Ultrastructure and phylogenetic position of Regin rotiferus and Otto terricolus genera et species novae (Bicosoecida, Heterokonta/Stramenopiles).

We describe two novel flagellates isolated from soil, Regin rotiferus and Otto terricolus, genera et species novae, which we cultivated and characterized by light and transmission electron microscopy and by 18S rDNA sequence analysis. Both strains exhibit the key characteristic structural feature of Bicosoecida; i.e. the L-shaped cytostomal root system with an x-fiber, used for feeding. Otto terricolus displays unique novel morphological traits; thus, it has a basal swelling on each flagellum, a root 3/root 2 distribution of 10 + 1 microtubules, and an amoeboid stage in its life cycle. Regin rotiferus has flagella without swellings and a root 3/root 2 distribution of 7 + 3 microtubules, a pattern commonly observed in the Bicosoecida. We present an updated exhaustive maximum likelihood phylogeny of 48 cultured, complete or nearly complete (+1600 bp) 18S rDNA Bicosoecida sequences. Both new species fall into a well-supported freshwater Siluaniidae clade, without being particularly closely related. The morphology and phylogeny do not conclusively support Rictus as a member of Bicosoecida.