The Pathogen's Cradle
In the cool, humid highlands of the Toluca Valley, a long and complex evolutionary battle has been playing out for thousands of years. This region is the center of origin for Phytophthora infestans, the notorious organism responsible for potato late blight. Despite its fungus-like behavior, P. infestans is not a true fungus. It is an oomycete, or water mold, a type of microorganism more closely related to brown algae and diatoms. Important differences are found in its cell walls, which are made of cellulose and glucans, whereas true fungi have cell walls composed mainly of chitin.
The Toluca Valley is also a center of diversity for wild, tuber-bearing relatives of the potato, such as Solanum demissum and Solanum stoloniferum. For millennia, P. infestans and these wild potatoes have been locked in a co-evolutionary arms race. The plants develop resistance genes (R genes) to detect and fight off the pathogen, while the pathogen evolves new ways to evade the plant's defenses. This constant back-and-forth has created an immense genetic diversity within the P. infestans population in this specific area. Both A1 and A2 mating types of the oomycete are found here in roughly equal numbers, allowing for frequent sexual reproduction. This process shuffles genes and creates new, aggressive strains, different from the clonal, asexual reproduction that long dominated P. infestans populations elsewhere in the world.
A Global Catastrophe
The pathogen's spread from its Mexican homeland to global infamy began in the 1840s. A single clonal lineage, later identified as HERB-1, is thought to have been transported out of the Americas on infected potatoes. It was first recorded in Europe near Belgium in June 1845 and spread with terrifying speed. The potato crop of Europe, primarily the single variety Solanum tuberosum, had no evolved resistance to this unfamiliar pathogen. It was a monoculture of defenseless hosts.
The consequences were most severe in Ireland, where a large portion of the rural population was completely dependent on the potato for sustenance. The resulting Great Famine, or An Gorta Mór, lasted from 1845 to 1852. During this period, the repeated failure of the potato crop led to the deaths of approximately one million people from starvation and disease. Another million were forced to emigrate, beginning a massive diaspora to North America and other parts of the world. The population of Ireland fell by 20-25% in just a few years. While other European countries also suffered crop losses and hardship, the socio-economic conditions in Ireland created a catastrophe on an unmatched scale. Today, the Toluca Valley is a "living laboratory" for scientists studying host-pathogen evolution, hoping to find new resistance genes in the wild potatoes that can protect one of the world's most important food crops.