Genome reduction and relaxed selection is associated with the transition to symbiosis in the basidiomycete genus Podaxis

Insights into the genomic consequences of symbiosis for basidiomycete fungi associated with social insects remain sparse. Capitalizing on viability of spores from centuries-old herbarium specimens of free-living, facultative, and specialist termite-associated Podaxis fungi, we obtained genomes of 10 specimens, including two type species described by Linnaeus >240 years ago. We document that the transition to termite association was accompanied by significant reductions in genome size and gene content, accelerated evolution in protein-coding genes, and reduced functional capacities for oxidative stress responses and lignin degradation. Functional testing confirmed that termite specialists performworse under oxidative stress, while all lineages retained some capacity to cleave lignin. Mitochondrial genomes of termite associates were significantly larger; possibly driven by smaller population sizes or reduced competition, supported by apparent loss of certain biosynthetic gene clusters. Our findings point to relaxed selection that mirrors genome traits observed among obligate endosymbiotic bacteria of many insects.

Automated summary

Insights into the genomic consequences of symbiosis for basidiomycete fungi associated with social insects are limited. Through genome analysis of termite-associated Podaxis fungi, it was found that the transition to termite association led to reductions in genome size and gene content. Termite specialists exhibited poorer performance under oxidative stress, while all lineages retained some capacity for lignin cleavage. Mitochondrial genomes of termite associates were significantly larger, potentially due to smaller population sizes or reduced competition.