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Wheat domestication accelerated evolution and triggered positive selection in the beta-xylosidase enzyme of Mycosphaerella graminicola - PubMed

  • ️Thu Jan 01 2009

Wheat domestication accelerated evolution and triggered positive selection in the beta-xylosidase enzyme of Mycosphaerella graminicola

Patrick C Brunner et al. PLoS One. 2009.

Abstract

Plant cell wall degrading enzymes (PCWDEs) of plant pathogens are receiving increasing interest for their potential to trigger plant defense reactions. In an antagonistic co-evolutionary arms race between host and pathogen, PCWDEs could be under strong selection. Here, we tested the hypothesis that PCWDEs in the fungal wheat pathogen Mycosphaerella graminicola have been positively selected by analyzing ratios of non-synonymous and synonymous nucleotide changes in the genes encoding these enzymes. Analyses of five PCWDEs demonstrated that one (beta-xylosidase) has been under strong positive selection and experienced an accelerated rate of evolution. In contrast, PCWDEs in the closest relatives of M. graminicola collected from wild grasses did not show evidence for selection or deviation from a molecular clock. Since the genealogical divergence of M. graminicola from these latter species coincided with the onset of agriculture, we hypothesize that the recent domestication of the host plant and/or agricultural practices triggered positive selection in beta-xylosidase and that this enzyme played a key role in the emergence of a host-specialized pathogen.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Predicted protein structure of β-xylosidase of M. graminicola.

Note that the amino acid sites under positive diversifying selection (yellow halos) are located on the surface of the protein.

Figure 2
Figure 2. Phylogenetic relationships among fungal samples from different hosts.

Maximum clade credibility (MCC) tree from Bayesian coalescent analysis depicting the phylogenetic relationships among fungal samples collected from domesticated bread wheat (Ta, Triticum aestivum) and wild grasses (Ar, Agropyron repens; Dg, Dactylis glomerata; Lm, Lolium multiflorum). Clade-credibility values are shown only for the three major lineages. Asterisks denote nodes within lineages that received >90% posterior support.

Figure 3
Figure 3. Accelerated rate of evolution in β-xylosidase.

Estimates of evolutionary rates for the positively selected β-xylosidase (circles), cellulase (squares), and the non-coding “STS2” nucleotide sequence (triangles) in Mycosphaerella graminicola.

Figure 4
Figure 4. Accelerated rate of evolution in M. graminicola.

Estimates of evolutionary rates for Mycosphaerella graminicola and its two closest relatives “S1 and “S2” (see Figure 2) under a relaxed clock model. Mean rates for internal branches of the β-xylosidase-based phylogeny are shown as filled circles for M. graminicola, squares for the lineage “S2”, and diamonds for the lineage “S1”, respectively.

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