DOI: 10.5061/dryad.j6q573nbw
title: Evolution of morphological but not aggressiveness‐related traits following a major resistance breakdown in the poplar rust fungus, Melampsora larici‐populina

description: Crop varieties carrying qualitative resistance to targeted pathogens lead
 to strong selection pressure on parasites, often resulting in resistance
 breakdown. It is well known that qualitative resistance breakdowns modify
 pathogen population structure but few studies have analysed the
 consequences on their quantitative aggressiveness-related traits. The aim
 of this study was to characterize the evolution of these traits following
 a resistance breakdown in the poplar rust fungus, Melampsora
 larici-populina. We based our experiment on three temporal populations
 sampled just before the breakdown event, immediately after and four years
 later. First, we quantified phenotypic differences among populations for a
 set of aggressiveness traits on a universally susceptible cultivar
 (infection efficiency, latent period, lesion size, mycelium quantity, and
 sporulation rate) and one morphological trait (mean spore volume). Then we
 estimated heritability to establish which traits could be subjected to
 adaptive evolution, and tested for evidence of selection. Our results
 revealed significant changes in the morphological trait but no variation
 in aggressiveness traits. By contrast, recent works have demonstrated that
 quantitative resistance (initially assumed more durable) could be eroded
 and lead to increased aggressiveness. Hence, this study is one example
 suggesting that the use of qualitative resistance may be revealed to be
 less detrimental to long term sustainable crop production.
description: We studied 54 strains from a historical collection of <i>M.
 larici-populina</i> that has been cryopreserved at -80°C at INRAE in
 Nancy (France). All strains were sampled in northern France and in Belgium
 among homogenous pathogen populations. All <i>M.
 larici-populina</i> strains were inoculated on the same cultivar to
 carry out a common garden experiment. Characterization of quantitative
 traits was performed on excised leaf disks of <i>Populus deltoides ×
 P. nigra</i> ‘Robusta’ which is known to be susceptible to all
 tested strains. We used a single-spore inoculation protocol to inoculate
 leaf disks. Strain replicates were arranged among inoculation plates to
 minimise confounding experimental effects. All aggressiveness traits
 classically studied in plant-pathogen systems were measured: infection
 efficiency, latent period, lesion size, and sporulation rate. Beyond these
 four classical traits, we also measured the quantity<i> </i>of
 mycelium <i>in planta</i> developing in an infected leaf disk,
 through qPCR analysis. We also measured one morphological trait: the
 volume of spores, which was computed from length and width of spores
 obtained from image analysis. All details of quantitative traits
 measurement are described in Maupetit <i>et al.</i>
 (2020).
description: Single-spore inoculation
 protocole lead to high rate of infection failure, hence missing data.
 Uneven infection efficiency led to large differences in the representation
 of strains within and among inoculation plates. In order to balance the
 experimental design and the number of replicates for all strains, 1000
 bootstrapped datasets were created by randomly selecting one replicate per
 strain and per plate. These bootstrapped datasets were used to compute
 phenotypic differentiation between population,
 <i>Q</i><sub>ST</sub>, broad-sense heritability
 (H²) and their confidence intervals. (see Maupetit <i>et
 al.</i> 2020 for details).