50 years ago, scientists created a wheat-rye hybrid prized for its resistance to the pathogenic powdery mildew fungus. At the turn of the century, however, wild fungus began to efficiently infect the crop, and researchers have recently uncovered its evolutionary feat.
Scientists often breed or engineer important food crops with the goal of producing a new variety capable of increased yield, drought tolerance, or pathogen resistance.
The globally important cereal grain called triticale was created in the 1960s by combining the genomes of wheat and rye into a single plant. It became popular because of its resistance to the powdery mildew fungus, which is able to infect wheat and rye and dramatically decrease yields.
However, reports of triticale powdery mildew infection started in the early 2000s and the pathogen is now widespread in Europe.
Studying the evolutionary process by which agricultural pathogens expand their host range is important to scientists because it can influence techniques for creating new genetic variants of crops.
In order to elucidate the mechanism that allowed powdery mildew to infect triticale, researchers sequenced the genomes of dozens of isolated fungal populations from fields across Europe. These included many wheat-, rye-, and triticale-infecting specimens.
As expected, the fungal genomes from different hosts were very similar to one another, confirming that they share a common ancestor. However, using sensitive bioinformatics techniques, the research team was able to identify thousands of nucleotide bases (the As, Ts, Cs, and Gs in DNA sequence) that differed between their isolates.
Using the information from these genetic comparisons, the researchers discovered that the triticale-infecting fungal genome is a hybridization of approximately 12.5% rye-infecting and 87.5% wheat-infecting powder mildews.
This natural hybridization is fascinating because it provides evidence that pathogens specialized to two distinct hosts are able to evolve to infect a hybrid of their hosts by becoming hybrids themselves.
Though they could not draw conclusive evidence based on their data, the researchers speculate that the process of fungal DNA exchange that created the hybrid must have occurred on a triticale plant that harbored both wheat- and rye-infecting powdery mildew cells.
Using genetic data to infer the evolutionary history and relatedness of all the fungal specimens, the authors also showed that this process of natural hybridization is how powdery mildew expanded to infect bread wheat at the dawn of agriculture 10,000 years ago.
The conclusions of this study have shed light on a major mechanism for pathogen host-range expansion. It is not clear how this information will influence our ability to create pathogen-resistant crops, but it does eloquently display that side-by-side evolution is a powerful force in host-pathogen interactions.
There may have been hundreds, thousands, or millions of unsuccessful rye and wheat mildew hybrid combinations that occurred in the many fungal generations since triticale fields started cropping up. With this raw genetic variation, natural selection blindly produced a powdery mildew variety that now threatens an entire industry. Pretty humbling.