Adaptation of the Australian Parastagonospora nodorum population to alterations in the host

Septoria nodorum blotch (SNB) is an economically important fungal disease of wheat caused by Parastagonospora nodorum . It is primarily controlled by the breeding of resistant wheat cultivars, but experience over the last 50 years shows that new pathogen populations soon evolve that are more virulent on the current popular cultivars. In this study, we assembled a panel of 360 P. nodorum isolates. The collection resolved into eight subpopulations. One core and seven transient populations were found possessing contrasting characters in term of spatial and temporal distribution, mating-type, effector haplotypes and patterns of intact and degraded copies of a Tc-1 mariner transposon, called Molly. Molly can proliferate and randomly insert throughout the fungal genome. Its multiplication in sexual population likely triggered RIP which partially explains the extensive genetic diversity. Molly activities, together with sexual reproduction and different selection pressures, are all part of evolutionary forces to shape the Australian P. nodorum population structure and explain a model where new lineages of this important pathogen of wheat could be formed. When tested on wheat, the recently emerged groups exhibited greater pathogenicity on modern elite cultivars consistent with the low-amplitude boom-and-bust cycle observed previously. This study identified and characterised potential mechanisms which may lead to the pathogen’s better adaptation and host specialisation. The study also suggests practical measures to improve the efficiency and longevity of resistance breeding for SNB.