Diversity and wiring variability of visual local neurons in the Drosophila medulla M6 stratum.
Bottom Line: Based on the segregation of putative dendrites and axons, these local neurons were classified into two families, directional and nondirectional.Neurotransmitter immunostaining suggested a signal routing model in which some visual information is relayed by directional M6-LNs from the anterior to the posterior M6 and all visual information is inhibited by a diverse population of nondirectional M6-LNs covering the entire M6 stratum.Our findings suggest that the Drosophila medulla M6 stratum contains diverse LNs that form repeating functional modules similar to those found in the vertebrate inner plexiform layer.
Affiliation: Institute of Biotechnology and Department of Life Science, National Tsing Hua University, Hsinchu, 30013, Taiwan.Show MeSH
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Mentions: We identified an independent Gal4 line, VT12760-GAL4, that showed GAL4 expression in only the nondirectional M6-LNs in the optic lobes. Compared with M6-LNs in the 30A-GAL4, those in the VT12760-GAL4 projected dense fibers to the anterior-dorsal and posterior edges but not the center field of M6 (Fig. 9A; n = 12). The terminals of putative dendrites and axons labeled by Dscam::GFP and Syt::HA, respectively, were small and mixed but did not colocalize (Fig. 9B), suggesting that VT12760-GAL4 expresses only nondirectional M6-LNs. This was confirmed by the following findings: 1) Cha-GAL80 inhibition reduced expression in the central brain (showing that the test was valid) but not in the anterior-dorsal and posterior edges of M6 (Fig. 9C) and 2) M6-LNs in the VT12760-GAL4 were all anti-GABA immunopositive (Fig. 9D). MARCM imaging collected at least six additional clusters of nondirectional M6-LNs (Fig. 10, Table12; n = 33). In addition, individual M6-LNs in the same cluster covered a similar number of retinotopic columns, with extensive areal overlapping between their axonal arbors, but made different choices for which specific columns to innervate (Fig. 11A,B). Combining several M6-LNs of the same cluster to the common M6 volume model showed that each cluster covered one distinct M6 domain (Fig. 11C,D). Brainbow labeling of multiple VT12760-GAL4 neurons with different colors in the same brain confirmed that M6-LNs exhibit individual variability limited within the same domain and domain specificity between different clusters (Fig. 11E–H).
Affiliation: Institute of Biotechnology and Department of Life Science, National Tsing Hua University, Hsinchu, 30013, Taiwan.