Figure 1. Drosophila larvae display innate, stimulus-specific responses to amino acids.

(A) Schematic of the larval gustatory choice assay. Groups of ~30 experimentally-naïve larvae are placed at the centre of a Petri dish divided in four quadrants, which contain control (agarose alone) or test (agarose + tastant) substrates. The Preference Index is calculated by counting the number of larvae on each substrate and dividing the difference by the total number of larvae; positive values reflect behavioural attraction towards a tastant-containing substrate. (B) Preference Indices of wild-type larvae (w¹¹¹⁸) for substrates containing individual L-amino acids (all at 50 mM, except for tyrosine, which was used at 2 mM because of its lower solubility). Essential amino acids are shown in blue. Grey boxes indicate amino acids that produce a Preference Index significantly different from 0 (t-test with Bonferroni correction, p < 0.0025). For boxplots in this and subsequent figures, the boxes represent the upper and lower quartiles, the thick line is the median value and the whiskers show the 5^th and 95^th percentiles. Circles outside the whiskers are outliers. (N ≥ 18 groups of ~30 larvae for each tastant). (C) Venn diagram (adapted from ref. 57) summarising the main physico-chemical properties of amino acids, indicated by single letter codes. Large black letters indicate those that are attractive to larvae; the sole aversive amino acid is shown in white.