Abstract
BackgroundPhytomyxids (plasmodiophorids and phagomyxids) are cosmopolitan, obligate biotrophic protist parasites of plants, diatoms, oomycetes and brown algae. Plasmodiophorids are best known as pathogens or vectors for viruses of arable crops (e.g. clubroot in brassicas, powdery potato scab, and rhizomania in sugar beet). Some phytomyxid parasites are of considerable economic and ecologic importance globally, and their hosts include important species in marine and terrestrial environments. However most phytomyxid diversity remains uncharacterised and knowledge of their relationships with host taxa is very fragmentary.ResultsOur molecular and morphological analyses of phytomyxid isolates–including for the first time oomycete and sea-grass parasites–demonstrate two cross-kingdom host shifts between closely related parasite species: between angiosperms and oomycetes, and from diatoms/brown algae to angiosperms. Switching between such phylogenetically distant hosts is generally unknown in host-dependent eukaryote parasites. We reveal novel plasmodiophorid lineages in soils, suggesting a much higher diversity than previously known, and also present the most comprehensive phytomyxid phylogeny to date.ConclusionSuch large-scale host shifts between closely related obligate biotrophic eukaryote parasites is to our knowledge unique to phytomyxids. Phytomyxids may readily adapt to a wide diversity of new hosts because they have retained the ability to covertly infect alternative hosts. A high cryptic diversity and ubiquitous distribution in agricultural and natural habitats implies that in a changing environment phytomyxids could threaten the productivity of key species in marine and terrestrial environments alike via host shift speciation.
Highlights
Phytomyxids are cosmopolitan, obligate biotrophic protist parasites of plants, diatoms, oomycetes and brown algae
We provide the first 18S rDNA sequences for Sorosphaerula viticola, Ligniera junci, Woronina pythii from the oomycete Pythium sp. (Figure 2b, c), and Plasmodiophora diplantherae from the angiosperm sea grass Halodule wrightii (Figure 2f)
We sequenced five isolates of Polymyxa graminis from the grass Poa sp., two of Plasmodiophora brassicae from the crucifers Sinapis sp. and Brassica sp., and a plasmodiophorid we identified as P. graminis from Poa sp. which formed a distinct clade branching basally to P. graminis, S. veronicae, P. betae, and S. viticola in the 18S phylogeny
Summary
Phytomyxids (plasmodiophorids and phagomyxids) are cosmopolitan, obligate biotrophic protist parasites of plants, diatoms, oomycetes and brown algae. Plasmodiophorids are best known as pathogens or vectors for viruses of arable crops (e.g. clubroot in brassicas, powdery potato scab, and rhizomania in sugar beet). Phytomyxea (phytomyxids) are a poorly known group of obligate biotrophic, endobiotic parasites of plants, diatoms, brown algae, and oomycetes [1,2]. Their phylogenetic position was long debated but molecular phylogenies robustly place them within the eukaryote supergroup Rhizaria, as sister group to the omnivorous vampyrellid amoebae [3,4]. Given the impact of plasmodiophorids on staple crops it is not surprising that research has been focussed on the species causing economic damage, while research on species from outside of agricultural environments has been rare
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