Rootlet homology, taxonomy, and phylogeny of bicosoecids based on 18S rRNA gene sequences

The cytoskeleton of Pseudobodo tremulans was investigated by transmission electron microscopy to resolve homologies among flagellar rootlets within the bicosoecids. The rootlet arrangement in P. tremulans is typical for bicosoecids. The basal body of anterior flagellum (bb2) has 2 rootlets: a dorsal rootlet of 2 microtubules and broad ventral rootlet (8+3 microtubules); the basal body of posterior flagellum (bb1) has two rootlets of 1 and of 2 microtubules respectively. There is a fibrillar rootlet that descends to the nucleus, and a set of fibrillar connectives around and between the basal bodies. The ventral rootlet is shown to originate from basal body 2, revealing that it is a homologue of rootlet 2 (not rootlet 3 as was accepted earlier for bicosoecids) of heteroconts. The fibrillar rootlets are not necessarily homologous in this order, as they may originate from either or both basal bodies. In general, all the flagellar microtubular rootlets of bicosoecids are homologues of those described from other stramenopiles. The bicosoecid rootlet 2 is much more developed than in colourless chrysophyceans. Rootlet 3 is often absent in bicosoecids while it is prominent in chrysophyceans as the ventral rootlet associated with feeding. Nuclear small subunit rRNA gene sequence (ssu rDNA) for 6 species of bicosoecids belonging to 2 families (Cafeteriidae and Siluaniidae) are partly congruent with the morphological characterstate data. In agreement with morphological data, the molecular trees robustly support a clade that includes Cafeteria and Symbiomonas (Cafeteriidae) and a clade composed of Adriamonas plus Siluania (Siluaniidae). The ssu rDNA trees also depict a paraphyletic Cafeteriidae (Pseudobodo and Cafeteria are not sister taxa) and a paraphyletic Siluaniidae (Adriamonas and Caecitellus are not sister taxa). The further investigation of the representatives of 2 other families (Pseudodendromonadidae and Bicosoecidae) by electron microscopy and molecular methods is required to clarify the phylogeny of bicosoecids.