William C. Beckerson, Ricardo C. Rodríguez de la Vega, Fanny E. Hartmann, Marine Duhamel, Tatiana Giraud, and Michael H. Perlin
mBio, 2019: Volume 10, Issue 6, e02391-19
Abstract: Plant pathogens utilize a portfolio of secreted effectors to successfully infect and manipulate their hosts. It is, however, still unclear whether changes in se- cretomes leading to host specialization involve mostly effector gene gains/losses or changes in their sequences. To test these hypotheses, we compared the secretomes of three host-specific castrating anther smut fungi (Microbotryum), two being sister species. To address within-species evolution, which might involve coevolution and local adaptation, we compared the secretomes of strains from differentiated popula- tions. We experimentally validated a subset of signal peptides. Secretomes ranged from 321 to 445 predicted secreted proteins (SPs), including a few species-specific proteins (42 to 75), and limited copy number variation, i.e., little gene family expan- sion or reduction. Between 52% and 68% of the SPs did not match any Pfam do- main, a percentage that reached 80% for the small secreted proteins, indicating rapid evolution. In comparison to background genes, we indeed found SPs to be more differentiated among species and strains, more often under positive selection, and highly expressed in planta; repeat-induced point mutations (RIPs) had no role in effector diversification, as SPs were not closer to transposable elements than back- ground genes and were not more RIP affected. Our study thus identified both con- served core proteins, likely required for the pathogenic life cycle of all Microbotryum species, and proteins that were species specific or evolving under positive selection; these proteins may be involved in host specialization and/or coevolution. Most changes among closely related host-specific pathogens, however, involved rapid changes in sequences rather than gene gains/losses.
Keywords: functional proteomics, effectors, small secreted proteins, host specificity, fungal pathogens