Hebberecht, L. | Search bioRxiv

bioRxiv2022

Rapid expansion and visual specialization of learning and memory centers in Heliconiini butterflies

How do neural systems evolve to support new behaviors? Changes in the abundance and diversity of neural cell types, and their connectivity, shape brain composition and provide the substrate for behavioral variation. We describe a striking example of neural elaboration in an ecologically diverse tribe of Heliconiini butterflies. By building extensive new datasets of neural traits across the tribe, we identify major bursts in the size and cellular composition of the mushroom bodies, central brain structures essential for learning and memory. These expansion events are associated with increased innervation form visual centers and coincide with enhanced performance in multiple cognitive assays. This suite of neural and cognitive changes is likely tied to the emergence of derived foraging behaviors, facilitated by localized specialization of neural networks. One-Sentence SummaryMajor shifts in brain composition and behavior in butterflies with unique foraging and dietary behaviors.

evolutionary biology↗

bioRxiv2022

A butterfly pan-genome reveals a large amount of structural variation underlies the evolution of chromatin accessibility

Despite insertions and deletions being the most common structural variants (SVs) found across genomes, not much is known about how much these SVs vary within populations and between closely related species, nor their significance in evolution. To address these questions, we characterized the evolution of indel SVs using genome assemblies of three closely related Heliconius butterfly species. Over the relatively short evolutionary timescales investigated, up to 18.0% of the genome was composed of indels between two haplotypes of an individual H. charithonia butterfly and up to 62.7% included lineage-specific SVs between the genomes of the most distant species (11 Mya). Lineage-specific sequences were mostly characterized as transposable elements (TEs) inserted at random throughout the genome and their overall distribution was similarly affected by linked selection as single nucleotide substitutions. Using chromatin accessibility profiles (i.e., ATAC-seq) of head tissue in caterpillars to identify sequences with potential cis-regulatory function, we found that out of the 31,066 identified differences in chromatin accessibility between species, 30.4% were within lineage-specific SVs and 9.4% were characterized as TE insertions. These TE insertions were localized closer to gene transcription start sites than expected at random and were enriched for several transcription factor binding site candidates with known function in neuron development in Drosophila. We also identified 24 TE insertions with head-specific chromatin accessibility. Our results show high rates of structural genome evolution that were previously overlooked in comparative genomic studies and suggest a high potential for structural variation to serve as raw material for adaptive evolution.

genomics↗