Genetic and phenotypic analyses of Pythium isolates from reed suggest the occurrence of a new species, P. phragmiticola, and its involvement in the generation of a natural hybrid

• A new species closely related to the reed pathogen Pythium phragmitis was detected.
• It differed consistently from P. phragmitis in ITS, β-tubulin and cox II sequences.
• It is probably the second parent of a previously described natural Pythium hybrid.
• It is formally described here as P. phragmiticola sp. nov.
• P. phragmitis and P. phragmiticola can outcross, resulting in sterile, interspecific hybrids.

A set of isolates closely related to the reed pathogen Pythium phragmitis was found among Pythium strains obtained from reed (Phragmites australis) soil and plant samples in Germany and Switzerland. These isolates consistently differed from P. phragmitis at 6, 6 and 3 nucleotide positions in the rDNA internal transcribed spacer region, the β-tubulin, and the cytochrome oxidase II gene, respectively. They are formally described here as Pythium phragmiticola sp. nov. An analysis of phenotypic traits like oospore size, growth and aggressiveness to reed showed that P. phragmiticola is almost indiscernible from P. phragmitis, as most features assessed overlapped considerably between species. Phylogenetic analyses provided evidence that P. phragmiticola is the second parent of a previously described Pythium hybrid involving P. phragmitis. The alloploid status of the hybrid, and the parents' reproductive separation was confirmed by molecular evidence and the high ratios of aborted oospores. In natural environments there obviously are two species closely associated with reed, P. phragmitis and P. phragmiticola which are homothallic, but can outcross, resulting in sterile, vegetatively propagated interspecific hybrids. Driving forces leading to the radiation of these two species are unknown, but might be connected with divergent parasitic adaptation to different host tissues or species.