Polymorphism and evolutionary origins of accessory chromosomes in the basidiomycete Tremella fuciformis

Accessory chromosomes are non-essential for growth but poorly characterized in basidiomycetes, unlike in Ascomycota. Here, we report whole-genome sequencing of 16 strains of the basidiomycete Tremella fuciformis (silver ear fungus), generating 27 complete haplotypes (5 monokaryons and 11 dikaryons, each contributing two distinct haplotypes). Genome size varied by over one-third, driven by accessory chromosomes and repetitive sequences in core chromosomes (essential for basic biology). Each strain harbored 8-10 core chromosomes (polymorphic via fusion/fission) and 2-10 accessory chromosomes (total 108), whose distribution reflects phylogeny and symbiotic specificity with the ascomycete Annulohypoxylon stygium. Accessory chromosomes are small, transposon-rich, gene-poor, and exhibit higher sequence similarity but more diverse structural variations than core chromosomes, with few shared genes across phylogenetic branches. Both chromosome types show frequent copy number variation during cell type transformation. Most accessory chromosome genes lack homologs in core chromosomes or existing gene databases. Our study reveals basidiomycete accessory chromosome diversity, suggesting an origin from unexplored species pre-dating T. fuciformis speciation.