Abstract
Even though the microevolution of plant hosts and pathogens has been intensely studied, knowledge regarding macro-evolutionary patterns is limited. Having the highest species diversity and host-specificity among Oomycetes, downy mildews are a useful a model for investigating long-term host-pathogen coevolution. We show that phylogenies of Bremia and Asteraceae are significantly congruent. The accepted hypothesis is that pathogens have diverged contemporarily with their hosts. But maximum clade age estimation and sequence divergence comparison reveal that congruence is not due to long-term coevolution but rather due to host-shift driven speciation (pseudo-cospeciation). This pattern results from parasite radiation in related hosts, long after radiation and speciation of the hosts. As large host shifts free pathogens from hosts with effector triggered immunity subsequent radiation and diversification in related hosts with similar innate immunity may follow, resulting in a pattern mimicking true co-divergence, which is probably limited to the terminal nodes in many pathogen groups.
Highlights
Two kingdoms, Fungi and Straminipila, include the vast majority of eukaryotic plant pathogens, of which many, like Phytophthora infestans of the oomycetes and Puccinia tritici of the rust fungi, are of great economic importance
Over the past 10 years, an increasing amount of genome data for well-known pathogens has been advancing the understanding of the biology and genetics of oomycetes and other pathogens
Many studies have addressed the phylogenetic relationships of pathogens, and especially in downy mildews, many groups can be considered to be well-studied in this respect
Summary
Fungi and Straminipila (mostly Oomycetes), include the vast majority of eukaryotic plant pathogens, of which many, like Phytophthora infestans of the oomycetes and Puccinia tritici of the rust fungi, are of great economic importance. In both fungi and oomycetes, species richness is high in obligate biotrophic pathogens. To maintain this biotrophic lifestyle, they have developed highly host-specific and sophisticated mechanisms to confront the resistance system of the host plants [1, 2]. PLOS ONE | DOI:10.1371/journal.pone.0133655 July 31, 2015
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