Positively Selected Effector Genes and Their Contribution to Virulence in the Smut Fungus Sporisorium reilianum

Plants and fungi display a broad range of interactions within natural and agricultural ecosystems ranging from symbiosis to parasitism. Pathogenic interactions are governed by secreted fungal effector proteins, which are thought to coevolve with their host targets. Biotrophic smut fungi which belong to the division of Basidiomycota are well-suited to investigate the evolution of plant pathogens, because several quality draft genomes and genetic tools are available for these species. Here, we used the genomes of Sporisorium reilianum f. sp. zeae and S. reilianum f. sp. reilianum, two closely related formae speciales infecting maize and sorghum, respectively, together with the genomes of Ustilago hordei, U. maydis and S. scitamineum to identify effector genes showing signs of positive selection. The largest numbers of such genes were identified in the two pathovariants of S. reilianum and between paralogues in U. hordei, where many belong to families showing species-specific expansions. Next, we assessed the contribution to virulence of candidate effector genes in S. reilianum f. sp. zeae by deletion of individual genes in a solopathogenic strain. While eight of nine deletions mutants were unaffected in virulence, one mutant had lost virulence. This shows that despite the relatively recent divergent time of the two formae speciales, a signature of positive selection in candidate effector genes in S. reilianum is a poor indicator for the identification of genes with virulence functions.