Genetic Variability and Distribution of Mating Type Alleles in Field Populations of Leptosphaeria maculans from France

Abstract

Leptosphaeria maculans is the most ubiquitous fungal pathogen of Brassica crops and causes the devastating stem canker disease of oilseed rape worldwide. We used minisatellite markers to determine the genetic structure of L. maculans in four field populations from France. Isolates were collected at three different spatial scales (leaf, 2-m 2 field plot, and field) enabling the evaluation of spatial distribution of the mating type alleles and of genetic variability within and among field populations. Within each field population, no gametic disequilibrium between the minisatellite loci was detected and the mating type alleles were present at equal frequencies. Both sexual and asexual reproduction occur in the field, but the genetic structure of these populations is consistent with annual cycles of randomly mating sexual reproduction. All L. maculans field populations had a high level of gene diversity (H ‫؍‬ 0.68 to 0.75) and genotypic diversity. Within each field population, the number of genotypes often was very close to the number of isolates. Analysis of molecular variance indicated that >99.5% of the total genetic variability was distributed at a small spatial scale, i.e., within 2-m 2 field plots. Population differentiation among the four field populations was low (G ST < 0.02), suggesting a high degree of gene exchange between these populations. The high gene flow evidenced here in French populations of L. maculans suggests a rapid countrywide diffusion of novel virulence alleles whenever novel resistance sources are used.

Population Diversity of Leptosphaeria maculans in Australia

“…Genetic differences between eastern and western Australian isolates have previously been found using microsatellites and minisatellites, which were attributed to the presence of arid desert between the coasts (Hayden, Cozijnsen, & Howlett, 2007). Minisatellite markers used to analyse four field populations in France found high levels of gene and genotypic diversity within populations and high gene flow between populations, consistent with randomly mating populations (Gout et al, 2006) Dilmaghani et al (2012) also used minisatellite markers to show that the L. maculans population in Western Canada comprises two genetically distinct populations. A further study implementing fourteen minisatellite markers also found clonal sub populations of this pathogen on B. oleracea in Mexico (Dilmaghani et al, 2013).…”

“…juncea, B. rapa and B. oleracea: canola, vegetable and mustard crops (West, Kharbanda, Barbetti, & Fitt, 2001). This ascomycete was first described in 1791 by Tode and 1849 by Desmaziéres (Gout, Eckert, Rouxel, & Balesdent, 2006). Severe epidemics of blackleg disease occurred in Australia during the 1970s, wiping out the nascent canola industry .…”

“…These include mating systems, patterns of speciation, dispersal, mutation, migration and selection etc. (Giraud, Enjalbert, Fournier, Delmotte, & Dutech, 2008;Gout et al, 2006).…”

Landscape epidemiology of plant diseases

“…tritici was responsible for several resistance genes becoming ineffective for the control of yellow rust in four northwest European countries. Similarly, long-distance dispersal has been proposed to be responsible for the rapid breakdown of Rlm resistance genes in oilseed Brassica crops by Leptosphaeria maculans, the causal agent of stem canker of crucifers (Gout et al 2006). It should then be theoretically possible to reduce the risk of emergence of a new virulent (or resistant) pathogen by designing management strategies that limits gene/genotype flows among pathogen populations.…”

Population diversity and genomics of the fungal pathogen of canola Leptosphaeria maculans

“…Genetic differences between eastern and western Australian isolates have previously been found using microsatellites and minisatellites, which were attributed to the presence of arid desert between the coasts (Hayden et al, 2007). Minisatellite markers used to analyse four field populations in France found high levels of gene and genotypic diversity within populations and high gene flow between populations, consistent with randomly mating populations (Gout et al, 2006a). Dilmaghani et al…”