QTL Mapping of Fungicide Sensitivity Reveals Novel Genes and Pleiotropy with Melanization in the Pathogen Zymoseptoria Tritici

A major problem associated with the intensification of agriculture is the emergence of fungicide resistance. Azoles are ergosterol biosynthesis inhibitors that have been widely used in agriculture and medicine since the 1970s, leading to emergence of increasingly resistant fungal populations. The known genetic mechanisms underlying lower azole sensitivity include mutations affecting the CYP51 gene that encodes the target protein, but in many cases azole resistance is a more complex trait with an unknown genetic basis. We used quantitative trait locus (QTL) mapping to identify genes affecting azole sensitivity in two crosses of Zymoseptoria tritici, the most damaging wheat pathogen in Europe. Restriction site associated DNA sequencing (RADseq) was used to genotype 263 (cross 1) and 261 (cross 2) progeny at ∼ 8500 single nucleotide polymorphisms (SNP) and construct two dense linkage maps. Azole sensitivity was assessed using high-throughput digital image analysis of colonies growing on Petri dishes with or without the fungicide propiconazole. We identified three QTLs for azole sensitivity, including two that contained novel fungicide sensitivity genes. One of these two QTLs contained only 16 candidate genes, among which four most likely candidates were identified. The third QTL contained ERG6, encoding another protein involved in ergosterol biosynthesis. Known genes in QTLs affecting colony growth included CYP51 and PKS1, a gene affecting melanization in Z. tritici. PKS1 showed compelling evidence for pleiotropy, with a rare segregating allele that increased melanization while decreasing growth rate and propiconazole sensitivity. This study resolved the genetic architecture of an important agricultural trait and led to identification of novel genes that are likely to affect azole sensitivity in Z. tritici. It also provided insight into fitness costs associated with lowered azole sensitivity and suggests a novel fungicide mixture strategy.