Genome reshuffling as a route to specialization? Chromosome-level insights from the bark beetle symbiont genus Geosmithia.
Evolution of genome architecture is increasingly recognized as a major driver of fungal adaptation. However, because most studies have focused on plant and human pathogens, the genomic mechanisms underlying adaptation beyond pathogenic lifestyles remain poorly understood. Here, we establish the bark beetle-associated fungal genus Geosmithia as a model for studying genome evolution during…
Evolutionary changes in genome structure play a crucial role in the adaptation of fungi, but the mechanisms behind such adaptations in non-pathogenic fungi are not well understood. A study has focused on the bark beetle-associated fungal genus Geosmithia to explore genome evolution during ecological transitions. Geosmithia species can adapt to various ecological niches, from generalists to specialists, with some being obligate nutritional symbionts of ambrosia beetles.
Researchers used chromosome-level genome assemblies of eleven Geosmithia species, each representing different ecological strategies, to investigate how genome architecture, repetitive DNA, and gene family evolution contribute to this diversification. The results showed frequent intra- and interchromosomal rearrangements even between closely related species, indicating that chromosome restructuring can accumulate over short evolutionary timescales.
Breakpoints often occurred in gene-rich regions rather than repeat-rich areas, which is different from the patterns typically seen in fungi associated with transposable elements.
Hi-C analyses revealed a typical Rabl chromosome organization but unusual centromeres that are repeat-poor, gene-rich, transcriptionally active, and conserved as syntenic blocks across species. The centromere identity may be determined by a divergent CenH3 variant. Specialists within Geosmithia had increased repetitive DNA content and widespread gene family contractions, which is indicative of metabolic streamlining during ecological specialization.
Despite having similar genome sizes, specialists displayed these changes, suggesting that genome evolution and ecological diversification are closely intertwined. The study demonstrates that extensive structural genome evolution can occur in gene-dense fungal genomes and establishes Geosmithia as a valuable model for studying the relationship between genome evolution and ecological diversification.
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