Hybrid inferiority and genetic incompatibilities drive divergence of fungal pathogens infecting the same host.

The article focuses on the genetic basis of reproductive isolation and speciation between two closely related fungal pathogens, *Pyrenophora teres* f. *teres* (Ptt) and *P. teres* f. *maculata* (Ptm), which both infect barley but produce distinct lesion types and rarely interbreed in nature. Through experimental crosses, whole-genome sequencing of hybrid and pure progeny, and population genomic analyses of field isolates, the study demonstrates that hybrids exhibit reduced pathogenic fitness and a high frequency of avirulence, indicating extrinsic postzygotic reproductive barriers. The authors identify genomic regions with Dobzhansky–Muller incompatibilities (DMIs), some overlapping with virulence-associated genes and quantitative trait loci (QTL), suggesting that divergent adaptation to host infection strategies contributes to speciation despite co-occurrence on the same host. Additionally, recombination rates are lower and recombination hotspots differ in hybrids compared to pure crosses, supporting the role of genetic incompatibilities rather than large-scale structural genome differences in maintaining species boundaries. This work highlights the importance of genetic incompatibilities linked to virulence in the divergence of fungal pathogens sharing a host and provides a foundation for further functional studies on speciation mechanisms in plant pathogens.