The evolutionary relationships of rotifers and acanthocephalans

Advances in morphological and molecular studies of metazoan evolution have led to a better understanding of the relationships among Rotifera (Monogononta, Bdelloidea, Seisonidea) and Acanthocephala, and their relationships to other bilateral animals. The most accepted morphological analysis places Acanthocephala as a sister group to Rotifera, although other studies have placed Acanthocephala as a sister taxon to Bdellodea or Seisonidea. Molecular analyses using nuclear 18S rRNA and mitochondrial 16S rRNA genes support Acanthocephala as a sister taxon to Bdelloidea, although no molecular data is available for Seisonidea. Combining molecular and morphological analyses of Bilateria leads to a tree with Platyhelminthes, Rotifera, Acanthocephala and Gnathostomulida (and probably Gastrotricha) as a sister group to the annelid-mollusc lineage of the Spiralia (Lophotrochozoa).