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Epichloe fungi are endophytes of cool season grasses, both wild species and commercial cultivars, where they may exhibit mutualistic or pathogenic lifestyles. The Epichloe -grass symbiosis is of great interest to agricultural research for the fungal bioprotective properties conferred to host grasses but also serves as an ideal system to study the evolution of fungal plant-pathogens in natural environments. Here, we assembled and annotated gapless chromosome-level genomes of two pathogenic Epichloe sibling species. Both genomes have a bipartite genome organization, with blocks of highly syntenic gene-rich regions separated by blocks of AT-rich DNA. The AT-rich regions show an extensive signature of RIP (repeat-induced point mutation) and the expansion of this compartment accounts for the large difference in genome size between the two species. This study reveals how the rapid evolution of repeat structure can drive divergence between closely related taxa and highlights the evolutionary role of dynamic compartments in fungal genomes. • Epichloe fungi have two-speed genomes with a bipartite structural organization. • Gene-rich genomic regions are highly syntenic between a pair of sibling species. • Repeat-rich regions have been targeted by RIP generating AT-rich compartments. • AT-rich compartments are highly diverged indicating distinct evolutionary histories. [ABSTRACT FROM AUTHOR]