CAREER: The effect of brain size on the evolution of neuronal circuits in the genus Drosophila

How brains evolve such that a species can adapt and survive in a new environment remains an open question in biology. Historically, this question has been confined to macroscopic approaches, thus limiting our knowledge of how neuronal circuits — the functional units of the brain — evolve. Recent advances in Drosophila research — aided by an array of sophisticated genetically-encoded tools — have enabled neuroscientists to understand, with cellular precision, how well-defined neuronal circuits detect sensory information and transform it to generate meaningful behaviors.

These tools can be easily applied to other Drosophila species, facilitating detailed comparative studies. In this project, these advances are used to address a fundamental question in evolutionary neuroscience: why do brains increase in relative size among related species? The research outlined in this proposal will break new ground into this question by bringing a neuronal circuit perspective to this long-standing problem.

The project also aims at showcasing the value of a humble fruit fly to scientific research — and in particular in the field of neuroscience — to the general public and to the next generation of biologists by pursuing different outreach activities that aim at increasing public awareness for the potential and importance of Drosophila research.

Why the brains of some species — including our own — grow larger and what advantages these larger brains may convey are questions is of immense interest. Drosophila pseudoobscura — a species that diverged from D. melanogaster about 25 million years ago — has an unusually large brain relative to its body size when compared to other Drosophila species.

This size difference is immediately noticeable in the antennal lobe, an olfactory processing center: while the overall organization of the antennal lobe is largely conserved in both species, some glomeruli are three times as large as their D. melanogaster counterparts and there are two D. pseudoobscura-specific glomeruli. These observations led to two hypotheses.

First, the large D. pseudoobscura antennal lobe might result from an increase in both the number of neurons associated with certain glomeruli as well as from an increase in the total number of glomeruli. Second, these differences might endow the D. pseudoobscura olfactory circuit with an increased detection bandwidth. This research project will test these hypotheses by quantifying which components of the olfactory circuit of D. pseudoobscura increase in number and by identifying the functional consequences of these quantitative changes.

These results will reveal which elements of a neuronal circuit are evolutionarily malleable and how such changes affect the function of a circuit. It is possible that from these analyses fundamental rules of neuronal circuit evolution can be delineated. This research project will also lead to the formulation of specific hypotheses regarding the potential adaptive significance of changes in neuronal circuits and lay the foundation to identifying the genetic underpinnings of brain evolution.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.