Origin and Domestication of the Fungal Wheat Pathogen Mycosphaerella graminicola via Sympatric Speciation

The Fertile Crescent represents the center of origin and earliest known place of domestication for many cereal crops. During the transition from wild grasses to domesticated cereals, many host-specialized pathogen species are thought to have emerged. A sister population of the wheat-adapted pathogen Mycosphaerella graminicola was identified on wild grasses collected in northwest Iran. Isolates of this wild grass pathogen from 5 locations in Iran were compared with 123 M. graminicola isolates from the Middle East, Europe, and North America. DNA sequencing revealed a close phylogenetic relationship between the pathogen populations. To reconstruct the evolutionary history of M. graminicola, we sequenced 6 nuclear loci encompassing 464 polymorphic sites. Coalescence analyses indicated a relatively recent origin of M. graminicola, coinciding with the known domestication of wheat in the Fertile Crescent around 8,000-9,000 BC. The sympatric divergence of populations was accompanied by strong genetic differentiation. At the present time, no genetic exchange occurs between pathogen populations on wheat and wild grasses although we found evidence that gene flow may have occurred since genetic differentiation of the populations.