New record and phylogenetic affinities of the oomycete Olpidiopsis feldmanni infecting Asparagopsis sp. (Rhodophyta)

A new geographic record of Olpidiopsis feldmanni infecting the tetrasporophytic stage of Asparagopsis sp. from the Adriatic Sea, confirmed through morphological identification, allowed us to expand previous observations of this organism. Ultrastructural investigations of environmental material showed a large central vacuole and a cell wall thicker than previously reported from other basal oomycete pathogens of algae. Phylogenetic analysis closely associates O. feldmanni to Olpidiopsis bostrychiae concurrent with structural observations. This constitutes the first genetic characterisation of an Olpidiopsis species that was initially described before 1960, adding to the genetic data of three other marine Olpidiopsis species established and genetically characterised since the millennium. The paper discusses concurrences of the ultrastructural observations made here and in previous studies of the marine Olpidiopsis species with those made on the freshwater studies.