Elongation factor 1-alpha sequences alone do not assist in resolving the position of the Acoela within the Metazoa

摘要

Following years of debate, and thanks largely to molecular systematic studies, the long-held idea that the phylum Platyhelminthes is the most basal branch of the bilaterian Metazoa has been widely abandoned (e.g., Haszprunar 1996; Carranza, Baguna, and Riutort 1997; Hausdorf 2000). This new status quo has been upset by a recent paper that has suggested that the P-latyhelminthes are polyphyletic and that a single "flatworm" clade, the Acoela, is the most basal extant bilaterian lineage distinct from the other platyhelminths (Ruiz-Trillo et al. 1999). The potential importance of the acoels being confirmed as the outgroup to all other bilaterians is that they would then be instrumental in determining the character states of the ancestral Bilateria through outgroup comparison and thus greatly further our understanding of the evolution of the animals. The currently recognized out-group--the diploblasts--are generally too evolutionarily and phenotypically distant for this sort of comparison to be informative. Although recent data support the molecular results (Henry, Martindale, and Boyer 2000), some systematists distrust the findings based on 18S rDNA and have claimed that long-branch attraction (LBA; Felsenstein 1978) affected the final analysis of these data (e.g., Adoutte et al. 2000). Furthermore, according to the same 18S rDNA study (Ruiz-Trillo et al. 1999), the supposed sister group of the acoels, the nemertodermatids (Smith and Tyler 1985; Smith, Tyler, and Rieger 1986; Tyler 2001), do not unite with the acoels at the base of the Bilateria, but instead fall within the turbellarian flatworms. This has suggested that the more controversial position of their sister group, the acoels, was erroneous.