Ribonucleosides are essential nutrients for rapid larval growth and survival.

<p>(a) Growth time in days from hatching of the first-instar larvae to eclosion (left) and survival rate (right) of larvae raised in different media shows that inosine and uridine are essential components. Larvae raised on HM grow slightly slower than, but have the same survival rate as, larvae raised on SCF. Replacing ribonucleosides with RNA (0.5 mg/mL) in HMΔ restores both growth time and survival rate, while replacing it with equimolar concentration of ribose fails to do so. Each bar represents the mean ± SEM (<i>n</i> = 4). Bars with different letters represent significant differences (two-tailed Mann-Whitney U test, <i>p <</i> 0.05). Genotype: <i>w</i>^<i>1118</i>. The underlying data can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2005570#pbio.2005570.s008" target="_blank">S4 Data</a>. (b) CaMPARI imaging of TO taste neurons shows that inosine and uridine, but none of the 3 other ribonucleosides, are potent ligands for Gr28 neurons. Uridine, cytidine (100 mM), and inosine (50 mM) were dissolved in water, while guanosine and adenosine were dissolved in DMSO and presented at concentration of 25 mM and 50 mM in water containing 25% and 10% f.c. DMSO, respectively. Each bar represents the mean ± SEM of ratios of red and green fluorescence intensities (<i>n</i> = 5–19). “*” represents significant differences between the preexposure (untreated) group to the groups with the indicated ligands applied (two-tailed Mann-Whitney U test, <i>p <</i> 0.05). Genotype: <i>w</i>^<i>1118</i><i>; UAS-CaMPARI/Gr28a-GAL4</i>. The underlying data can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2005570#pbio.2005570.s007" target="_blank">S3 Data</a>. (c) Two-choice preference assay shows that larvae require the <i>Gr28</i> genes to exhibit preference for uridine (50 mM, <i>n =</i> 12–24) and inosine (100 mM, <i>n</i> = 12–18). “*” represents significant difference between the genotypes (two-tailed Mann-Whitney U test, <i>p <</i> 0.05). All the genotypes are compared to control. Genotypes: <i>w</i>^<i>1118</i>, <i>w</i>^<i>1118</i>; Δ<i>Gr28/</i>Δ<i>Gr28 and w</i>^<i>1118</i>; Δ<i>Gr28/</i>Δ<i>Gr28; genGr28/+</i>. The underlying data can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2005570#pbio.2005570.s005" target="_blank">S1 Data</a>. f.c., final concentration; HM, holidic medium; SCF, standard cornmeal food; TO, terminal organ.