Pirsonia

The genus Pirsonia and its type species Pirsonia guinardie were described in 1990 by E. Schnepf, G. Drebes and M. Elbrächter. The species was discovered parasitizing the centric marine diatom Guinardia flaccida in the North Sea. Due to its unique life cycle, with differentiation into trophosome and auxosome, it was left with no certain taxonomic position (as Mastigophora incertae sedis).^[1] Electron microscopy observations revealed that the flagellate stages of Pirsonia present stramenopile flagella, with tripartite hair-like extensions (mastigonemes). However, there were some differences with the typical stramenopile organization of the flagellar cytoskeleton, leading the authors to wait for genetic analyses to establish its position within the stramenopiles. Six species were added to the genus: P. diadema, P. eucampiae, P. formosa, P. mucosa, P. punctigerae and P. verrucosa.^[2][3]

The first molecular phylogenetic analysis that included species of Pirsonia was published in 2004. It confirmed the position of all species firmly within the stramenopiles, near the hyphochytrids, with the exception of P. mucosa which was revealed to be a cercozoan and placed under a separate genus Pseudopirsonia.^[4] A second analysis in 2006 confirmed these findings, and the genus was placed into its own family Pirsoniaceae and order Pirsoniales (under the botanical code, Pirsoniida under the zoological code).^[5][6] Pirsonia was the sole genus of Pirsoniales until 2022, when additional genera Feodosia and Koktebelia were described, followed by Bordeauxia, Bullionia and Noirmoutieria in 2024. In these studies, another species P. chemainus was described, bringing the total to six.^[7][8]