Spontaneous loss of Yr2 avirulence in two lineages of Puccinia striiformis did not affect pathogen fitness

Abstract

Fitness costs associated with the emergence of virulence (loss of avirulence) have been a subject of much debate in plant pathology. Here, differences in fitness between two pairs of wild types and spontaneous virulence mutants in Puccinia striiformis were studied. The mutants differed from their respective wild types in virulence corresponding to the Yr2 resistance gene in wheat. The wildtype and mutant pairs represented different genetic lineages and virulence phenotypes. Colony size, latent period, lesion growth rate and uredinium density and size were assessed on susceptible and Yr2 resistant wheat varieties. No significant difference was observed for any of the evaluated parameters within wildtype and mutant pairs, i.e. the results did not support a hypothesis of a fitness cost for acquiring virulence in P. striiformis. In contrast, significant differences were found for most of the parameters when isolates were compared according to genetic lineage, which confirm that the applied methodologies were appropriate for detecting differences in pathogen fitness.

The hard life of Phytophthora infestans: when trade‐offs shape evolution in a biotrophic plant pathogen

“…The experimental issue requires comparative infection experiments on hosts with no known effective R gene(s), with isolates: (i) as ‘isogenic’ as possible except for the virulence gene, to ensure that the effect measured is indeed due to the virulence factor considered (see for instance Vera Cruz et al ., in Xanthomonas oryzae or Bahri et al ., in Puccinia striiformis of wheat); (ii) as numerous as possible, to buffer interactions between virulence determinants and peculiar configurations of the genetic background into which they are placed, as well as plastic responses for life history traits (Reece et al ., ); and (iii) as ‘natural’ as possible, to ascertain the viability of these isolates in outdoor, and not just in laboratory conditions (e.g. Sørensen et al ., ). All these conditions are rarely met together, but populations evolving through mutations in large clonal lineages offer the possibility to collect suitable sets of near‐isogenic isolates differing for specific virulence, and comparing pathogenicity between virulent and avirulent bulks.…”

The role of effectors in nonhost resistance to filamentous plant pathogens

“…Whereas most effectors are thought to serve important functions during infection and seem to be indispensable, loss of effectors does not always lead to reduced fitness, most likely due to functional redundancy. For example, P. striiformis lacking effectors recognized by the wheat resistance protein Yr2 perform equally well on susceptible and Yr2 plants ( Sørensen et al, 2013 ). Presence/absence polymorphisms are also common in Phytophthora RXLR and CRN effectors and in some cases, effector loss does not obviously affect fitness ( Shan et al, 2004 ).…”

Temporal and Spatial Variability of Fungal Structures and Host Responses in an Incompatible Rust–Wheat Interaction

“…The two virulence mutant isolates, which were collected on adult plants of Skater in field trials, shared 100% identity with their respective wild type isolate based on AFLP markers ( Sørensen et al, 2013 ). Based on a differential set including three varieties with Yr 2 resistance (Kalyansona, Heines VII, and Heines Peko) the mutant isolates only differed from the wild types with respect to virulence to Yr 2.…”

“…Infections emerging on Skater during the trial were taken to the laboratory for virulence and amplified fragment length polymorphism (AFLP) phenotyping according to Justesen et al (2002) . These tests identified the Yr 2 virulence mutant isolates, which shared AFLP fingerprint with their respective wild type isolates when screened by 20 AFLP primer combinations producing c. 1400 AFLP fragments (for more details, see Sørensen et al, 2013 ).…”

“…Here we used two independent pairs of avirulent wild type and virulent mutant isolates. The mutant isolates, which were collected from field trials designed to select for spontaneous virulence mutants, only differed from the respective wild type in being virulent on wheat varieties carrying Yr 2 resistance ( Sørensen et al, 2013 ). The objective of the study was to carry out a histological characterization of the temporal and spatial variability of the incompatible interaction between the wild type isolates and wheat varieties with Yr 2 resistance.…”