View Preaxostyla Tree

The taxon Preaxostyla was established to recognize the well-supported relationship of Trimastix and oxymonads in molecular phylogenetic analyses (Simpson, 2003). Prior to the use of molecular data, both Trimastix and oxymonads were hypothesized to be related to a variety of other eukaryotes, but none with much certainty (see Dacks et al., 2001 for discussion). SSU rRNA gene phylogenies including Trimastix and the oxymonad Pyrsonympha showed the first strongly supported position for either lineage, as sisters to one another (Dacks et al., 2001). This has been further supported by other single- and multi-gene phylogenies (Hampl et al., 2005; Hampl et al., 2009; Slamovits and Keeling, 2006).

The relationship of Preaxostyla to other eukaryotes remains less certain. They were proposed to be members of the Excavata based on the typical excavate structure of the cytoskeleton of Trimastix (O’Kelly et al., 1999; Simpson et al., 2000), remnants of which were subsequently identified in the oxymonad Monoceromonoides (Simpson and Patterson, 2001; Simpson et al., 2002), Recent multi-gene molecular phylogenies have supported the monophyly of most, and probably all, Excavata, including Preaxostya (Hampl et al., 2009). The weight of molecular phylogenetic evidence suggests that Preaxostyla is most closely related to parabasalids and Fornicata, the other major groups of Excavata that lack classical mitochondria (Cavalier-Smith, 2003; Simpson et al., 2008; Hampl et al., 2009), however a closer relationship with Malawimonas remains plausible (see Simpson et al., 2006).