Co-evolution and circuit integration of mushroom bodies and central complex

• Applicant: Dr. Max Stephen Farnworth
• Subject Area: Systematics and Morphology (Zoology) ; Evolutionary Cell and Developmental Biology (Zoology) ; Cognitive, Systems and Behavioural Neurobiology
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 450708951

Species differ drastically in brain structure and function, reflecting how they have adapted to their environment. However, evolutionary modifications of the brain need to overcome functional constraints by which modifications can only occur when keeping other functions undisturbed. I want to explore the interplay between such functional constraints and adaptability of the brain, using the Heliconiini tribe of Neotropical butterflies as model system. Heliconius butterflies are uniquely qualified for such questions, as they strongly vary in brain size, are closely related and exhibit a behavioural innovation unique to butterflies. Specifically, they exhibit a pollen feeding behaviour that is associated with massively expanded mushroom bodies (MBs). To feed on pollen they need to memorize the spatial location of feeding plants, necessitating visual stimuli to be perceived and memorized. In Heliconiini this task is performed by the MBs. In other insects, such as Drosophila, spatial memory is processed by the central complex (CX), and ancestrally, the CX is involved in spatial processes. Hence, we suspect that MBs and CX co-evolve, or at least integrate signals in a common neural circuit, to enable pollen feeding in Heliconius. Curiously, while Heliconius MBs are 4X larger than their closest relatives’, CX size seems to not vary. Hence, we want to investigate how these neuropils co-evolve, using much finer measures than size, to identify patterns of adaptability and constraints, thus offering empirical data to the concept of functional constraints. I have three objectives to address this. First, I will investigate global patterns of volumetric investment in MBs and CX in 45 species (imaging data already available). Using 3D reconstruction and subsequent phylogenetic comparative analysis I will reveal co-adaptive patterns in the Heliconiini tribe to which results of the following two objectives can be assigned to. In objectives 2 and 3, I will select four of the most diverse species in MB and CX size. In objective 2 I will label synapsin and conserved neuroactive substances, such as Serotonin, to determine differences in synaptic density and fluorescence intensity. In addition, differences in the pattern of neurons expressing such neuroactive substances will be quantified by combining neuron registration and morphometric analyses. For objective 3, I will perform novel Neurobiotin microinjections into characteristic locations, and then determine the connections between the neuropils using light sheet microscopy, which will be quantified analogous to objective 2. Hence, objectives 2 and 3 will reveal species differences in synaptic density, neuromodulator expression and connectivity that will be contextualized with volumetric differences of objective 1. This collection of objectives promises to reveal on an unprecedented level where brain areas might co-evolve, how functional constraints act in evolutionary processes and where adaptability is reflected.

• DFG Programme: WBP Fellowship
• International Connection: United Kingdom
• Host: Dr. Stephen H. Montgomery