Comparative pangenomics of Trypanosoma brucei reveal host specialization is driven by targeted or segmental genome streamlining in human and equine-restricted lineages

How the eukaryotic parasite Trypanosoma brucei remodels its genome during host specialization remains a fundamental question in evolutionary biology. In contrast to the uniform, systemic gene loss seen in resident bacterial genomes, the structural variations driving niche restriction in extracellular pathogens are poorly defined. Here, we establish a high-resolution comparative structural pangenomic map of the T. brucei complex. By analyzing the human-restricted specialist DAL972 and the equine-restricted specialist Trypanosoma equiperdum IVM-t1 against the conserved TREU927 backbone, we determine how host-vector constraints dictate eukaryotic genome architecture. Our analyses reveal that host specialization is governed by a coordinated spatial blueprint that segregates structural variation into two distinct evolutionary pathways: targeted perimeter streamlining and systemic core erosion. The human specialist DAL972 preserves its core metabolic toolkit under intense purifying selection, restricting genomic variation to the compartmentalized trimming of its sub-telomeric variant surface glycoprotein (VSG) archives. Conversely, the equine specialist IVM-t1, which has completely abandoned cyclical tsetse fly transmission, displays multi-focal core erosion across internal domains. We demonstrate that vector abandonment forces a shift to strict asexual clonal population dynamics. This transition eases purifying selection, allowing obsolete, insect-stage flagellar and cytoskeletal structures to decay through neutral genetic drift. At the chromosomal perimeters, this structural allocation unmasks an unannotated sub-telomeric reservoir of 86 profile-validated retrotransposon hot spot (RHS) and catalytic DDE transposase domains organized into 23 discrete loci. Operating under adaptive positive selection in the human specialist, these silent, repetitive elements function as physical recombination anchors that stabilize strand exchange. This mechanism drives rapid antigenic diversification while insulating the core genome from dangerous macro-rearrangements. Importantly, we bridge this macro-scale chromosomal architecture to micro-scale proteomic execution through tertiary modeling of these unannotated specialized loci. This structural translation unmasks a biophysical mechanism of structural sequestration, where lineage-specific mutations and specificity-determining positions are deeply buried within hydrophobic protein cores to shield functional divergence from host immune surveillance. By pairing these rigid, hidden core packing networks with hyper-variable surface tiles and dynamic boundary hinges, the parasite deploys tiered molecular mimicry to selectively manipulate host cellular networks. Together, these findings redefine our understanding of structural and non-coding architecture as mechanical drivers of parasitic adaptability, uncovering a direct pipeline from pangenomic spatial variations to stable, structure-based allosteric drug targets.