Comparative Development of the Ant Chemosensory System: Current Biology | Current Biology
Anna R. Ryba, Sean K. McKenzie, Leonora Olivos-Cisneros, E. Josephine Clowney, Peter Mussells Pires, Daniel J.C. Kronauer
The appearance of observable AL structure coincides at P4 with the growth of OSN axons into the lobe, suggesting that glomerular division is contingent on OSNs. Indeed, previous findings suggest that this is the case in other insects [
Construction of a protoglomerular template by olfactory axons initiates the formation of olfactory glomeruli in the insect brain.
]. We investigated the consequences of ablating OSNs at P0, before sensory neurons sprout to the antennal lobe. Most projections to the AL come from OSNs housed under sensory sensilla in the antennal club. Furthermore, projections from ant antennae are entirely ipsilateral [
Transcriptomics and neuroanatomy of the clonal raider ant implicate an expanded clade of odorant receptors in chemical communication.
]. Thus, we surgically removed the right antennal club at P0, unilaterally ablating sensory neurons. Aging manipulated animals to eclosion before collecting and imaging their brains allowed us to compare morphology between normally developing and deafferented ALs within single animals. Lobes that received no antennal input were markedly reduced in volume as compared to intact lobes and had many fewer glomeruli (Figures 3A and 3B ). In fact, reconstructions of deafferented lobes from two animals showed 90 and 91 large glomeruli, respectively, which exactly replicates the reduced antennal lobe structure in Orco loss-of-function mutants [
In flies, OSNs from the palps innervate a sizeable fraction of AL glomeruli, as do OSNs from the antennae that express ionotropic receptors (IRs) and gustatory receptors (GRs), two other types of chemosensory receptors that do not rely on Orco [
Molecular, anatomical, and functional organization of the Drosophila olfactory system.
, ]. One possible explanation of our results is therefore that the reduced AL structures we see in Orco mutants and surgically denervated animals arise because different populations of OSNs survive each manipulation. That is, the structures look similar largely by coincidence. For this to be true, some combination of ca. 90 populations of non-OR OSNs would have to survive Orco loss of function, whereas ca. 90 populations of AL-innervating OSNs in the palps would escape antenna ablation. However, analysis of previously published gene expression data [
Comparative genomics and transcriptomics in ants provide new insights into the evolution and function of odorant binding and chemosensory proteins.
] showed that only 4 IRs and GRs are expressed in ant heads (including palps, but excluding antennae), 13 in antennae, and 7 in both. Furthermore, backfills from antennae of wild-type adults showed that every glomerulus in the AL receives input from antennal neurons (Figure S2A), while backfills from Orco loss-of-function mutants showed that only six glomeruli in the T7 cluster continue to receive input from the antennae (Figure S2B). Finally, α-Orco staining of wild-type adult ALs revealed that all glomeruli except the T7 glomeruli are innervated by Orco-expressing OSNs (Figure S3A). Taken together, these results suggest that only the six glomeruli in the T7 cluster are innervated by OSNs expressing non-OR chemosensory receptors. Our genomic analysis also showed that only 12 ORs are expressed in ant heads (including palps, but excluding antennae). Finally, α-Orco staining of brains from adults that received unilateral ablation of the right antenna at P0 showed no glomeruli innervated by Orco-expressing OSNs (Figure S3B). These data, in combination with backfills from antennae in wild-type adults, suggest that few if any glomeruli are uniquely innervated by OSNs from the palps. Therefore, survival of non-OR and/or non-antennal OSNs can explain at most a small subset of the 90 glomerular structures in either Orco loss-of-function mutants or animals with antenna ablations. Instead, we conclude that the reduced AL morphologies that result from either manipulation are phenocopies of one another, probably resulting from an early loss of antennal OSNs in Orco mutants. Importantly, Orco loss of function results in a significant loss of OR-expressing OSNs in the antennae of ants, but not flies [
An engineered orco mutation produces aberrant social behavior and defective neural development in ants.
]. This suggests that the differences in AL phenotype between Orco mutants in ants and flies can at least partly be explained by the difference in OSN development or survival during the pupal stage.
The 90 remaining structures in deafferented antennal lobes could represent structures that develop before normal AL development arrests or could result from aberrant, retarded development unique to the deafferented lobe. To compare early changes in normally developing and deafferented antennal lobes, we removed the right antennal club at P0 and then collected and imaged brains at P2, P4, and P10. In the absence of input from its ipsilateral antenna, the size and complexity of a developing antennal lobe was retarded early in development. Deafferented lobes showed reduced volume as compared to intact lobes as early as P2, a disparity that increased as pupae aged (Figure 3D). Additionally, by P4, the number of observable glomeruli in deafferented versus intact lobes was markedly reduced (Figure 3E). These results show that, in ants, proper AL development is contingent on OSN innervation. However, even in the absence of sensory neuron input, a reduced structure forms that ultimately contains ca. 90 glomeruli, perhaps a template from which OSN-dependent elaboration of antennal lobe structure would normally progress.