Abstract
The gene coding for the small subunit ribosomal RNA molecule was sequenced in three marine spirotrichs:
Pseudoamphisiella lacazei,
P. alveolata, and
Parabirojimia similis, the systematic positions of which remain unclear in spite of recent progress. Phylogenetic trees were constructed by means of neighbor-joining (NJ), least-squares (LS), maximum parsimony (MP), and Bayesian inference (BI) methods in order to test previously inferred phylogenetic assignments of
Pseudoamphisiella and
Parabirojimia based on morphological and morphogenetic data. The main results are that: (1) all trees constructed have similar topologies in which the genus
Parabirojimia derives at the base of the class Stichotrichia, which suggests that it might represent a unique taxon at about suborder or even-order level near urostylids; (2) the present study supports the conclusion reported by previous researchers that urostylids could be a paraphyletic assemblage and that
Uroleptus and
Paruroleptus should be removed from the order Urostylida; (3) Pseudoamphisiellidae/
Pseudoamphisiella always clusters with
Prodiscocephalus borrori rather than with urostylids, which indicates that they might represent an ancestral form for both discocephaline and urostylid species; (4) the monophyly of the traditional suborder Discocephalina and typical euplotids is consistently rejected. Based on both morphological/morphogenetical and molecular data obtained, a new suborder Parabirojimina n. subord. is suggested, which currently includes one family, Parabirojimidae n. fam., and the single genus
Parabirojimia.