Analysis of Leptosphaeria maculans Race Structure in a Worldwide Collection of Isolates

Abstract

Leptosphaeria maculans, the causal agent of stem canker of oilseed rape, develops gene-for-gene interactions with its hosts. To date, eight L. maculans avirulence (Avr) genes, AvrLm1 to AvrLm8, have been genetically characterized. An additional Avr gene, AvrLm9, that interacts with the resistance gene Rlm9, was genetically characterized here following in vitro crosses of the pathogen. A worldwide collection of 63 isolates, including the International Blackleg of Crucifers Network collection, was genotyped at these nine Avr loci. In a first step, isolates were classified into pathogenicity groups (PGs) using two published differential sets. This analysis revealed geographical disparities as regards the proportion of each PG. Genotyping of isolates at all Avr loci confirmed the disparities between continents, in terms of Avr allele frequencies, particularly for AvrLm2, AvrLm3, AvrLm7, AvrLm8, and AvrLm9, or in terms of race structure, diversity, and complexity. Twenty-six distinct races were identified in the collection. A larger number of races (n = 18) was found in Australia than in Europe (n = 8). Mean number of virulence alleles per isolate was also higher in Australia (5.11 virulence alleles) than in Europe (4.33) and Canada (3.46). Due to the diversity of populations of L. maculans evidenced here at the race level, a new, open terminology is proposed for L. maculans race designation, indicating all Avr loci for which the isolate is avirulent.

Insights into fighting against blackleg disease of Brassica napus in Canada

“…However, a few limited PG cannot fully illustrate population variations of the pathogen. To better address population variation of L. maculans, a new term, race structure was introduced by Balesdent et al (2005) to describe population structures of L. maculans populations. To date, at least 14 avirulence (Avr) genes have been identified in L. maculans and a few of them have been cloned (Liban et al 2016).…”

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

“…Races of the L. maculans isolates were determined by inoculating the 401 isolates onto a B. napus differential set, comprising fixed lines or commercial cultivars with as few Rlm alleles as possible (8). Cotyledons of the fully susceptible cv.…”

“…There are two types of disease resistance in B. napus: polygenic quantitative resistance (29,30) or specific resistance, which involves major resistance gene(s) (19,39). Nine avirulence genes (AvrLm genes), namely, AvrLm1 to AvrLm9, have been identified in L. maculans (2,(6)(7)(8), and the corresponding nine major resistance genes (Rlm1 to Rlm9) have been described in Brassica sp. (13), as expected in a host-pathogen system with gene-for-gene interactions (17).…”

“…Our objective here was to assess genetic variation in French populations of L. maculans with minisatellite loci (15), mating type, and virulence markers (8). We tested the hypotheses that the field populations were randomly mating, that populations from different fields were genetically separable, and that epidemics are initiated by ascospores.…”

Interaction between plant genotype and the symbiosis with Epichloë fungal endophytes in seeds of red fescue (Festuca rubra)

“…The discussion, led by Dr Thierry Rouxel, highlighted the various issues with nomenclature whereby the same gene has been given multiple names by various groups, for example the confusion around the resistance gene(s) in Surpass400 which have been identified as LepR3, RlmS, Rlm1, BLMR1 and BLMR2 (Balesdent et al 2005; Van de Wouw et al 2009, 2014Long et al 2011;Larkan et al 2013). In addition, it was highlighted that we now have the added confusion that the same nomenclature used for major resistance genes, Rlm (resistance to L. maculans), has been used for quantitative or minor gene resistance, with the first locus of this kind identified and named Rlm12 (Raman et al 2016).…”