The diatom parasite Lagenisma coscinodisci (Lagenismatales, Oomycota) is an early diverging lineage of the Saprolegniomycetes

Diatom pathogens are potentially of great ecological importance as they might play a role in the breakdown of plankton blooms. Even though several diatom-infecting oomycetes have been described, none of them has so far been included in molecular phylogenetic investigations. One of the most enigmatic species of these pathogens is Lagenisma coscinodisci, which infects centric diatoms of the genus Coscinodiscus. Its life-cycle and assumed mode of sexual recombination suggested a basal position for this pathogen. In the current study, the phylogeny of this pathogen was investigated using SSU and cox2 sequences. Phylogenetic reconstructions revealed a position basal in Saprolegniomycetes, together with Atkinsiella, while Olpidiopsis and other marine holocarpic pathogens were in an even more basal position. As the formation of oospores has been reported in Peronosporomycetes and Saprolegniomycetes, as well as in some Olpidiopsis species, the unique mode of sexual reproduction observed in Lagenisma either represents an alternative way of exchanging genetic material, rather than representing an ancestral state or suggests that oospores have evolved several times independently from resting zygotes.