Additive and epistatic QTL contribute to the adaptation of the fungus Leptosphaeria maculans to Brassica carinata

Leptosphaeria maculans is a plant-pathogenic fungus that infects Brassica species, including Brassica napus (oilseed rape). Breeding oilseed rape varieties with genetic resistance is an efficient way to control the disease; however, L. maculans can adapt and overcome these resistances. Understanding the mechanisms that enable L. maculans to adapt is crucial for managing the emergence of better-adapted isolates. Brassica carinata , the Ethiopian Mustard, although closely related to B. napus , is considered a nonhost species of L. maculans because this fungus cannot infect it. Despite the extreme resistance of B. carinata , one natural L. maculans isolate has been identified as unable to infect B. napus , causing moderate and atypical symptoms on this species. We performed a cross between this isolate and an isolate adapted to B. napus , followed by a QTL analysis, which identified seven QTL, each encompassing candidate genes involved in L. maculans adaptation to B. carinata or B. napus . Additionally, we observed transgression in the progeny, wherein a few strains caused significantly more or less aggressive symptoms on both species of Brassica . We found that epistasis within the L. maculans genome contributes to the observed transgression. These initial findings provide further opportunities to study the adaptive capacities of L. maculans , as well as data to initiate analysis of the extreme resistance of B. carinata to L. maculans .