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Phytophthora: an ancient, historic, biologically and structurally cohesive and evolutionarily successful generic concept in need of preservation
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Phytophthora: an ancient, historic, biologically and structurally cohesive and evolutionarily successful generic concept in need of preservation
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Journal Article
Phytophthora: an ancient, historic, biologically and structurally cohesive and evolutionarily successful generic concept in need of preservation
2022
Overview
The considerable economic and social impact of the oomycete genus
Phytophthora
is well known. In response to evidence that all downy mildews (DMs) reside phylogenetically within
Phytophthora
, rendering
Phytophthora
paraphyletic, a proposal has been made to split the genus into multiple new genera. We have reviewed the status of the genus and its relationship to the DMs. Despite a substantial increase in the number of described species and improvements in molecular phylogeny the
Phytophthora
clade structure has remained stable since first demonstrated in 2000. Currently some 200 species are distributed across twelve major clades in a relatively tight monophyletic cluster. In our assessment of 196 species for twenty morphological and behavioural criteria the clades show good biological cohesion. Saprotrophy, necrotrophy and hemi-biotrophy of woody and non-woody roots, stems and foliage occurs across the clades. Phylogenetically less related clades often show strong phenotypic and behavioural similarities and no one clade or group of clades shows the synapomorphies that might justify a unique generic status. We propose the clades arose from the migration and worldwide radiation ~ 140 Mya (million years ago) of an ancestral Gondwanan
Phytophthora
population, resulting in geographic isolation and clade divergence through drift on the diverging continents combined with adaptation to local hosts, climatic zones and habitats. The extraordinary flexibility of the genus may account for its global ‘success’. The 20 genera of the obligately biotrophic, angiosperm-foliage specialised DMs evolved from
Phytophthora
at least twice via convergent evolution, making the DMs as a group polyphyletic and
Phytophthora
paraphyletic in cladistic terms. The long phylogenetic branches of the DMs indicate this occurred rather rapidly, via paraphyletic evolutionary ‘jumps’. Such paraphyly is common in successful organisms. The proposal to divide
Phytophthora
appears more a device to address the issue of the convergent evolution of the DMs than the structure of
Phytophthora
per se. We consider it non-Darwinian, putting the emphasis on the emergent groups (the DMs) rather than the progenitor (
Phytophthora
) and ignoring the evolutionary processes that gave rise to the divergence. Further, the generic concept currently applied to the DMs is narrower than that between some closely related
Phytophthora
species. Considering the biological and structural cohesion of
Phytophthora
, its historic and social impacts and its importance in scientific communication and biosecurity protocol, we recommend that the current broad generic concept is retained by the scientific community.
