Abstract
The ectomycorrhizal (ECM) symbiosis has independently evolved from diverse types ofsaprotrophic ancestors. In this study, we seek to identify genomic signatures of the transitionto the ECM habit within the hyperdiverse Russulaceae.We present comparative analyses of the genomic architecture and the total and secretedgene repertoires of 18 species across the order Russulales, of which 13 are newly sequenced,including a representative of a saprotrophic member of Russulaceae,Gloeopeniophorellaconvolvens.The genomes of ECM Russulaceae are characterized by a loss of genes for plant cell wall-degrading enzymes (PCWDEs), an expansion of genome size through increased transposableelement (TE) content, a reduction in secondary metabolism clusters, and an association ofsmall secreted proteins (SSPs) with TE ‘nests’, or dense aggregations of TEs. Some PCWDEshave been retained or even expanded, mostly in a species-specific manner. The genome ofG.convolvenspossesses some characteristics of ECM genomes (e.g. loss of some PCWDEs, TEexpansion, reduction in secondary metabolism clusters).Functional specialization in ECM decomposition may drive diversification. Accelerated geneevolution predates the evolution of the ECM habit, indicating that changes in genome archi-tecture and gene content may be necessary to prime the evolutionary switch