-protein-coupled catecholamine receptor related to vertebrate adrenergic receptors. tyra-3 acts in sensory neurons that detect environmental cues, suggesting that the internal catecholamines detected by tyra-3 regulate responses to external conditions. These results indicate that genetic variation and environmental cues converge on common circuits to regulate behaviour, and suggest that catecholamines have an ancient role in regulating behavioural decisions.

Figure 1. Lawn-leaving behaviour varies between wild-type C. elegans strains

a) Lawn-leaving assays. Top: Six adult HW hermaphrodites on a bacterial lawn. One animal has left the lawn and one is leaving. Bottom: Track of a HW animal during 5 min of an assay; colour shows passage of time. The border of the lawn is outlined. Scale bar, 6 mm. b) Leaving rates of six wild-type strains. c) Leaving rates of 91 N2-HW recombinant inbred advanced intercross lines (RIAILs) and parental strains. d) QTL analysis of RIAILs shown in c. The horizontal line denotes the P<0.01 genome-wide significance threshold. Error bars indicate s.e.m.

Figure 2. N2 and HW tyra-3 alleles differentially affect leaving rates

a) Dissection of the QTL on X into two loci: leav-1 (4.70–4.78 Mb) and leav-2 (4.78–5.75 Mb). ‘Genotype’ shows chromosomes; thick line is X chromosome. Blue denotes HW DNA, red denotes N2 DNA, and yellow denotes the tyra-3(ok325) null mutant. In heterozygous strains, both X chromosomes are diagrammed. b) tyra-3 genomic fragments (Fig. 3a) reduce HW leaving rates. Blue, HW transgenes; red, N2 transgenes. Two-way ANOVA showed significant effects of both transgene concentration and DNA strain of origin. c) Effect of tyra-3 RNAi. Error bars indicate s.e.m. * P<0.05, ** P<0.01, or *** P<0.001 by t-test or ANOVA with Dunnett test.

Figure 3. Noncoding changes in tyra-3 affect its activity and expression level

a) HW polymorphisms in the tyra-3 locus relative to N2. tyra-3 encodes three predicted G protein-coupled receptors. The genomic region examined in Fig. 2b and the 4.9 kb promoter used in Figs. 3b and 4a are indicated. b) Leaving rates of transgenic HW animals with tyra-3b promoters fused to tyra-3b cDNAs. Error bars indicate s.e.m. ** P<0.01 by two-way ANOVA; no statistical interaction between the promoter and the cDNA. c) Relative amounts of tyra-3 isoform mRNAs in HW, N2, and leav-2 strains (Fig. 2a). Error bars indicate s.d. ** P<0.01 compared to HW, ANOVA with Dunnett test.

Figure 4. tyra-3 acts in ASK and BAG sensory neurons

a) Expression of 4.9 kb N2 tyra-3b promoter::GFP fusion (Fig. 3a) in HW animal; HW tyra-3b promoter::GFP is expressed in the same cells. Posterior signal is gut autofluorescence. Scale bar = 20μm. b) Leaving rates of HW strains expressing tyra-3b in specific cells. c) Left: GFP fluorescence intensity in ASK of HW animals with a MosSCI insertion of N2 or HW 4.9 kb tyra-3b promoter::GFP. Right: Schematic of MosSCI technique . d) Leaving rates after killing ASK or BAG in HW and leav-2 strains (Fig. 2a). Error bars indicate s.e.m. * P<0.05, ** P<0.01, or *** P<0.001 by t-test or ANOVA with Dunnett test.