Metamorphosis of memory circuits in Drosophila reveals a strategy for evolving a larval brain
- Howard Hughes Medical Institute, United States
Abstract
We have focused on the mushroom bodies (MB) of Drosophila to determine how the larval circuits are formed and then transformed into those of the adult at metamorphosis. The adult MB has a core of thousands of Kenyon neurons; axons of the early-born g class form a medial lobe and those from later-born a'b' and ab classes form both medial and vertical lobes. The larva, however, hatches with only g neurons and forms a vertical lobe 'facsimile' using larval-specific axon branches from its g neurons. Computations by the MB involves MB input (MBINs) and output (MBONs) neurons that divide the lobes into discrete compartments. The larva has 10 such compartments while the adult MB has 16. We determined the fates of 28 of the 32 types of MBONs and MBINs that define the 10 larval compartments. Seven larval compartments are eventually incorporated into the adult MB; four of their larval MBINs die, while 12 MBINs/MBONs continue into the adult MB although with some compartment shifting. The remaining three larval compartments are larval specific, and their MBIN/MBONs trans-differentiate at metamorphosis, leaving the MB and joining other adult brain circuits. With the loss of the larval vertical lobe facsimile, the adult vertical lobes, are made de novo at metamorphosis, and their MBONs/MBINs are recruited from the pool of adult-specific cells. The combination of cell death, compartment shifting, trans-differentiation, and recruitment of new neurons result in no larval MBIN-MBON connections persisting through metamorphosis. At this simple level, then, we find no anatomical substrate for a memory trace persisting from larva to adult. For the neurons that trans-differentiate, our data suggest that their adult phenotypes are in line with their evolutionarily ancestral roles while their larval phenotypes are derived adaptations for the larval stage. These cells arise primarily within lineages that also produce permanent MBINs and MBONs, suggesting that larval specifying factors may allow information related to birth-order or sibling identity to be interpreted in a modified manner in these neurons to cause them to adopt a modified, larval phenotype. The loss of such factors at metamorphosis, though, would then allow these cells to adopt their ancestral phenotype in the adult system.
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All data generated or analyses in this study are included in the manuscript and the supporting images
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Funding
Howard Hughes Medical Institute
- James W Truman
Howard Hughes Medical Institute
- Tzumin Lee
The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.
Copyright
© 2023, Truman et al.
This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.
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Metamorphosis of memory circuits in Drosophila reveals a strategy for evolving a larval brain
eLife 12:e80594.
https://doi.org/10.7554/eLife.80594
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