Expression of the Gr28 genes in the larval sensory organs.
A) Schematic representation of the larval chemosensory system. Three external sensory organs (DO, TO and VO) hold collectively the dendritic extensions of neuronal cell bodies in the respective ganglia (DOG, TOG and VOG). Three clusters of sensory neurons (DPS, VPS and PPS) reside along the pharynx. The antennal nerve (AN) connects the DOG neurons to the subesophageal zone (SEZ). The TOG and VOG neurons project along the maxillary nerve (MN) to the SEZ. The DPS and PPS neurons project axons to the SEZ via the labral nerve (LRN). The VPS neurons project to the SEZ through the labial nerve (LBN). Note that olfactory neurons are omitted in the schematic here and in E (below).
B) Structure of the Gr28 locus. The six Gr28 genes are clustered within 10 kilobases. The five Gr28b genes are transcribed from distinct promotors, with first unique exons that are spliced to common second and third exons. Gr28a is downstream of the Gr28b genes. The Gr28 mutation (ΔGr28) used in this study lacks the shared common exons of the Gr28b genes and the entire Gr28a gene.
C) Expression of the Gr28 genes in the external sensory organs. Note that images show only one of the bilaterally symmetrical organs. Co-expression between Gr66a (a marker for bitter taste GRNs) and different Gr28 genes (in “Merge” panel) was assessed using GAL4 and LexA drivers for Gr28 genes and Gr66a, respectively. For Gr28b.a, Gr28b.c and Gr28b.e co-expression was observed in each case. However, Gr66a and Gr28a are expressed in different GRNs. Asterisks refer to a GRN expressing both Gr66a and the indicated Gr28 gene. Scale bars are 5 μm.
D) Expression of the Gr28 genes in internal sensory organs. Note that the bilaterally symmetrical halves of the DPS/VPS are physically close to each other, and the images includes both halves, while the image of the PPS shows only one side of the bilaterally symmetrical organ. Gr28b.c, but none of the other Gr28b genes, is co-expressed with Gr66a in the DPS/VPS. In the PPS, none of the Gr28b genes is found, while Gr28a and Gr66a are partially co-expressed, but each gene is also expressed exclusively in a subset of GRNs. Asterisks refer to a GRN expressing both Gr66a and the indicated Gr28 gene. Scale bars are 5 μm.
E) Expression summary: Only relevant neurons in one of the paired taste organs, are shown with total number of neurons indicated in parenthesis. The cartoon summarizes the immunostainings shown in C) and D). The VOG is not shown as none of the Gr28-GAL4 drivers is expressed there. Immunostaining was performed on whole-mount preparations from larvae heads of the following genotypes: UAS-mCD8:RFP lexAop-rCD2:GFP;Gr66a-LexA/Gr28a-GAL4, UAS-mCD8:RFP lexAop-rCD2:GFP;Gr66a-LexA/Gr28b.c-GAL4, UAS-mCD8:RFP lexAop-rCD2:GFP;Gr66a-LexA/Gr28b.e-GAL4 and UAS-mCD8:RFP lexAop-rCD2:GFP;Gr66a-LexA/+; Gr28b.a-GAL4/+. The Gr43aGAL4 GRNs, which do not overlap with Gr28a-GAL4 neurons (Mishra et al., 2018) are shown for reference to experiments described in Figures 2 and 7.