In plant-associated fungi, the epigenome is increasingly recognized as an important regulator of the expression of genes involved in interaction with the host plant. Leptosphaeria maculans ‘brassicae’ (Lmb) and Leptosphaeria maculans ‘lepidii’ (Lml) are closely-related phytopathogenic species that exhibit a large macrosynteny but contrasting genome structure. Lmb has more than 30% of repeats clustered in large repeat-rich regions, while the Lml genome has only a small amount of evenly distributed repeats. Repeat-rich regions of Lmb are enriched in effector genes, expressed during plant infection. The distinct genome structures of Lmb and Lml provide an excellent model for comparing the organization of pathogenicity genes in relation to the chromatin landscape in two closely related phytopathogenic fungi. Here, we performed chromatin immunoprecipitation (ChIP) during axenic culture, targeting histone modifications typical for heterochromatin or euchromatin, combined with transcriptomic analysis to analyse the influence of chromatin organisation on gene expression.
In both species, we found that facultative heterochromatin are enriched with genes lacking functional annotation, including numerous effector and species-specific genes. Notably, orthologous genes located in H3K27me3-domains are enriched with effector genes. Compared to other fungal species, including Lml, Lmb is distinct in having large H3K9me3-domains associated with repeat-rich regions that contain numerous species-specific effector genes.
Discovery of these two distinctive heterochromatin landscapes now raises questions about their involvement in the regulation of pathogenicity, the dynamics of these domains during plant infection, and the selective advantage to the fungus to host effector genes in H3K9me3- or H3K27me3-domains.