Effector genes with signatures of positive selection in smut fungi
To study the evolution of effector genes as well as their contributions to virulence, speciation and host specificity, we employed the genome of the recently sequenced strain Sporisorium reilianum f. sp. reilianum SRS1_H2-8 (host plant sorgum) together with the genomes of U. maydis (host plant maize), Ustilago hordei (host plant barley), Sporisorium scitamineum (host plant sugar cane) and S. reilianum f. sp. zeae (host plant maize) (Figure 5) to identify potential effector genes with signatures of positive selection. To determine whether such candidate effectors play a role in virulence, they were individually deleted in S. reilianum f. sp. zeae and the phenotype of the deletion strain assessed after infection of maize. Only one of nine tested candidate effectors showed a strong contribution to virulence, suggesting that most genes with signatures of positive selection contribute to virulence under conditions not tested here or are selected for traits not directly related to virulence in S. reilianum f. sp. zeae.