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Chemical tricks in new hosts expand pathogen variety

Mirabilis jalapa (the four o'clock flower). This plant is the host of Phytophthora mirabilis, the sister species of the Irish potato famine pathogen Phytophthora infestans.
[Image courtesy of Sophien Kamoun]

A pathogen that has spent generations living comfortably in one host can leap to and successfully inhabit an entirely different one, and now a new report suggests the chemical changes it makes in the new host allow it to make this jump.

As part of the process, the pathogen is changed, too -- leading to new and diverse pathogen species over time.

The discovery is reported in the 31 January issue of the journal Science.

Scientists have lacked a clear understanding of how new pathogens that crop up stay in the population; understanding any molecular changes these parasites undergo, and how they are preserved over time, could help.

Norwich Research Park scientist Suomeng Dong and colleagues used comparative genomics to compare the genes of the pathogen responsible for potato blight with those of a closely related pathogen that infects "four-o'clock flower." The flower pathogen came from the same parasite family, but split off from the main potato pathogen over 1,000 years ago.

The potato and flower pathogens both cause infection in their host by disabling enzymes involved in plant defense. But their attacks are specific to their hosts; neither pathogen can bother the other's host organism.

Dong and colleagues tried to trace back the events that led to these pathogens' split. Taking a look at the structure of each one, the researchers found a small change in the amino acid sequence of the flower pathogen compared to the potato pathogen -- and, critically, a corresponding change in the flower pathogen's target enzyme.

This change, they say, helped the pathogen that came to affect the four-o'clock flower function effectively there.

The work of Dong et al. suggests that biochemical changes made after a pathogen jumps hosts help it adapt to its new niche.