Rapidly Evolving Genes Are Key Players in Host Specialization and Virulence of the Fungal Wheat Pathogen Zymoseptoria tritici (Mycosphaerella graminicola)

Abstract

The speciation of pathogens can be driven by divergent host specialization. Specialization to a new host is possible via the acquisition of advantageous mutations fixed by positive selection. Comparative genome analyses of closely related species allows for the identification of such key substitutions via inference of genome-wide signatures of positive selection. We previously used a comparative genomics framework to identify genes that have evolved under positive selection during speciation of the prominent wheat pathogen Zymoseptoria tritici (synonym Mycosphaerella graminicola). In this study, we conducted functional analyses of four genes exhibiting strong signatures of positive selection in Z. tritici. We deleted the four genes in Z. tritici and confirm a virulence-related role of three of the four genes ΔZt80707, ΔZt89160 and ΔZt103264. The two mutants ΔZt80707 and ΔZt103264 show a significant reduction in virulence during infection of wheat; the ΔZt89160 mutant causes a hypervirulent phenotype in wheat. Mutant phenotypes of ΔZt80707, ΔZt89160 and ΔZt103264 can be restored by insertion of the wild-type genes. However, the insertion of the Zt80707 and Zt89160 orthologs from Z. pseudotritici and Z. ardabiliae do not restore wild-type levels of virulence, suggesting that positively selected substitutions in Z. tritici may relate to divergent host specialization. Interestingly, the gene Zt80707 encodes also a secretion signal that targets the protein for cell secretion. This secretion signal is however only transcribed in Z. tritici, suggesting that Z. tritici-specific substitutions relate to a new function of the protein in the extracellular space of the wheat-Z. tritici interaction. Together, the results presented here highlight that Zt80707, Zt103264 and Zt89160 represent key genes involved in virulence and host-specific disease development of Z. tritici. Our findings illustrate that evolutionary predictions provide a powerful tool for the identification of novel traits crucial for host adaptation and pathogen evolution.

A fungal avirulence factor encoded in a highly plastic genomic region triggers partial resistance to septoria tritici blotch

“…Two highly conserved lysin motif (LysM) effectors, Mg1LysM and Mg3LysM, prevent fungal recognition and shield the fungal cell wall from degradation by host hydrolytic enzymes (Marshall et al ., 2011). The other known effectors of Z. tritici , Zt80707, AvrStb6 and Zt_8_609, are rapidly evolving small secreted proteins (Poppe et al ., 2015; Hartmann et al ., 2017; Zhong et al ., 2017; Kema et al ., 2018). The latter two were identified because they are specifically recognised by certain wheat cultivars, and they were found to be located in transposable element‐rich genomic regions (Brading et al ., 2002; Hartmann et al ., 2017; Zhong et al ., 2017).…”

The Evolution of Orphan Regions in Genomes of a Fungal Pathogen of Wheat

“…Characterization of the genome and transcriptome of Z. tritici revealed a large number of putative effector genes encoding small secreted proteins (SSPs) that are likely involved in virulence (32, 33). Genes encoding SSPs evolved more rapidly than the genomic background (25, 34) and were more likely to lack orthologs in closely related species (33). Genomic analyses suggested that numerous genes (including effector genes) were deleted in at least some isolates of the species (31, 35).…”

Pathogen effectors and plant immunity determine specialization of the blast fungus to rice subspecies

“…However, although it is important to elucidate the origin and maintenance of variations in pathogen fitness on different hosts for developing durable means of controlling disease, most current understanding is still largely based on theoretical predictions (Brown and Tellier, 2011; Laine and Tellier, 2008; Schulze-Lefert and Panstruga, 2011; Thrall et al, 2015). Thus there is a lack of studies investigating the molecular or physiological bases of variation in pathogen fitness across pathogen populations, especially in fungi (Poppe et al, 2015). …”