Evidence for the occurrence of natural hybridization in reed-associated Pythium species

A comparison of oomycete diversity in reed stands (Phragmites australis) of Lake Constance, Germany, and maize fields close by provided evidence for the occurrence of natural hybridization between Pythium phragmitis, a newly described reed pathogen, and an as-yet unknown Pythium species closely related to P. phragmitis and P. arrhenomanes. Internal transcribed spacer and beta-tubulin sequences of a large set of Pythium isolates from reeds showed dimorphic signals at several positions, indicative of a mixture of characters of two parent species. Involvement of P. phragmitis in the putative hybrid species was confirmed after cloning and sequencing of ITS regions and beta-tubulin genes of the hybrid isolates. Mitochondrially inherited cox II gene sequences did not show dimorphic sites and suggested that the hybridization event was relatively ancient, or that other species might be involved. Intermediate habitat preferences, morphological characters and aggressiveness towards reeds and other grasses confirmed the suggestion that these isolates comprise a natural hybrid between two Pythium species. Pythium arrhenomanes, likely to be involved in the putative hybrid's evolution, was repeatedly isolated from maize fields adjacent to P. phragmitis-infested reed stands. The interface between natural habitats with established oomycete communities and agricultural fields with potentially introduced pathogens might constitute an evolutionary hot-spot giving rise to new species with as-yet unknown host ranges. As indicated by inoculation tests, the hybrid was significantly more pathogenic towards reed rhizomes than P. phragmitis, which caused no damage to these organs. This is apparently the first report of the occurrence of natural hybridization in Pythium.